Habilitationsschriftzur

Erlangung der Venia legendi

fur das Fach Astronomie

der

Ruprecht - Karls - Universitat

Heidelberg

vorgelegt von

Silke Britzen

aus Trier

2004

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High energy radiation from AGN

and radio jets on pc- and kpc-scales

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Nur die Fulle fuhrt zur Klarheit ...

aus Schiller: Spruche des Konfuzius

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Contents

1 Preface 6

2 Introduction 7

3 The Current Paradigm of AGN 9

4 Radio surveys for pc-scale structure 13

4.1 The CJF survey . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4.1.1 Homogeneity and statistical validity . . . . . . . . . . . 15

5 Jets and jet components 17

5.1 pc-scale morphologies of jets in CJF-AGN . . . . . . . . . . . . 17

5.2 kpc-scale morphologies of CJF-AGN and misalignment betweenpc- and kpc-scale structure . . . . . . . . . . . . . . . . . . . . 21

5.3 pc-scale proper motion . . . . . . . . . . . . . . . . . . . . . . 25

5.4 Beaming, bulk relativistic motion and their effects . . . . . . . . 34

6 Multi-wavelength aspects 40

6.1 The need for multifrequency observations of AGN . . . . . . . . 40

6.2 X-ray emission processes . . . . . . . . . . . . . . . . . . . . . 41

6.2.1 ROSAT observations of the CJF sources . . . . . . . . . 42

6.2.2 The soft X-ray properties of AGN from the CJF sample . 44

6.2.3 The determination of beaming parameters based on X-ray and radio data . . . . . . . . . . . . . . . . . . . . . 46

6.2.4 Searching for a possible correlation between the largescale structure of AGN and X-ray emission . . . . . . . . 51

7 Gamma-ray emission mechanisms 53

7.1 The exceptional SEDs of Blazars, Evidence for RelativisticallyBeamed Gamma Rays . . . . . . . . . . . . . . . . . . . . . . . 53

7.2 The γ-bright blazars in the CJF survey . . . . . . . . . . . . . . 55

7.2.1 The typical radio properties of γ-bright CJF sources . . . 55

7.2.2 Curvature and Paths - Evidence for Binary Black Holes? 57

8 Unification of AGN 58

8.1 Current unification scheme . . . . . . . . . . . . . . . . . . . . 58

8.2 Low- and high-redshift BL Lac Objects . . . . . . . . . . . . . . 59

8.3 CSS and GPS . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

8.4 Testing unification theories with superluminal motion statistics . 60

8.5 Is the current beaming model sufficiently complex? . . . . . . . 61

8.6 Grand Unification . . . . . . . . . . . . . . . . . . . . . . . . . 62

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9 Conclusions 63

Acknowledgements 65

References 66

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1 Preface

This habilitation thesis is based upon the Caltech-Jodrell Bank Survey for flat-spectrum radio sources (called CJF hereafter) and its analysis. While surveys ingeneral play an important role in astronomy as the so called ’bread and butter’work - everybody uses them - not many scientists are inclined to perform thenecessary time consuming work.Cosmological questions have traditionally been the science drivers in survey pro-posals for telescope time. This was true also for the CJF-survey, whose maingoals were to explore the evolution of compact radio sources with cosmic epoch,to test for cosmological evolution in the velocity distribution, and to place firmlimits on cosmological parameters such as the deceleration parameter q (viasuperluminal motion studies, e.g., Vermeulen & Cohen 1994; Vermeulen 1995,1996) and the Hubble constant H (via gravitational lenses, e.g., Wilkinson etal. 1994, 1996). But in addition, survey work has proven to be very useful forfurthering - and sometimes initiating - astrophysical research in general.In principal, there are two main types of surveys:1) General surveys searching the sky or parts of it for sources (ideally previ-ously unknown ones) with unexpected characteristics. These surveys are es-sential when new wavelength bands become available for astronomical researchor where improvements in technology allow access to fainter objects at earliercosmological epochs – thus the more distant objects.2) Surveys for specific properties of samples of known objects.The CJF-survey belongs to the latter category: It investigates the physical prop-erties in flat radio spectrum sources, which are at the same time those sourcesthat contain highly compact components in their core regions. These sources,also called ’Active Galactic Nuclei’ (called AGN hereafter) comprise radio galax-ies, quasars, BL Lacertae objects, and the so-called ’Blazars’. They are amongthe most powerful objects known in the universe whose principle energy sourceis thought to be the famous and still mysterious ’Black Holes’, which were firstpostulated on theoretical grounds by Schwarzschild (1916).The very fact that AGN are found in general at higher redshifts allows the studyof their cosmological evolution. With the fully accomplished and analyzed CJFsample, a complete, flux-density limited VLBI1 (Very Long Baseline Interferom-etry) survey of 293 AGN is available to test current paradigms of AGN- andjet-physics. This data base is unprecedented in its statistical validity due toa fully homogeneous observational strategy, data reduction, and data analysis.The CJF survey is the first available survey that provides several epochs of VLBIobservations of a complete, large sample.In this thesis I briefly outline the current status of AGN jet research, presentthe results derived from the analysis of the Caltech-Jodrell flat spectrum sample

1VLBI is at present still the only method to achieve a sufficient resolving power for astro-

nomical instruments to observe very distant objects in detail. The resolving power of the VLBI

arrays used for this work is about 1–1.5 mas.

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and discuss the implications of these results on our understanding of jet- andAGN-physics.Our picture of AGN is the result of an interplay between the intrinsic propertiesof these objects and the enhancement and dilution via several factors that areextrinsic to the source (like beaming, obscuration, scattering). The inherentproblem of AGN investigation is that the selection of objects is biased and pos-sible conclusions are always affected by these selection effects.The aim of this thesis is to-describe the current paradigm-introduce the CJF survey and compare it with other new VLBA surveys-present and discuss the results of the CJF survey with regard to

• jet physics: the main emphasis lying on the innermost regions, i.e. theparsec-scale (pc-scale hereafter) regions of the AGN jets.

• multi-wavelength aspects (X-ray and γ-ray observations): data only takenat radio wavelengths can never reveal the whole picture of an AGN. Forinstance, there is no possibility to determine reliably distances to suchobjects by radio astronomical observations alone.

• unification scenarios: these try to reduce the variety of objects to a smallnumber of source classes and object types

In the framework of this habilitation thesis I will not present the extensivetabular and graphical material which describes and presents quantitatively theresults of my work on the CJF-sample. This material can soon be found in aseries of papers in the journal ’Astronomy & Astrophysics’ and ’Astronomy &Astrophysics Supplements’ (Britzen et al., in prep.).

2 Introduction

The progress in our understanding of AGN has been a story of consolidationand integration. Previously separate phenomena have been shown to be dif-ferent manifestations of the same underlying physical process, distinguished byluminosity, observer’s distance or orientation, or environment. There is everyindication that this progression will continue.In the early 1900’s Heber Curtis (Curtis, 1918) found while observing M87 ”acurious straight jet ... apparently connected with the nucleus by a thin line ofmatter”. These optical observations were not followed up by Curtis and only dueto the development of radio astronomy in the 1960s, did jets emanating fromthe nuclei of certain galaxies became a major theme of research in astrophysics.The detection of apparent superluminal motion in jets, helical trajectories, one-sidedness of jets, etc. belong to the major achievments of VLBI. With theupgraded VLBA, eMERLIN, VSOP2, ALMA, and later SKA, the potential ofVLBI will even be greatly enhanced.Realistic physical models can be tested and constrained through in-depth studies

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of the observational results of prototypical objects, in particular when VLBI ob-servations are combined with information from other spectral regimes, e.g., withoptical and γ-rays (e.g., Britzen et al. 2000, 2001 for PKS 0420-014; Britzenet al. 1999b for PKS 0528+134), and with X-rays (e.g., Otterbein et al. for0836+714). While detailed studies of individual objects are undoubtedly impor-tant for understanding the origin and collimation of the jets and their emissionmechanisms, a full understanding will only come from the supplementary studyof large, well-defined samples that can be subjected to statistical analysis. Inparticular, if multi-wavelength observations, making use of new observing facil-ities offering higher resolution and spectral capabilities will be available, a morecomplete understanding of the AGN phenomenon – including the nature of the’central engine’ – might be a realistic goal.In the following parts of this habilitation thesis I will first outline the currentstatus of AGN research and thereafter describe the status of the work on theCJF-sample, raise some questions which can be attacked, and discuss the presentresults. My analysis mainly deals with the kind of AGN found in the CJF-sample:radio galaxies, radio quasars2, and BL Lac objects.

2Only about 10% of the quasars are radio-loud

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Jet

Obscuring Torus

Black Hole

Narrow Line Region

Broad Line Region

Accretion Disk

Figure 1: The figure at the top shows an artists impression of the new ’unifiedmodel’ for the different kinds of quasar activity (Hasinger et al., 2002). For awell-fed black hole part of the matter streaming towards the center never actuallyreaches the black hole, but is blown apart in a bipolar cone-like outflow, drivenby the strong radiation pressure of the central object. In the case of APM0827+5255 we are incidentally looking down the gas stream which is ”X-rayed“by the central light source (Graphic: Max-Planck Institute for Astrophysics /Spruit). A schematic diagram of the current paradigm for radio-loud AGN isshown at the bottom (not to scale; Urry & Padovani 1995).

3 The Current Paradigm of AGN

Although a commonly accepted description of how AGN function does not existyet, some parts of the complete scenario are widely accepted. Accretion into avery deep potential well is the only mechanism that appears to be sufficientlyefficient (easily >5%) in converting matter into the energy to power the mostluminous AGNs. According to the generally accepted relativistic jet model (e.g.,Blandford & Konigl 1979) a supermassive black hole (SMBH) surrounded byan accretion disk, is the primary energy source of an AGN (Rees 1971; Scheuer1974; Blandford & Rees 1974). The SMBH is also surrounded by fast-movingclouds — probably under the influence of the strong gravitational and the mag-netic field — emitting Doppler-broadened lines. The emission in the inner parts

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Figure 2: A schematic diagram of the inner jet and disk of a radio-loud AGN(not to scale; S. Britzen).

of an AGN is highly anisotropic (see Fig.1, Urry & Padovani 1995). More distantclouds emit narrower lines. Absorbing material in some flattened configuration(usually idealized as a torus) obscures the central parts. As a consequence forlines of sight in the plane of the torus only the narrow-line emitting clouds areseen (narrow-lined of Type 2 AGN), whereas the near-IR to soft-X-ray nuclearcontinuum and broad-lines are visible only when viewed face-on (broad-linedor Type 1 AGN). Unified models also require the presence of a thick dustytorus outside the accretion flow (on the scale of several parsecs) that can ab-sorb enough soft X-ray, UV, and optical radiation to hide both the direct corecontinuum emission and the broad emission line region if viewed from a smallenough angle (Padovani, 1997).One of the first discoveries made about compact radio sources was that many

of them varied in intensity and (later also found in) structure. The deducedtimescale gave a lower bound on the brightness temperature of the sourcewhich worked out to be in excess of the inverse Compton limit of ∼ 1012K(Pauliny-Toth & Kellermann, 1966). Rees realised that this problem could beameliorated by relativistic expansion and predicted that sources would appearto expand faster than the speed of light (Rees, 1967).Hints of superluminal motion were found as early as in 1969, two years after theinvention of VLBI. But the first definitive measurements were not made until1971, when two teams, one headed by Shapiro (Whitney et al., 1971), the otherby Cohen (Cohen et al., 1971) found evidence for the predicted high-velocityoutflow in 3C273 and 3C279.The development of VLBI imaging revealed not only that jets are commonamong compact radio sources but also that jets on parsec scales tend to bewell-collimated, one-sided, and apparently superluminal. The relativistic jet inradio-loud objects is oriented roughly perpendicular to the disk (Blandford &

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Payne 1982). The commonly observed one-sidedness of the pc-scale core jetis produced by relativistic boosting, amplifying the intensity of the jet orientedtowards the observer. Relativistic beaming (Shklovskii 1963) is the dominanteffect on the appearance of these objects (e.g., Eckart et al., 1986; Witzel,1987; Witzel et al., 1988; Readhead, 1993; Zensus & Pearson 1987), and hasmotivated the development of the so-called “unified theories” of quasars andradio galaxies (Orr & Browne 1982) that explain the appearances of sub-classesof AGN as a result of differences in the aspect angle viewing a ’universal proto-type’.In general, different components of the AGN are dominant at different wave-lengths. Jets are seen most strongly in radio, optical, and X-ray frequencies, theaccretion disk is thought to be a strong optical/UV/soft X-ray emitter, whilethe absorbing material will emit predominantly in the IR (Padovani, 1998). Aproper understanding of AGN will therefore only come through multifrequencystudies.Several models have been proposed to explain the origin of jets - all of thesemodels involve the expulsion of a certain fraction of the matter being accretedby the supermassive black hole (SMBH). The major difference between the mostimportant classes of jet launching scenarios is whether or not magnetic fields areassumed to be primarily responsible for the expulsion of jet plasma. Since jetsemit via the synchrotron mechanism it is clear that magnetic fields are involved,and several plausible ways to use magnetic fields to accelerate and collimateflows have been studied.The advantage of magnetic acceleration mechanisms is that they can simulta-neously produce relativistic velocities, narrow jets and large momentum fluxes.This is supported by the discovery of powerful, dynamical magnto-hydrodynamic(MHD) instabilities that rapidly build strong fields in accretion disks. Rees(1971) proposed the idea that jets are predominatly a Poynting flux with littlemass loading. Most of the magnetically accelerated jet models rely on eitherextracting energy and angular momentum through magnetic fields anchored inthe disk (e.g., Blandford & Payne 1982), or by extracting the spin energy ofthe BH itself, through magnetic fields threading its horizon (e.g., Blandford &Znajek 1977).Some authors have argued that the magnetic field is always primarily poloidaland any toroidal field that is generated by the rotation will quickly vanish throughreconnection. At the other extreme it has been proposed that rotation domi-nates and the magnetic field lines behave like a coiled spring pushing the jetsout along the spin axis. Both components are relevant to centrifugal modelswhere the inertia of the outflowing plasma plays a crucial role. Finally, there aremodels without any long range order to the field and the local anisotropy asso-ciated with loops of magnetic flux creates the collimation (Blandford, 2002).As shown in sections 5 and 6, many AGN reveal complex jet structures even onlarge (kpc-) scales. Once launched, the key question therefore becomes: howcan these theoretical jets survive to the distances demanded by observations,

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where jet hot-spots are often much narrower than 1% of the jet length?It could be shown, by stability analyses of hydrodynamical jets and Kelvin-Helmholtz instabilities, that faster jets, particularly relativistic ones, could sur-vive longer (e.g., Turland & Scheuer 1976; Hardee, 1982, 1987; Ferrari et al.1980). Various assumptions about magnetic field geometry have been studiedwithin an ideal MHD framework (e.g., Hardee & Norman 1988).A study of rotating jets confined by toroidal fields has shown that rigid rotationtends to stabilize, while differential rotation destabilizes the jet in a way similarto the magneto-rotational instability which is now believed to dominate viscosityproduction in accretion disks (Hanasz et al. 2000). No specific model for thepoduction of cosmic jets is absolutely compelling. While it is highly likely thatMHD processes are of importance in the launching and inital collimation of jets,the details of these processes remain extremely controversial (Blandford, 2002).One cause for this is the fact that the region immediately around the BH isnot yet accessible for direct observation: VLBI in the cm-regime, even includingspace based antennas cannot image these parts due to optical depth effects. Butfor mm-VLBI these regions are transparent. Very recently, VLBI experimentsat 3mm wavelength reached a resolution of about 15–20 Schwarzschild radii inM87 (Virgo A) corresponding to 2×1012 km (Krichbaum et al. in prep.). Theresolution achieved at 1 mm on the BH in our Galactic Center is better than 1astronomical unit (Krichbaum et al. 1998) and about 6×1011 km in Virgo A(in 3 mm observations).But these observations are still far from the imaging quality reached by conven-tional VLBI due to the lack of a large number of participating telescopes. Wetherefore have to rely on indirect methods including, especially, multi-wavelengthmonitoring to understand their structure.

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4 Radio surveys for pc-scale structure

The CJF survey is the largest VLBI survey in terms of the number of sourcesand epochs of observations that has ever been available for studies of AGNphysics. In addition, it is the first homogeneous data base that provides an-gular resolution and dynamic range data necessary to identify and trace in-dividual jet components reliably across the epochs. Due to its completeness,statistical statements can be made concerning the distributions of velocities,bending, pattern motions, and changes in the brightness of jet componentsand their dependance on the core separation. We hope to have produceda body of astrophysical data that can be used to develop and test physicaltheories of active nuclei in a more reliable way than has been possible up tonow.Extensive VLBI surveys in the past have provided a morphologic classifica-tion of compact radio sources (e.g., Wilkinson 1995 and references therein),motion studies yielded apparent velocity and Lorentz factor statistics thatcan be compared to other indicators of relativistic motion (Ghisellini et al.1993, Vermeulen 1995). In addition, these VLBI surveys have provided newperceptions for unification models and cosmology (Vermeulen 1995, Keller-mann 1993, Gurvits 1994).

Several attempts have been made to investigate the pc-scale structures of sam-ples of AGN by using the ’Very Long Baseline Array’ (VLBA), an array consistingof 10 (25m) radio antennas spread throughout the USA.10 –20% of the VLBA observing time is currently allocated to survey propos-als. While it would be of great importance to monitor a statistically meaningfulnumber of AGN with high time sampling over periods of years, practical consid-erations force compromises in the number of sources and observations. Thus, itis either possible to perform a detailed monitoring of a small number of sources,or observe a larger sample of objects in several observing runs with less densesampling in time.While detailed VLBI monitoring with dense sampling in time of single sources hasdiscovered so far unknown physical processes (e.g., the detection of componentsresembling ”trailing” shocks in a 16 month sequence of monthly polarimetric 43GHz images of 3C 120 by Gomez et al. 2001), less frequent monitoring of large,complete samples can address the mechanisms of AGN physics on a statisticalbasis.In the following, I briefly introduce newer surveys of AGN. While taking noteof important surveys obtained with other interferometric networks (VLA, MER-LIN, EVN), I here concentrate on the large surveys either under way or currentlyperformed with the VLBA.The goal of VLBA surveys is manifold: exploring the polarisation (e.g., Pollacket al. 2003), studying the radio morphologies of sources detected in other wave-length regimes (e.g., VLBA observations of SiO masers, Desmurs et al. 2000),

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population studies (e.g., The COINS Sample: VLBA Identifications of Com-pact Symmetric Objects, Peck & Taylor 2000), providing structural informationfor upcoming new-instrument studies (e.g., The VSOP 5 GHZ Continuum Sur-vey: The Prelaunch VLBA Observations; Fomalont et al. 2000), searching forgravitational lenses (e.g., A VLBA 15 GHz Small Separation Gravitational LensSurvey, Patnaik et al. 1996), etc. In addition, calibrator surveys like the VLBACalibrator Survey (VCS1; Beasley et al. 2002), which consists of astrometricallyderived positions accurate to a level of a milliarcsecond for 1332 extragalacticradio sources, provide an archive not only for astrometric research, but also forstudies of AGN and cosmological questions. The S5 polar cap phase-connectedastrometry program aims at an improvement of astrometric precisions at cen-timeter wavelengths (Ros et al. 2001).In 1994, a multi-epoch VLBA survey program started with observations at 15GHz (Kellermann et al. 1998; Zensus et al., 2002). The aim was to studythe outflow in radio jets ejected from quasars and active galaxies. The originalsample was based on the ”1 Jy catalog“ (Kuhr et al. 1981), which lists allextragalactic radio sources with flux-densities >1 Jy at 6cm wavelength. Theselected sources have a flat spectral index (α > −0.5 for Sν ∼ να) above 500MHz, and a total flux-density at 15 GHz greater than 1.5 Jy for sources northof the celestial equator, or greater than 2 Jy for sources between declinations 0

and -20. By adding sources of special interest, the final sample now contains96 sources suited for velocity determinations (Kellermann et al. 2004).Smaller samples of AGN have been observed at different radio frequencies bye.g., Jorstad et al. (2001) (at 8.4, 15, 22, and 43 GHz) with the advantage ofbeing able to study wavelength dependent effects.Geodetic VLBI campaigns monitor polar motion and UT1 by observing a celestialreference system using AGN. In a series of simultaneously performed campaigns(starting in the 1960’s), a large and important archive of VLB data of AGN hasbeen built up. This serves as a basis for astrometric studies but can as wellbe explored for astronomical research (see e.g., Charlot 1990; Fey et al. 1996;Piner & Kingham 1997; Britzen et al. 1999b, 2000).Many objects appear in several of the above surveys (and also in the CJF). Com-bination of the data from different surveys obviously enhances the possibilitiesto study time- and wavelength dependent effects.

4.1 The CJF survey

The CJF survey integrates the Caltech-Jodrell Bank (CJ) surveys in a complete,flux-density limited survey. For all the CJ surveys, sources were selected fromthe region of sky with declination > 35 and galactic latitude |b| > 10. Theoriginal sample (”Pearson-Readhead”, PR sample), is a complete sample of 65sources with flux density S5GHz ≥1.3 Jy, part of which were imaged with VLBIat 5 and 1.6 GHz (Pearson & Readhead 1981, 1988; Polatidis et al. 1995). TheCJ1 extended the PR sample down to S5GHz ≥0.7 Jy (total of 200 sources)

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(Polatidis et al. 1995; Thakkar et al. 1995; Xu et al. 1995). For CJ2 thelimit is S5GHz ≥0.35 Jy with the restriction that the sources should have a flatspectrum (α > −0.5) (total of 193 sources; Taylor et al. 1994; Henstock et al.1995).CJF, defined by Taylor et al. (1996), is a complete flux-limited VLBI sample of293 flat-spectrum radio sources, drawn from the 6 cm and 20 cm Green BankSurveys (Gregory & Condon 1991; White & Becker 1992) with selection criteriaas follows: S(6 cm)≥ 350 mJy, α6

20≥−0.5, δ(1950)≥35 , and |bII|≥10.This sample is mostly a superset of the flat-spectrum sources in the Pearson-Readhead Survey (Pearson & Readhead 1981) based on the 6 cm MPI-NRAO5 GHz surveys (e.g., Kuhr et al., 1981), the First Caltech-Jodrell Bank Survey(CJ1: Polatidis et al. 1995; Thakkar et al. 1995; Xu et al. 1995), and the SecondCaltech-Jodrell Bank Survey (CJ2: Taylor et al. 1994; Henstock et al. 1995).Optical identifications have been made for 97% of the CJF sample, and red-shifts are known for 94% of the objects (e.g., Stickel & Kuhr 1994; Stickel etal. 1994; Xu et al. 1994; Vermeulen & Taylor 1995; Vermeulen et al. 1996). Apublication presenting the optical information on the CJF sources is in prepara-tion.The composition of the CJF is 66.9% quasars, 18.4% radio galaxies, 11.3%BL Lac objects, and 3.4% still unclassified objects. Between 5 and 10% ofthe sources in the CJ samples are compact symmetric objects (Pearson et al.1998). An overview sumarizing existing investigations of first- and second-epochobservations of subsamples of the CJF and references is given in Pearson et al.(1998). Preliminary results for selected samples of CJF-sources have been pub-lished in Britzen et al. (1999a; 2001a) and Britzen (2002).VLBI observations of the CJF sources have been performed since 1990. Subsam-ples were observed in several global VLBI observations and in VLBA snapshotruns at 6cm wavelength between March 1990 and December 2000. The VLBAsnapshot runs of CJF sources started in 1998. The observational strategy wasto observe the sources in eight 5.5-minute snapshots per epoch and to recordthe data over 32MHz total bandwidths broken up into 4 baseband channels,with 1-bit sampling. The data were correlated in Socorro, NM, USA.We aimed at a minimum of three epochs for every source since we believe thatfor the unambiguous determination of the jet component position and motionparameters, it is necessary to have at least three observing epochs spread overroughly 4 years. The last and most recent epoch for a subsample of 34 sourceshas been obtained in December 2000. For details concerning the observationsand data reduction see Britzen et al. (in prep.)

4.1.1 Homogeneity and statistical validity

A problem in almost all of the VLBI-survey work is the necessity to distribute thesurvey work among the collaborators. This leads to inhomogenities due to dif-

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ferent data reduction and analysis strategies (personal biases). The CJF surveyis the first survey to overcome this problem by using a fully homogeneous andcomplete reduction and analysis. For large surveys, the question concerning thestatistical relevance of the results and the homogeneity of the different stagesof data reduction and analysis is of utmost importance.In order to create a homogeneous, statistically valid database, I have done asystematic (re-)analysis of all epochs for all sources obtained since 1990, simul-taneously with the aquisition and analysis of new data.All sources and epochs have been analyzed in the same standardized way, in-cluding calibration (in order to enable a reliable calibration of the sources, weincluded in each observing run at least one calibrator source, 3C279, which wasobserved at similar uv ranges at each epoch) and automated mapping withindifmap (v.2.4b, M.C. Shepherd 1997) with the help of the automatic mappingscript muppet (G. Taylor).A critical analysis of the VLBI jet components and a comparison of jet prop-erties requires a quantitative determination of the jet components’ features. Itherefore fitted Gaussian model components directly to the observed visibili-ties (real and imaginary parts) using the Levenberg-Marquardt non-linear leastsquares minimization technique (program ’modelfit’ within difmap), comprisingthe brightness, sizes and positions of the individual jet components.All data sets were model-fitted, starting from a point source and using circularGaussian components. We calcualted (statistical) uncertainties for the fittedGaussian parameters for each source at each epoch via the covariance matrixcalculated in difmap (for details see Britzen et al., in prep.).The complete survey consists of 293 objects, nine of which appear to be point-like in at least three of the epochs. Seven sources were either too faint duringthe time of the observations or wrong coordinates were used. For 94% of thesources the redshifts are available. Thus, for the following analysis 269 sourceswere statistically investigated and the results are presented in the followingchapters.

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5 Jets and jet components

pc-scale jets have been studied extensively with all available VLBI networks(even incorporating the space radio telescope VSOP, Hirabayashi 1996) atevery available wavelength from mm to 20-cm (see Zensus 1997, and refer-ences therein). Of prime interest have been morphological classifications ofcompact radio sources and studies of the statistics of apparent (often faster-than-light) jet component motions. With improved data quality additionalquestions concerning the physical processes in jets can be addressed.For a small number of sources, detailed information on pc-scale jet and com-ponent physics has been derived via dedicated monitoring programs withdense time sampling. On the other hand for the majority of known AGN,only a crude picture of jet component motion has been obtained by justa small number of observations performed over large time intervals. Thus,the current situation is that our knowledge about pc-scale jets is biased byobservational constraints. The CJF now fills the gap between these on theone hand in detail studied objects and the objects studied just occasionallyon the other hand. The main goal of the CJF is to provide comparabledata in quality, number and timespan covered, as well as in data reduction,and analysis techniques for a large number of AGN in a complete survey.Only by this strategy can comparable processes in pc-scale AGN be studiedon a similar observational basis. Above all, the determination of apparentsuperluminal motions is strongly dependent on uniformly distributed obser-vations of similar resolution and dynamic range. Differences in data qualityeasily lead to over- or underestimated component motions and thus preventany reliable statistical conclusion.Before I concentrate on the analyis of my CJF-based results on AGN jets,I briefly introduce our current conceptional view of pc-scale jets and thenature of jet components. Then I outline the panorama of different mor-phologies in the CJF. The relation between the position angle of the pc-scaleand kpc-scale jet of an AGN enables the estimation of parameters that areotherwise complicate to assign, e.g., providing hints on the viewing angle.The distribution of the proper motions can yield information on the cosmol-ogy. The apparent superluminal motions of different classes of objects asdetermined by their optical properties are confronted with the predictions ofunified schemes. The observational results are discussed in light of the rel-ativistic beaming model that explains kinematic properties of pc-scale jetsvia shocks in an underlying continuous jet flow.

5.1 pc-scale morphologies of jets in CJF-AGN

pc-scale jets are complex physical entities; the details of their nature are stillunder investigation. Higher dynamic range VLBI observations start to unravelthose physical processes that take place beyond simple outward motion of the

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main constituents, the so-called ”components”. In some objects, filaments,limb-brightening, and edge-brightening have been reported. State-of-the-artimages in addition to the main components tend to reveal weaker featuresand often directly the underlying continuous jet emission. The investigation ofthese complex jet processes, taking place at faint flux levels, requires the high-est resolution observations, which only can be provided in a series of dedicatedexperiments.Not only does the resolution and the observational frequency determine thekind of processes we can resolve, but by choosing the frequency we also makea decision about the closeness of the jet-part we intend to study to the centralenergy engine.The scientific goal of the CJF survey is a statistical analysis of the basicproperties of jet components and their motion in the pc-scale jets. Snapshotobservations at 5 GHz performed with the VLBA provide this kind of informa-tion. Flux-density limited surveys – as the CJF – are biased towards selectingpredominantly strongly beamed objects, and indeed the majority of the CJFAGN reveal ”core-jet“ type structures. They show a steeper-spectrum jet-likecontinuous extension, which contains distinct structure ”components”, on oneside of an unresolved flat-spectrum ”core”. The cores are mainly determinedby their compactness, and flat or inverted radio spectra. Phase-referencing ob-servations have seldom been performed and only in a few cases demonstratedlittle movement (Bartel et al. 1986) of the core component.Some AGN also show evidence for jet emission on both sides of the core oreven more complex, disrupted jet structures. Fig. 3 shows images (the Gaussianmodel components superimposed) of selected AGN. The variety of pc-scale jetmorphologies is obvious. In addition, the jets are seldom straight: many of theobserved jets are curved, and in some cases quasi-oscillatory trajectories or ridgelines have been observed (e.g., 1803+784, Fig. 6).Whenever a jet is present in an AGN, localized patches of relatively high inten-sity (hot spots, knots or components) along their lengths occur. Rees (1978)identified the knots in M87 as internal shocks which develop from irregularitiesin the flow speed of the jet. Today, the main components of the jets are gen-erally associated with shock waves that either are generated as a consequenceof fluctuations at the source or arise from the development of large-amplitudeinstabilities. The general picture that has emerged is that relativistic electronsare accelerated, mainly at shock fronts, and that they emit synchrotron radia-tion at low frequencies and inverse Compton emission at high frequencies. Thesynchrotron radiation provides the soft photons for the inverse Compton emis-sion in low power sources and the disk ultraviolet emission supplies the softphotons in high power sources. Shock models have been studied in great detail(e.g., Hughes et al. 1989; Konigl 1981; Marscher 1980; 1992; Marscher & Gear1985).While this is generally accepted, the nature of the positively charged compo-nent of the jet plasma remains unknown. So far, it has been universally assumed

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that protons provide the positive particles (e.g., Celotti & Fabian 1993).Theirmuch greater masses require extra energy to accelerate, but they provide littleradio emission. Several other possibilities have been discussed: an e+e− plasmahas been suggested very early (e.g., Kundt & Gopal-Krishna 1980; Roland &Hermsen 1995).Blandford (2002) suggests that jets start off in a predominately electromagneticform. At some finite distance from the black hole, jets metamorphose into apair plasma. At a yet greater distance, these jets should ultimately interactstrongly with their surroundings as they become radio sources and decelerate.Presumably, when the jet is powerful, the outflow can remain relativistic and bean FR2 source; when the jet is weak and decelerates to a subsonic speed, anFR1 source results. B.L. Fanaroff and J.M. Riley (1974) found that the relativepositions of regions of high and low surface brightness in the lobes of extra-galactic radio sources are correlated with their radio luminosity. The luminositythat divides the objects into FR1 (low power) and FR2 (high power) sources is:L(178MHz) ≤ 2 · 1025h−2

100W Hz−1str−1.

19

Figure 3: An overview of pc-scale jets in selected AGN from the CJF survey.Model fits convolved with the interferometric beam and with the residual mapadded to them are shown with the Gaussian model components superimposed.The positions and sizes of the jet components are indicated by circles withdifferent sizes.

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5.2 kpc-scale morphologies of CJF-AGN and misalignment between pc-

and kpc-scale structure

It is generally believed that the small-scale jets observed in VLBI images are thesources of, and are continuous with, the large scale jets that supply the outerlobes. The large-scale structure of AGN is assumed not to be beamed and thusrepresents the intrinsic extended structure of the sources.One important qestion that I address with the VLBA observations of the CJFsurvey is the orientation distribution of core-dominated radio sources. Thesuperluminal proper motions of components within pc-scale jets provide infor-mation on this question (Cohen, 1989). Another independent, albeit crude,measure of source orientation is the apparent misalignment between the posi-tion angles of the pc- and kpc-scale jets (hereafter ∆PA). Pearson & Readhead(1988) found in the distribution of ∆PAs a highly unexpected bimodal dis-tribution of relatively well aligned and roughly orthogonal jets. More currentinvestigations of larger samples by e.g., Conway & Murphy (1993) proved thisexcess to be statistically significant compared to the predictions of simple mod-els. The so-called ’misaligned population’ of core-dominated AGN reveals a∆PA of 70 to 90 (’secondary peak’; Appl et al. 1996). (The position angledistributions of large samples of AGN have been studied in detail by Pearson &Readhead, 1988; Wehrle et al. 1992; Conway & Murphy 1993; Xu et al. 1994and Appl et al. 1996.)Small apparent misalignments can be explained by small random bends. Smallintrinsic bends between pc- and kpc-scales will give the large ∆PAs that areobserved if sources are viewed almost along the direction of the VLBI jet. As-suming similar intrinsic bends, sources in which the VLBI jet is oriented closerto the line of sight should show more extreme misalignment angles. The orthog-onal misalignments however can not be explained by these processes (Conway& Murphy 1993).The CJF survey offers the unique opportunity not only to study the misalign-ment distribution in this sample but also to investigate the specific propertiesof the misaligned objects. Several ∆PA-distributions of different AGN-sampleshave been published; however, no compilation of ∆PA values is currently avail-able. I performed a literature search for kpc-scale morphological information onthe CJF sources. To build up a homogeneous database for this kind of analysis,I redetermined the orientation of the large-scale structures (in Britzen et al., inprep.) from published maps and compared them with the pc-scale orientationderived directly from the CJF VLBI survey results. This sample is the largesthomogeneous sample that has so far been available for this kind of misalignmentstudies in AGN. Detailed information on the ∆PAs and the extended morphol-ogy can be found in Britzen et al. (in prep.).117 of the 293 CJF-sources have a point-like VLA structure and do not con-tribute to this analysis. In Fig. 4 I show the distribution of ∆PA for those CJFsources that reveal kpc-scale extended emission. The figure clearly shows the

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expected peak around 0 (and smaller peak around 150-180) for the alignedobjects and the ’secondary’ peak around 75 for the misaligned objects. Asdiscussed below, primarily driven by the BL Lac objects.In Fig. 5 I show the misalignment distribution for the three classes of objects:quasars (a), BL Lac objects (b), and galaxies (c). As obvious from the figures,the three classes clearly show a different ∆PA distribution. While the galaxiestend to show only aligned kpc- and pc-scale jets, a prominent fraction of theBL Lac objects consists of misaligned sources. The distribution for the galaxies(c) shows evidence for strong alignment: the 0 and 150 peak are well defined,while the secondary misaligned peak is completely missing. This ’misaligned’peak however, is quite prominent in the distribution for the BL Lac objects (b).The quasar distribution (a) contains more objects and is broader, covering thecomplete range of misalignment angles. The primary and secondary peak aresignificant.

22

This result for the CJF sources is in agreement with Conway & Murphy (1993),

(a)

Center at RA 18 03 39.17700 DEC 78 27 54.2900

CONT: 1803+784 IPOL 1660.000 MHZ 1803+784.ICLN.56PLot file version 8 created 23-NOV-1998 16:44:54

Cont peak flux = 1.9643E+00 JY/BEAM Levs = 1.3750E-04 * ( -1.00, 1.000, 2.000, 4.000, 8.000, 16.00, 32.00, 64.00, 128.0, 256.0, 512.0, 1024., 2048.)

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Figure 6: The morphology of S5 1803+784 (member of the CJF sample) isshown on different angular scales. (a) shows the pc-scale structure as observedwith a global VLBI array (Britzen et al., in prep.). (b) shows a MERLIN mapof the intermediate-scale structure (Britzen et al. 1999c), and (c) shows thelarge-scale VLA structure in a map by Antonucci et al. (1986).

who consider the properties of the aligned and misaligned sources and find atendency for those objects with more extreme blazar properties to belong to themisaligned population.Appl et al. (1996) on the other hand find no definite significant correlationbetween morphological criteria and other properties of misaligned sources. Theonly common feature seems to be the obvious helical form of the jet.In Fig. 6, I show an example for a misaligned CJF BL Lac object showing helical

23

structures: S5 1803+784 (Britzen et al. 1999c). This source has also beenstudied in detail by e.g., Eckart et al. (1986, 1987), Strom & Biermann (1991),Gabuzda (1999), Gabuzda & Cawthorne (2000), Ros et al. (2000, 2001). GlobalVLBI observations reveal the pc-scale jet to be oriented in East-West direction(Fig. 6(a), Britzen et al. in prep.), while the large-scale structure of the samesource comprises a dominant core component and a weak secondary component∼ 45′′ away at position angle Θ ≈ −166 south-south-west of the core, whichis presumably physically related to the BL Lac object (Antonucci et al. 1986).The jet flow has been redirected, as can be seen in the MERLIN-map (Fig. 6(b),Britzen et al. 1999c). At a core separation of ∼ 0.5 arcseconds, the jet changesits orientation and bends by about 90. The bent jet continues in southwarddirection towards the southern component.The jet of 1803+784 is an example for a self-similar morphology: Sinusoidalbending is observed in 1803+784 on all observable scales (Britzen et al, inprep.); the 90 degree bending towards the South however, only appears once inthe jet of 1803+784, as seen in the MERLIN only map. Tateyama et al. (2002)discuss the overall radio morphology of this source as described by two domi-nant components: a narrow helical jet of 6 kpc in extension and a much largernorth-south component perpendicular to the helical jet as a “dogleg” structure.This “dogleg” structure might be attributed to a collision between the jet anda massive cloud (Stocke et al. 1985).S5 1803+784 is an exceptional case for a misaligned source, since we here de-tected the transition region where the change in jet orientation takes place. Afew other misaligned objects have been studied in detail in the literature and areparticularly well mapped at the intermediate scale of 10 to 100 milliarcsecondswith MERLIN or the European VLBI Network (3C216, 3C309.1, 3C345, andMkn501). Rather than building a homogeneous sample, they all show pecu-liarities and require specific comments which have suggested various ideas forcurvature modelling. In the case of 3C345, an archetype source whose VLBIjet components are following different quasi-helical 3-dimensionally bent trajec-tories (Kollgaard et al. 1989), two outer knots aligned with the nucleus havesuggested a sudden change in the orientation of the central engine.For most other misaligned sources, the relation between pc-scale and kpc-scalestructure is not known. Several solutions to solve the misalignment problemhave been discussed (e.g., Strom & Biermann 1991). Possible explanations in-clude the following possibilities: more than one jet exists, we don’t see the samejet with different angular resolution, the jet flow has been interrupted at sometime in the past and we possibly see the disturbed remaining bits or, the jetflow has been redirected.Helicity seems to be a common feature of the misaligned sources and is a com-mon phenomenon in pc-scale AGN jets. According to Appl et al. (1996) onecan distinguish essentially two main types of mechanisms for the deformation ofjets from a “straight” path, namely a) interaction of the flow with the ambientmedium, and b) a change of orientation of the jet source. The former comprise

24

bending by ram pressure, pressure gradients and Kelvin-Helmholtz instabilities,while examples of the latter are precession of the axis of the central black hole orof the accretion disk. A model explaining how the transition between the wig-gling on different scales can be caused by helical distortions (Kelvin-Helmholtz,Hardee) has been presented by Conway & Wrobel (1995, applied for Mkn 501).This geometrical model requires a helical jet viewed close to the line of sight inwhich the helical amplitude saturates. The kpc-scale appearance is the resultof emission from saturated helical components moving parallel to the cone axis.Combined with the selection effects due to relativistic beaming this scenariocan, under certain conditions, naturally explain these orthogonal sources. Inorder to fit the presently observed misalignment distribution, the simplest suchhelical model requires that misaligned sources must have core bulk Lorentz fac-tors γ > 20.Several authors have argued that jets which are initially straight and pressure-confined should naturally develop helical distortions due to the effects of Kelvin-Helmholtz instabilities (Birkinshaw 1991 and references therein). The clear di-vision between the aligned and the orthogonally misaligned source populationscould indicate that the former have straight jets, while the latter have helicallydistorted jets (Conway & Murphy 1993). A step further is the speculation byConway & Murphy that the bimodality could easily be explained if helicallydistorted jets occurred in binary black hole sources and straight jets in sourcescontaining a single black hole.An alternative model has been suggested by Appl et al. 1996: twisted accretiondisks which should be even more frequent than binary black holes in the centralengines of active galactic nuclei. Outflows can then be stabilized along twodifferent axes, orthogonal to inner and outer parts of the disks.

5.3 pc-scale proper motion

From the CJF observations, I made CLEAN maps and derived the parame-ters of Gaussian-component models. In most of the sources, it is possible totrace the same components in the jets across different epochs. The proper mo-tions of these jet components can be calculated. This proper motion analysisis complicated by the fact that different jet components in the same sourcecan move with different velocities, components can merge or split, can dim orbrighten in flux-density, and appear at different position angles (curved paths).In Fig. 7, and 8 I show examples for jet component motion in three of the 293CJF sources: 0711+356 (quasar at z=1.620), 1106+380 (galaxy at z=2.29),and 2138+389 (unclassified object at z=1.306, tentative redshift, R. Vermeulenprivate communication). The jet components (denoted by the letter ’C’ plusnumber according to increasing separation from the core; ’CC’ denotes for acounter-jet component) that can be followed across the epochs are connectedby a dashed line in the same color as used in the following figures (Fig. 9–12).

25

We call the most compact feature the reference point (’core’). Only phase refer-ence observations could determine whether the core remains at a fixed position.The position of the reference point (r) is marked by a solid black line.The following figures show different aspects of the motion of the identifiedjet components: Fig. 9 shows the jet components in the xy-plane, and theirmotion (lines) calculated based on the different positions and weigthed by theflux-density of the individual components; In Figs. 10, 11, and 12 the x- andy-positions as function of the epoch are displayed; In addition, the radial- andazimuthal proper motions as function of the core separation (r) are shown.I obtained similar results for the complete CJF. The results presented here arebased on a careful identification of the components across the epochs and mul-tiple checks of the resulting motions in the xy-plane. It turned out that not alljet components yield proper motion values of equal significance. This qualityscheme adopted takes the following properties of each jet component into ac-count:-the jet component has to be clearly separated from other components or thecore-brighter components are more reliable-the component should not merge or split (in different epochs)-the component should be visible in at least three epochsSingle, bright jet components that are clearly separate in all epochs were givena quality 1, while all sources that merge or split were assigned a quality 3. Withthis additional quality criteria, an additional selection ensures that the most re-liable jet component proper motions enter further calculations and conclusions.

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Figure 7: Model Fits convolved with the interferometric beam and with theresidual map added to them are shown for the quasar 0711+356 and the galaxy1106+380 at three epochs each. The model components are superimposed.Circles mark the sizes and the positions of the Gaussian components. Theidentified jet components that can be traced across the epochs are connectedby dashed lines in different colours (for better comparison, I here use the samecolours as in Fig. 9–11).

27

C3 CC1

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Figure 8: Four epochs of the unclassified object 2138+389 are shown. Thesame notation as in Fig. 7 applies. The colours for the components are identicalin Fig. 12. 28

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Figure 9: The jet component identification and motion of the three AGN (shownin Figs. 7 and 8) in the xy-plane. Colours mark the different components, anddifferent symbols are used to indicate the time order of the observations (tri-angle: first epoch, square: second epoch, diamond: third epoch, cross: fourthepoch). The proper motion model, calculated based on the individual jet com-ponent and epoch data, is shown as a line in the components colour. Elementson this line show model positions corresponding to each of the observed epochsby shape.

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Figure 10: The jet components positions in x and y for 0711+356 as functionof the epochs are shown. In addition, the radial (vr) and azimuthal (vθ) propermotions as function of the core separation (r) of 0711+356 are shown.

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Figure 11: The same relations as in Fig. 10 are shown for 1106+380.

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Figure 12: The same relations as in Fig. 10,11 are shown for 2138+389.

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Figure 13: The histograms show the distribution of proper motions in the jetsof the CJF sample sources. In (a)–(c) the distributions for all jet componentsin the three different classes, quasars (a), BL Lac objects (b), and galaxies (c)are shown. Figs. (d)–(f) show the relations for the subsamples consisting of themost reliable jet components in the different classes (please note the differentx-axis scales, especially for the BL Lac objects).

In Fig. 13, the proper motion distributions of the three optical classes are shown.(a)–(c) show the results for all jet component proper motions. (d)–(f) show theresults for the most reliable jet components (quality 1) only. A general trendis that in the higher quality data for all classes the fast jet component motionsare reduced or removed. The shape of the distributions remain to be the samefor the quasars and the galaxies. The median proper motion values based onthe most reliably determined data are listed in Table 1. We find similar propermotion values for jet components in quasars, BL Lac objects, and galaxies. Thedispersion is largest for the galaxies, and quasars. Jet components in BL Lacobjects show a smaller divergence from the median value.In Fig. 14, I show the total proper motion as function of the redshift for the

different classes of objects. All classes reveal a trend towards slower propermotion values with increasing redshift.

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Table 1: The median proper motion values calculated based on the CJF sub-sample consisting of the most reliably determined jet components (quality 1)only.

Class median±uncertainty percentage uncertainty[mas/year]

Quasars 0.064±0.068 106%BL Lac objects 0.079±0.049 62%

Galaxies 0.078±0.082 105%

5.4 Beaming, bulk relativistic motion and their effects

An emitting plasma, moving with velocity β = v/c at an angle θ to the line-ofsight reveals an apparent transverse velocity of βapp = (β sin θ)/(1 − β cos θ).βapp can exceed 1 if β ≈ 1 and θ is small. This ”superluminal” illusion (Rees,1966) is present in extragalactic radio sources expanding with space velocitieswithin one percent of the speed of light. Very fast motions, implying bulkLorentz factors, 2< Γ = (1 − β2)−1/2 < 10, naturally explain the asymmetryfound in the radio emissivities of jets: The jet with a component directed towardsthe observer is Doppler boosted, while the (usually unobserved) counter-jet isDoppler dimmed. The amount of beaming contributes to the morphology ofradio loud AGN, the spectral energy distribution (SED), the velocity of jet com-ponents, the amplitude and frequency of flux-density variability, the evolution ofmulti-frequency flaring, etc. Thus, the consequences of the anisotropic beamedradiation pattern are considerable, introducing significant selection effects inalmost any flux-limited sample.Structural variability and, in particular, apparent superluminal motion are fre-

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quently observed in AGN (e.g., Zensus & Pearson 1987; Witzel 1988; Vermeulen1995; Zensus 1997). In general, the observed apparent motions are too slowto explain the observation of intraday variability (IDV; Wagner & Witzel 1995and references herein), and the direct observations of the source sizes suggestthat the Lorentz factors could be much larger (the source size yields a directdetermination of the brightness temperature which is boosted by a factor 2Γfrom its value in the comoving frame which, in turn, cannot be much larger thanthe traditional inverse Compton unit of ∼ 1012 K; hence the minimum Lorentzfactor). On the other hand, if the Lorentz factor becomes too large, typicallylarger than ∼ 30, the radiative efficiency becomes unreasonably small from thetheoretical point of view. Coherent processes (Benford & Lesch 1998) couldprovide a solution to this problem and VLBI circular polarization observationsin the future might help to investigate this question.Straight jets are rare cases in the CJF. Most sources reveal curved jet ridge linesand in several objects jet components have been traced on curved paths (Zensus1997 and references herein). So far, in most VLBI observations, only the radialvelocity (core separation as function of time) has been calculated to determinethe apparent velocity. If there is a significant non-radial component to the mo-tion, then the measured radial speed underestimates the true apparent speedof the component. To estimate the contribution of the non-radial (azimuthal)velocity component, we also calculated the azimuthal apparent velocities basedon the position at the mean epoch.We have determined the radial and azimuthal proper motion of each jet com-ponent by performing a least squares fit to the measured component positions,relative to the reference point (core). We calculated the apparent velocities onthe basis of the derived proper motion values. The cosmological parameterswe assume are taken from the WMAP data (Spergel et al., 2003) (h=0.71,Ωbh

2=0.0224, Ωmh2=0.135).Fig. 15 shows histograms of the total apparent superluminal motion from the

q1 (quality 1) jet components in quasars (a), in BL Lac objects (b), and ingalaxies (c). The median values for the three distributions are listed in Table 2.We find some evidence for lower values for the jet component motions in BLLac objects than for the quasars, which reveal the highest apparent velocities.Our results show a trend for the galaxy subsample to have the lowest values butthe widest spread in data.Our result is confirmed by Jorstad et al. (2001) who find different apparent

velocities for quasars (10.6±6.2h−1c) and for BL Lac objects (4.2±3.1h−1c),using q=0.1, Λ=0, and H = 100h km s−1Mpc−1, although the average val-ues are higher than those obtained by us for the CJF sample. A wavelengthdependent effect is possible, with higher apparent velocities at higher observingfrequencies (Jorstad et al. 2001 observed mainly at 43 and 22 GHz). This re-sult, that typical VLBI component speeds in BL Lac objects are systematicallylower than those in quasars, was previously found by Gabuzda et al. (1994) andWehrle et al. (1992) at 3.6 and 6cm. It was noted before that the expansion

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Table 2: The median βapp-values calculated based on the quality 1 sources only.

Class median±uncertainty percentage uncertainty[c]

quasars 4.48±4.20 94%BL Lac objects 3.11±2.78 89%

galaxies 1.24±1.87 151%

velocities of BL Lacertae objects are smaller than those of quasars, which wasexplained either by assuming that they are extremely aligned sources but havethe same Lorentz factor as the other superluminal sources (e.g., Roberts & War-dle 1987; Cohen, 1989), or by assuming that they have smaller Lorentz factors(e.g., Mutel et al. 1990). Statistical results from the study of motions in largesamples of superluminal sources have been discussed among others by Ghiselliniet al. (1993), Vermeulen & Cohen (1994), and Vermeulen et al. (1995). Theseprevious studies have been hampered by the fact that they relied on smallerand/or inhomogeneously sampled data collections, i.e., taken from literaturesearches.Ghisellini et al. (1993) instead find that the mean apparent speeds of BL Lacobjects is similar to that of quasars, but smaller than that of HPQs (highlypolarized quasars).For a sample of 81 flat-spectrum objects — a subsample of the CJF sample— Vermeulen (1995) found no evidence for intrinsically different populations ofgalaxies, BL Lac objects, and quasars.In Fig. 16(a) I show the distribution of the azimuthal jet component speeds

for the quasars and the BL Lac objects. Although curvature plays an importantrole in some of the objects the contribution of the apparent azimuthal speed to

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the total apparent speed is ∼20%, thus the dominant contribution to the totalapparent speeds in AGN comes from the radial motion of the jet components.The azimuthal velocity component not only reflects the role curvature plays inAGN, but also uncertainties in the position of a component are reflected in thespread of the position angles. Curvature seems to play a more prominent role inthe inner regions of the AGN, as also seen in the mm-regime, where the degreeof bending increases towards the innermost regions.In Fig. 16(b) the βapp-z relation is shown. We find an increase for the mea-sured total apparent velocities with cosmic epoch for all the classes of objects.We find an upper limit to the measured apparent velocities: 0.35 mas propermotion per year seems to be the fastest velocity we can observe. This valuecould represent a maximum value for AGN velocities. The apparent velocitieswe can determine are constrained on the one hand by the time span betweenthe observations and the beam size of the VLBA beam. Within two years (thetypical time span between two epochs in the CJF), the components must haveseparated by 0.7 mas, to be clearly distinguishable. On the other hand, jet com-ponents with high apparent velocities might need more frequent observationsto be properly identified. But frequent VLBI monitoring of AGN did not provehigher but slower apparent velocities. With higher angular resolution and athigher frequencies, faster components could be detected. Thus, we cannot ruleout faster motions; however, they would not appear in our results.Jet components show a wide range of apparent velocities and different com-ponents in the same source can show different velocities. Very seldom do jet

37

components in the same source show similar velocities. We find acceleration ordeceleration of jet components along the jet. In many cases two jet componentsmerge or one splits. This can be the result of comparing observations with dif-ferent dynamic range or angular resolution, however, J.-M. Marti (priv. comm.)could show that one jet component can be accompanied by fainter componentsthat disappear. In quite a number of sources we find jet components that donot seem to move at all within our uncertainties. These ”stationary” featurescoexist with moving components.In addition, we find jet components approaching the core. Apparently negativevalues can be produced if the beam can not resolve a newly ejected componentand the center of brightness is thus shifted towards the core. Another expla-nation is, that the true core is hidden and the brightest component is part ofthe jet. It is also possible, that intrinsic bending mimics a decreasing separationbetween the jet component and the core.No distribution of relativistic Lorentz factors predicts a significant fraction ofmotions near zero (Vermeulen & Cohen 1994). Various explanations to ex-plain ”true” stationary components have been suggested. These componentsare either standing recollimation shocks caused by pressure imbalances at theboundary between the jet fluid and the external medium, they represent sitesof maximized Doppler beaming where a curved jet points most closely alongthe line of sight, or these are stationary shocks where the jet bends abruptly,presumably as a result of striking an obstacle (dense cloud) that deflects it. Sta-tionary components and possible explanations have been discussed by Gomezet al. (2001), Agudo et al. (2001), and Jorstad et al. (2001).It is debatable whether the components are discrete objects following ballisticpaths. Detection of superluminal motion does not necessarily imply that thesource of radiation is moving at near-relativistic speeds. It only requires a rela-tivistic phase or pattern speed, which could be different from the bulk velocityof the radiating plasma itself. The occurrence of different component speeds,as in M 87 (Biretta & Junor 1995 ), 3C 120 (Gomez et al. 2001), or Cygnus A(Krichbaum et al. 1998; Bach et al. 2002) — if not interpreted as geometric ef-fect — might be caused by variable pattern speeds (Vermeulen & Cohen 1994).For example, some theoretical studies predict interacting instability modes thatcan produce such variable pattern speeds (Hardee et al. 1995).Whether the pattern speed inferred from superluminal motion differs from thebulk motion of the plasma, as predicted by jet models that include relativisticshocks (Lind & Blandford 1985) is a matter of debate (Ghisellini et al. 1993;Vermeulen & Cohen 1994; Kollgaard, 1994).The jet can not be characterized by a single speed. There are good reasons for itto accelerate and decelerate along its length and even more reasons for it to es-tablish a transverse velocity profile.The jet parts that we see are probably shockwaves and this introduces additional kinematic complexity. In particular, thedirection and the speed of the emitting plasma behind a shock front must differfrom the kinematic speed of the shock. Higher angular resolution VLBI observa-

38

tions will be necessary to sort all this out in individual sources (Blandford, 2002).

39

6 Multi-wavelength aspects

Radio data alone are not sufficient to explain even the basic properties ofan AGN. What is needed is as much multiwavelength information as pos-sible. Only then it is possible to determine the origin and nature of theradiation processes. However, it is impossible to perform simultaneous mul-tiwavelength observations for a sample of 293 AGN. Although simultaneitycan not be achieved, all the CJF sources have been observed as part of theROSAT all sky survey. We lack simultaneous observations when comparingthe radio and the ROSAT data, but we do have the possibility to check thebasic concepts of beaming with the largest, uniform sample so far. Sincepart of the X-ray emission is isotropic and part is beamed inverse Comptonradiation from the radio jet, we can place limits on the inverse ComptonDoppler factor δIC.A subsample of the CJF sources has also been detected in the γ-ray regime(EGRET, Hartman et al. 1999). For those sources our picture of an AGN,although not complete, gains in clarity. The fact that sources have been de-tected in the X-ray and/or the γ-ray band immediately poses the questionwhy these source have and other objects have not been detected. Dedicatedintensive multi-wavelength campaigns as already performed for 3C279 (e.g.,Maraschi et al. 1994; Hartman et al. 2001) are urgently needed for a largernumber of AGN in order to be able to detect and investigate the temporalordering of events seen in different wavelength regimes.

6.1 The need for multifrequency observations of AGN

Although AGN form the ideal class of objects for multi-wavelength studies – astheir emission can cover almost 20 orders of magnitude in frequency from theradio to the γ-ray band – our knowledge about their physics is limited by obser-vational constraints and the inherent complex physical processes. Studies withmultifrequency coverage for a single object are rare (e.g., NGC 3783, 3C273,3C279). In addition, the objects studied are not necessarily representative oftheir class and they are usually relatively local. While single-object studies areof particular importance to test specific models, the general properties of theAGN population and their emission in various bands have to be studied on astatistical basis.The multifrequency spectrum of AGN is not that well known. The biggestgaps in our knowledge about AGN are in the far/mid-IR, UV, hard X-ray, andγ-ray bands. Only about 10% of AGN have data from the radio/optical/softX-ray bands (Padovani, 1998). The major hurdles studying AGN are: they arerare, making up only about 1% of all bright galaxies (although low luminosityAGN could be relatively more numerous); they are hard to find, especially inthe optical. Only ∼10% are radio-loud objects. The complete samples, whichare needed to study AGN evolution, are still quite small, especially the radio-

40

selected ones, and only less than 300 BL Lac objects are known over the wholesky (Padovani & Giommi 1995).Many models accounting for the observed broadband spectra of blazars havebeen developed. Most of them attribute the radio through optical emission tosynchrotron radiation, and X-ray through γ-ray emission to Compton scattering(e.g., Marscher, 1980; Konigl, 1981; Sikora et al. 1994). The models differ inthe location and structure of the acceleration and emission region(s).

6.2 X-ray emission processes

Only recently have CHANDRA observations been able to clarify the dominantemission mechanism for some individual sources and tripled the number of AGNwith X-ray jets (from seven to at least 19; Harris & Krawczynski 2002). 37radio galaxies and quasars with known X-ray emission from jets or hotspotsare now known (http://hea-www.harvard.edu/XJET/). However, the nature ofthe emission process responsible for the X-rays in most of the sources is still aquestion of debate and a statistical treatment of this phenomenon for a largersample of AGN is still missing.Generally there is little doubt that thermal bremsstrahlung is not a major contrib-utor to the X-ray emsision from most of the jet features (Harris & Krawczynski2002), although X-ray emission from hot gas may be closely associated withjets in some cases.The X-ray emission from knots in radio jets can mainly be attributed to syn-chrotron emission. Convincing evidence for this has been found in the opticalpolarization of M87, suggesting that the optical emission as well as the radioemission are produced via the synchrotron process (Harris et al. 1998; Biretta etal. 1991). On the other hand, X-ray intensities that lie well above the extrapo-lation of the radio/optical synchrotron spectrum are taken to be strong evidenceagainst the ”simple” synchrotron model. In addition, every synchrotron sourcemust also produce IC emission from at least the CMB and the synchrotron pho-tons themselves.Before physical parameters for a given feature can be determined, it is necessaryto identify the emission process. The important question thus is, which processdominates the X-ray emission. To decide on the basis of the X-ray spectralindex has become a common way – although not considered to be definitive –to differentiate IC from synchrotron emission. If αx ≤ αr, the IC emission isindicated, if αx ≥ αr, then synchrotron losses are invoked (Harris et al. 2003).To briefly summarize the current status concerning the most likely X-ray emis-sion processes:

• Inverse Compton emission does not require extremely high energies for theemitting electrons. Instead, both sufficient electrons and energy densityin photons are necessary to produce the desired scattered photons arenecessary.

41

• SSC emission occurs in all synchrotron sources, but since usually the pho-ton energy density is small compared to the energy density of the magneticfield, the major energy loss for all electrons happens via synchrotron emis-sion, squelching the SSC process.

• IC emission from relativistic electrons scattering off the CMB is - exceptfor high-z sources - in general incapable of producing the observed X-rayintensities.

• The ”beaming model” by Tavecchio et al. (2000) and Celotti et al. (2001)offers an escape from this dilemma. This model explains an enhancementof the X-ray emission relative to the synchrotron (radio/optical) by as-suming a relativistic bulk velocity of the jet fluid even on kiloparsec scales.

Thus, the leading contender for most of the X-ray jets appears to be a sin-gle or multiple spectral-component synchrotron model augmented by a modestbeaming with Γ < 10 to account for the one-sidedness of most of the recordedX-ray features. The IC/beaming hypothesis can consistently explain the emis-sion from several jets, and the low required beaming factors for some sourcesmake it probable that this emission component contributes substantially to theobserved X-ray fluxes of at least some of the observed X-ray features. Higherredshifts and steeper radio spectra appear to favor beaming, while low-z sourceswith flatter radio spectra are probably dominated by synchrotron emission. Al-though most of the radio sources with jet-related X-ray emission display spatialcoincidence among the radio, X-ray, and optical morphologies, there are a fewsources where this is not the case (Cen A, 3C 66B, and PKS 1127).While CHANDRA observations determined the X-ray production mechanismsin 19 sources, this infomation is not available for all sources in a large surveysuch as the CJF. However, assuming that the dominant mechanism is IC emis-sion, lower limits on beaming parameters – derived from ROSAT observationsfor the complete CJF – can be derived. So far this kind of analysis has reliedon smaller samples and/or data taken from the literature searches. With theROSAT observations of a complete and homogeneous VLBI survey, beamingindicators, relying on radio and X-ray data, can for the first time be estimatedon a significantly improved data basis. This enables an unprecedented study ofthe soft X-ray properties and possible radio/X-ray correlations of a large AGNsample, and might help to answer the question what role beaming plays in theproduction of X-ray emission. In addition, the cosmological evolution of theseproperties can be studied.

6.2.1 ROSAT observations of the CJF sources

The ROSAT All-Sky Survey was the first soft X-ray survey of the whole skyusing an imaging telescope (Trumper, 1983). It was performed from August 1,1990 to February 1, 1991 and yielded ∼ 125000 X-ray sources with a positional

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accuracy such that 68% of the sources are found within 20 arcsecs of (a) corre-sponding optical counterpart(s) (Voges et al. 2000) and a limiting sensitivity ofa few times 10−13 in the 0.1 - 2.4 keV energy band, depending on the spectralform and the amount of galactic absorption. The survey was followed by a pe-riod of pointed observations lasting until February 12, 1999; the PSPC detector(Pfeffermann et al. 1986) exhausted its gas supply earlier in September 1994.About 50% of the sources found in the ROSAT All-Sky Survey are expected tobe AGN, the majority of them quasars.It is not currently known which intrinsic properties are responsible for the X-rayloudness of these radio objects. Vice versa, it remains unclear why exactly the’non-detected’ objects are not seen in X-rays. As this distinction applies notonly to quasars but even more strongly to radio galaxies as well, orientation andbeaming effects are perhaps not the main cause. Environmental and evolution-ary effects might dominate the X-ray properties of the objects (Brinkmann etal. 1994).The complete CJF has been observed in either the ROSAT All-Sky surveyand/or pointed PSPC observations.The vast majority of the CJF sources are quasars or blazars, detectable in allwavebands of the electromagnetic spectrum accessible for astrophysical investi-gations. The redshifts of the identified quasars in CJF range from z=0.227 toz=3.889, with an average of 16 quasars per redshift interval of 0.2 in the rangez=0.6–2.6. This provides us with the opportunity to investigate possible corre-lations over a broad range in redshifts and to address important cosmologicalquestions, such as the evolution of AGN with cosmic epoch. The CJF is nowknown to contain some 25 galaxies and 11 BL Lac objects at z >0.6, enoughto allow a meaningful comparison of the properties of these source classes atthe same redshift and luminosity.In Fig. 17, I show the redshift distribution of the CJF sources detected byROSAT (solid black line) and those CJF sources that have not been detectedby ROSAT (hatched). The two distributions show a quite similar relation withredshift. The biggest difference between the two distributions is among thenearby objects: lower z objects (0< z <0.5) seem to have a higher likelihoodto be detected by ROSAT.Fig. 18 shows the distribution of Fig. 17 separately for the three different classesof objects. In the case of the quasar distribution, less low-z quasars are (pref-erentially) detected. BL Lac objects show the highest rate of detections amongthe low-z objects. However, BL Lac objects are less numerous and only appearin a smaller redshift range than the quasars. The number of low-z detected andnot detected galaxies is identical.

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Figure 17: The redshift distribution of all CJF sources. The source sample thathas been detected by ROSAT is indicated by a solid black line whereas thedistribution of the non-detected source sample is shown hatched.

6.2.2 The soft X-ray properties of AGN from the CJF sample

Details concerning the data extraction from the ROSAT All-Sky Survey (RASS)and the application of maximum-likelihood analysis are presented in detail inBritzen et al. (in prep.). The CJF contains 196 quasars, of which 94 were notdetected in the survey; 54 galaxies (including Seyferts), of which 33 were notdetected; 33 BL Lac objects (9 non detections); and 10 objects which have notyet been classified. The highest rate of non detection is amongst the galaxies,whereas most of the BL Lac objects have been found as strong X-ray emitters.From the cross-correlation of a source catalog from the ROSAT All Sky Surveywith existing radio surveys that resulted in a list of more than 2500 extragalac-tic objects (Brinkmann & Siebert 1994), it is known that BL Lac objects areradio- and X-ray loud and therefore they are over-represented among the brightsources in the survey. All but three galaxies are associated with clusters ofgalaxies or are even the cluster central galaxies. At least a certain part of theX-ray emission thus may originate from thermal radiation within the intraclustermedium or even, as shown for NGC 1275 in the Perseus cluster (Boehringer etal. 1993), from an interaction of the galaxy with the ambient medium. Wehave excluded these cases from our statistical analysis.The association of galaxies with clusters raises the fundamental question whetherthe X-ray emission attributed to radio galaxies is predominantly cluster emissionor whether the cluster environment is the evolutionary cause for active galaxieswith high X-ray and radio emission.

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Figure 18: The histograms show the different z-N distributions for the threeclasses of objects: (a) shows the quasars, (b) the BL Lac objects, and (c) thegalaxies; the detected objects are shown by the black line, the non-detectionsare hatched.

It should be noted that some quasars/BL Lac objects, observed repeatedly inpointed observations, show variations of their count rates by a factor of two tothree, often accompanied by spectral changes as well. For example, both thetwo extreme BL Lac objects Mrk 421 (1101+384) and Mrk 501 (1652+398)are known to show flux variations by about 50% in different ROSAT observa-tions. In Fig. 19(a) we plot the K-corrected monochromatic X-ray luminosities(at 2 keV) as a function of the 5 GHz radio luminosities for different classes ofobjects in the sample. The arrows denote upper limits of non detected objects.Interestingly, most of the objects (the galaxies included) exhibit a nearly linearrelation between X-ray and radio luminosities. Far above this general trend inFig. 19(a) we find the three extreme BL Lac objects Mrk 421, Mrk 501 (atlow radio luminosities) and 3C 66A. The three Seyfert galaxies 21116+818,0402+379, and 0309+411 show excess X-ray emission at low radio luminosi-ties. There is an obvious correlation between the X-ray luminosity and theradio-luminosity, with galaxies populating the low luminosity region and quasarsthe high luminosity end. Radio galaxies and quasars have nearly the same ratiosof X-ray to radio luminosities. The impressive correlation between Lx and Lr

over many decades of both luminosities strongly suggests a similar origin for theradiation in all radio sources. The lx − lo diagram for the sample shows, thatgalaxies and quasars have different slopes (in the lx − lo plane).Brinkmann et al. (1994, 1995) find strong correlations among luminosities inthe radio, optical, and X-ray bands, which differ for quasars and radio galaxies.The data strongly suggest that the X-ray, optical, and radio emission are trulyphysically connected. Zamorani et al. (1984) concludes that there are at leasttwo components contributing to the X-ray emission of quasars, and Browne &Murphy (1987) present a model of a ’canonical quasar’ where the optical and X-ray emission both contain beamed and unbeamed contributions. Even in radio

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(a) (b)

Figure 19: (a): The monochromatic X-ray luminosity (at 2 keV) as a functionof the radio luminosity. The arrows denote the upper limits of not detectedobjects. (b): The monochromatic X-ray luminosity (at 2 keV) as a functionof the optical luminosity. The arrows denote the upper limits of not detectedobjects.

galaxies the origin of the X-ray emission in the radio-jet region of the nucleusseems to be established (Fabbiano et al. 1984).Flat spectrum radio quasars are brighter in X-rays than are steep spectrum ob-jects at comparable optical luminosities (Brinkmann et al. 1997). The X-rayloudness αox = −0.384 log (l2 keV/l

2500A) has been used frequently in the

past for the discussion of the relative fraction of X-ray to optical emission in anevolving quasar source population. The average value 〈αox〉 = 1.28 found forthe CJF quasars is nearly identical to the result of Brinkmann et al. (1997) forthe much larger sample of flat spectrum quasars. However, the dispersion inFig. 20 is rather large and the upper limits with αox > 1.7 indicate that thesequasars are either highly variable or X-ray quiet, like BAL quasars (Brinkmannet al. 1999). A further study of these sources is highly desirable.

6.2.3 The determination of beaming parameters based on X-ray and radiodata

A fundamental parameter that describes relativistic motion in AGNs is theDoppler factor of the flow,

δ = [Γ(1 − β cos φ)]−1, (1)

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Figure 20: The X-ray loudness αox of the quasars as a function of the opticalluminosity. The arrows denote the upper limits of not detected objects.

where β is the speed (in units of the speed of light) Γ = (1 − β2)−1/2 is theLorentz factor of the flow, and φ is the angle between the direction of the flowand the line of sight.Several Doppler factors can be calculated. Assuming that the observed X-raysare of inverse Compton origin, one can compute the inverse Compton Dopplerfactor δIC (e.g., Jones et al. 1974, Marscher 1987). δIC equals the real Dopplerfactor δ of the source only if all of the observed X-ray flux is produced throughinverse Compton scattering by the component in question. If part of the X-rayflux is produced in other components or by some other mechanism, then δIC

is a lower limit to δ. Comparing the observed and the predicted X-ray flux byassuming the observed X-rays to be of inverse Compton origin, I compute theDoppler factor δIC for the CJF sources. In addition, I calculate the equipartitionDoppler factor δEQ (Guijosa & Daly, 1996; Readhead 1994) and compare thetwo Doppler factors. I further compare the Doppler factors with other beamingindicators derived from the VLBI observations, such as the value of the expansionvelocity and discuss whether bulk plasma speeds alone suffice to explain theobservations.

Synchrotron Self-Compton Limit

I use the formula of Ghisellini et al. (1993) and Guijosa & Daly (1996) to derivethe inverse Compton Doppler factor. The formula assumes the ideal case ofa uniform spherical source of angular diameter θd, where the radiating parti-cles have a power-law energy distribution and move in a tangled homogeneousmagnetic field (in their rest frame). One then can predict the expected inverse

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Compton X-ray flux density, given the relevant radio and X-ray data.

δIC = f(α)Sm[ln(νb/νop)ν

αx

fxθ6−4αd ν5−3α

op]1/(4−2α)(1 + z) (2)

Where fx is the observed X-ray flux density (in Jy) at frequency νx (keV), νop

is the observed frequency at the radio peak (in Gigahertz), θd is the angulardiameter of the source (in milliarcseconds), νb is the synchrotron high-frequencycutoff (assumed to be 105 GHz), and f(α) ' −0.08α+0.14 according to Ghis-ellini et al. (1987). We assume α = −0.75 for all the sources. The flux densitySm is the value that would be obtained at νop by extrapolating the opticallythin spectrum (Marscher 1987). For α=-0.75, this is about a factor of 2 largerthan the observed peak flux density Sop (Marscher 1977, 1987).In Fig. 21, I compare the apparent velocities for the CJF objects (average val-ues per source determined from only the q1 components in each source) withthe values for δIC derived from the SSC argument for the same componentsper source. Two scenarios can be adopted: (a) δIC for a spherical source (sin-gle blob assumption), (b) δIC for continuous jets (continuous jet assumption).From this figure we expect to see whether beaming, i.e., the bulk velocity, issufficient to explain the observed X-ray flux, since the pattern velocity does notcontribute to this Doppler factor.We find the highest δIC for the quasars, then BL Lac objects, and finally thesmallest values for the galaxies. This is confirmed by Ghisellini et al. (1993),who derived Doppler factors for about 100 sources with known VLBI struc-tures by comparing the predicted and observed X-ray fluxes in the synchrotronself-Compton model. The main results agree with those from other beamingindicators (superluminal motion and core-to-extended flux-density ratio) andsupport a simple kinematic model of ballistic motion of knots in relativistic jets.The derived Doppler factors are largest for core-dominated quasars, intermedi-ate for BL Lacertae objects, and smallest for lobe-dominated quasars and radiogalaxies. For a subsample of 39 superluminal sources, Ghisellini et al. (1993)find that apparent expansion speeds and Doppler factors correlate and havesimilar average numerical values.Fig. 21 (a) and (b) show the δIC-βapp relation for the three classes of objects,for sphere-like and continuous jets respectively. We find a similar range of valuesof δIC and βapp for the quasars and the BL Lac objects (see Table 3). Thissupports the conclusion that δIC and βapp are of similar value and that thereis no need to invoke other scenarios. Some galaxies reveal very small δIC andsignificantly higher βapp values. For these sources beaming seems not to be suf-ficient to explain the observed X-ray fluxes. Since cluster emission determinesthe X-ray emission in many galaxies, this can be easily explained. Due to thelarge dispersion, significant conclusions cannot be drawn, but some trends seemto emanate, nevertheless.

Equipartition Doppler Factor

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Figure 21: The panels show the relation between βapp and δIC for sphere-like(a) and for continuous jets (b) in those by ROSAT detected CJF sources withmost reliably determined jet component motions. In both images, the quasarsare shown as black filled squares, the BL Lac objects as filled red circles, andthe galaxies as filled green triangles.

The equipartition Doppler measures the ratio of the particle and magnetic energydensities. By definition is δEQ = δIC if the source is at equipartition. If thisis not the case, the ratio δEQ/δIC is a measure of the source’s deviation fromequipartition. δEQ can be calculated from single-epoch radio observations byassuming that the particles and magnetic field are in equipartition (Readhead1994). We use the formula for the equipartition Doppler factor by Readhead(1994):

δEQ = [(103F (α)]34([1 − (1 + z)−1/2]/2h)−2

(1 + z)(15−2α)

Table 3: The median δIC- and βapp-values for those sources with the mostreliably determined jet components (quality 1) only.

Quasars BL Lac Objects Galaxies

Number of objects 66 11 11

for sphere-like jetsmedian δIC± uncertainty 3.29±4.95 2.53±3.92 0.30±3.98median βapp± uncertainty 4.65±3.29c 3.11±2.37c 1.89±3.79c

for continuous jetsmedian δIC± uncertainty 4.28±8.55 2.72±2.68 0.23±6.37median βapp± uncertainty 4.65±3.29c 3.11±2.37c 1.89±3.79c

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Figure 22: The relation between the Equipartition δEQ and Inverse ComptonDoppler δIC factor for spherical and continuous jets for quasars (black filledsquares), BL Lac objects (red filled circles), and galaxies (green filled triangles).The continuous-jet case is displayed in grey filled symbols (same symbol formsas for the detections).

× S16opθ−34

d (νop × 103)−(2α+35)]1/(13−2α)

The equation as well as a graph for F (α) are given in Scott & Readhead (1977).Here we only need F (−0.75) = 3.4.This formula is calculated assuming a single blob. For most AGN assuming acontinuous jet might be a more realistic assumption. In this case, according to

Ghisellini et al. (1993) δcontinuous = δ4−2α/3−2αsphere .

In Fig. 22, I show the relation between δIC and δEQ, calculated for sphere-likejets and continuous jets respectively. We find a high correlation between bothDoppler factors, especially for those calculated assuming continuous jets. Thusboth present reliable estimates of the true Doppler factor. Guijosa & Daly(1996) confirm this high correlation between δEQ and δIC.

The βapp-relation for ROSAT detected and non-detected objects

We find similar median values of βapp for those quasars detected by ROSAT

(4.65±3.29c) and those that were not detected (4.56±3.84c). There is only aslight hint that the median values of the detected BL Lac objects (3.11±2.37c)are higher than those of the non-detected BL Lac objects (1.06±0.18c). How-ever, the number of the non-detected BL Lac objects is rather small, whichaggrevates a comparison. The median values for the galaxies again yield similarresults for the detected (1.89±3.79c), and non-detected (1.56±2.98c) objects.Thus, we find similar median values for the apparent velocities of quasars andBL Lac objects in the detected and non-detected samples.

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Figure 23: The βapp-z relation is shown for the detected sources (quasars asfilled black squares, BL Lac objects as filled red circles, and galaxies as filledgreen triangles) and non-detected objects (symbols and colours identical butopen symbols). The histograms (a) – (c) show the βapp distribution of the jetcomponents in quasars (a), BL Lac objects (b), and galaxies (c). The non-detections are shown hatched.

6.2.4 Searching for a possible correlation between the large scale structure ofAGN and X-ray emission

The X-ray emission from radio-loud AGN is thought to arise from the jet, be-cause radio-loud objects have stronger X-ray emission and rather different X-rayspectra than radio-quiet objects (Mushotzky 1993 and references therein). I de-scribed the connection between the pc-scale jet structure and the kpc-scale jetstructure for the CJF sources. Correlation studies between X-ray observationsand large radio surveys yield some evidence that the X-ray emission is relatedto the kpc-scale morphology. In this section, I therefore contrast the kpc-scalemorphology of the ROSAT detected and non-detected objects to search for thisassumed correlation.117 of the 293 CJF-sources reveal a point-like VLA structure. Among those,

51

72 have not been detected by ROSAT, but 45 have been detected. Thus wefind significant evidence that ROSAT-detected sources tend to show extendedradio emission on large scales. Besides two objects (0014+813, 1246+586),the point-like sources reveal relatively low X-ray fluxes. Fig. 24 shows the

50 100 150 200Position angle difference [Degrees]

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(a)

Figure 24: The distribution of the misalignment angle for the detected (solidblack line) and non-detected CJF sources is shown.

misalignment-angle distribution of the objects detected and not detected byROSAT. The detected and undetected populations reveal different misalign-ment distributions. While the misalignment angles are more or less randomlyspread among the undetected objects, the primary and secondary peak of thebimodal misalignment distribution can clearly be seen in the sample of the de-tected objects.In order to be able to classify the extended morphology of the CJF kpc-scalejets, I adopted a classification scenario. The large-scale structures appear tobe either unresolved, slightly resolved, jet-like and extended, double, or morecomplex. Sometimes, (complex) jet- and counter-jets are visible in the large-scale maps; halo emission can appear along with jets or be the only large-scalecomponent.To quantify the large-scale structure, I adopted complexity factors, where thenumber increases with the complexity of the morphology: the unresolved sourceswere classified as 0, the slightly resolved sources as 1, sources with a clearlyresolved jet as 2, double-source morphologies as 3, jet- counter-jet structuresas 4, and the most complex morphologies as 5.In Fig. 25 (a), I show in a histogram the distribution of these complexity-factorsfor the detected CJF sources and the non-detections. There are significant dif-ferences between the two distributions: the non-detected objects tend to showless complex kpc-scale structures, while the by ROSAT detected CJF sources

52

are less likely to show point-like structures but rather to have more complexkpc-scale structures. The most complex large-scale structures are only foundamong ROSAT detections.The large-scale jet structure might play an important role in contributing tothe X-ray emission. Jets are very likely relativistic on kiloparsec scales as well.Large-scale relativistic proper motions have been directly observed in the nearbyradio galaxy M87 (Biretta et al. 1995). The most plausible explanation of someof the newly discovered extended X-ray jets requires that the plasma has bulkrelativistic motions on scales of hundreds of kiloparsecs (Tavecchio et al. 2000;Celotti et al. 2001; Sambruna et al. 2002). More observations are definitelyrequired to search for and confirm large scale motion in these ROSAT detectedquasars and BL Lac objects.Fig. 25 (b) shows the relation between the complexity of the large-scale mor-phology and the misalignment angle. As shown already in Fig. 24, the extendedkpc-scale sources detected by ROSAT mainly concentrate at angles of 0 andaround 75.

0 50 100 150 200Position angle difference [Degrees]

0

1

2

3

4

5

com

plex

ity o

f the

larg

e-sc

ale

stru

ctur

e (V

LA)

(a)

Figure 25: The figure shows the relation between the complexity of the large-scale structure and the misalignment angle (stars for the detected objects, greyfilled circles for the non-detections). The criteria for the complexity system(from pointlike 0, to very complex 5) are explained in the text.

7 Gamma-ray emission mechanisms

7.1 The exceptional SEDs of Blazars, Evidence for Relativistically Beamed

Gamma Rays

Among AGN, blazars represent the most extreme and powerful sources. Becausethe plasma moves relativistically in a jet close to the line of sight, beaming is

53

Table 4: The table lists the numbers of objects with the complexity factorsdescribing the kpc-scale structure. The numbers are given for the sample ofCJF sources detected by ROSAT and for the non-detections.

0 1 2 3 4 5

detected by ROSAT 45 11 28 25 20 16not-detected by ROSAT 72 11 38 14 1 1

an important factor controlling observational properties of blazars as a class.With 66 high-confidence blazar identifications of sources detected by EGRET(Hartman et al. 1999), it has become increasingly clear that GeV γ-ray sourcesare preferentially radio-bright, compact-core, flat-spectrum sources, many ofwhich have been classified as optically violent variables (OVV) or blazars. Theactive galaxies Mrk 421, Mrk 501, and — at a lower level of significance — afew other AGN (some are members of the CJF sample) have been detected byground-based Cherenkov telescopes at the highest energy end (TeV-regime) ofthe electromagnetic spectrum accessible with current technology. The bulk oftheir radiative output is emitted in the γ-ray range. The γ-ray luminosity above100 MeV ranges from less than 3 x 1044 erg/s to more than 1049 erg/s (assum-ing isotropic emission). Superluminal motion is common to all γ-ray sourcesstudied in sufficient detail (e.g., Barthel, 1995; Bower et al., 1997; Britzen etal. 1999b, 2000; Jorstad et al. 2001). Many sources also exhibit a parsec-scalejet that is bent or is misaligned with the kiloparsec-scale jet (e.g., Bower etal., 1997). As pointed out, for instance, by Fichtel et al. (1994), these factssuggest that strong γ-ray emission and blazar properties are physically related.Additional support is provided by γ-ray variability. Many of these sources arestrongly variable in the γ-ray band on timescales from days to months (Mukher-jee et al. 1997). As discussed by von Montigny et al. (1995), all of the brightersources are clearly variable from one observing epoch to the next, on time scalesof a few days, but large flux variability on short timescales of <1 day is alsodetected (Mukherjee et al. 1997). This rapid variability leads to a largely model-independent argument that the γ-rays, at least, must be relativistically beamed.The difference in γ-ray properties may then be related to radio loudness, whichin turn must be closely associated with relativistic beaming. Although suchrapid γ-ray variability may result from shocks that are reasonably far down thejet, the possibility that the emission comes from the innermost region of thejet, close to the central energy source, is not excluded. On the other hand, ifthe γ-rays are produced by the inverse Compton process, the production regionshould be in the denser regions inside the jet.The temporal relation between γ- and radio flares is a matter of importance indetermining the γ-ray production mechanism (see, e.g., Marscher, 1996; Britzenet al. 1999b for 0528+134; Britzen et al. 2000 for PKS 0420-014). The delay

54

of flares, going from high to low frequencies, coupled with the knowledge ofother parameters, is crucial to locate its position within the radio source.It has been suggested that relativistic beaming may explain the observationthat although all EGRET-identified AGNs are radio loud with flat spectra, notall radio-loud flat-spectrum AGNs are detectable γ-ray sources (von Montignyet al. 1995). An explanation for this could be that the beaming cone for γ-rayemission is narrower that that for radio emission. The γ-ray emission would thenbe beamed away from the line of sight, but the radio emission is still Dopplerboosted enough due to its wider beaming cone (von Montigny et al. 1995).Various models for γ-ray emission have been proposed:

• The inverse Compton process on the external photons (ECS), in whichthe soft photons come directly from a nearby accretion disk (Dermer et al.1992) or from disk radiation reprocessed in some region of the AGN, e.g.,the broad emission line region (Sikora et al. 1994; Blandford & Levinson1995)

• The synchrotron self-Compton model (SSC), in which the soft photonsoriginate as synchrotron emission in the jet (e.g., Marscher & Gear 1985;Maraschi et al. 1992; Ghisellini & Madau 1996).

• Synchrotron emission from ultrarelativistic electrons and positrons pro-duced in a proton-induced cascade (PIC) (Mannheim & Biermann 1992;Mannheim, 1993).

Estimates of the position of location of the γ-ray production vary: ∼100 Rg(Hartman et al. 1996), 205 Rg (Xie et al. 1998), and hundreds of Schwarzshildradii (Ghisellini & Madau, 1996) from the central black hole.

7.2 The γ-bright blazars in the CJF survey

The EGRET detected sources (Hartman et al. 1999) form a subsample of 14sources within the full CJF sample. Within this thesis I particulary concentrateon the analysis of those properties that are considered to be essential for theγ-ray production, e.g., superluminal motion, bending, and misalignment.In Table 5, I list those CJF sources that have been detected by EGRET accordingto the third EGRET catalog (Hartman et al. 1999) and their properties. MKN501 has first been detected in the TeV-regime and later by EGRET.

7.2.1 The typical radio properties of γ-bright CJF sources

The γ-bright subsample reveals quite slow apparent velocities on average forthe BL Lac objects (1.51c for 6 BL Lac objects), whereas the values for the

55

Table 5: Sources in CJF that have been detected by EGRET. I list the EGRETname (Hartman et al. 1999) based on the J2000 coordinates for the bestposition of the source, followed by the source name in IAU naming convention,the radio identification, redshift, optical classification, total apparent velocity, anindication of whether the source is ROSAT detected, the misalignment angle,the complexity-factor, and the amount of curvature on pc-scales. For threeγ-bright objects the radio identification is questionable.

EGRET name radio source Radio z optical βapp ROSAT PA VLA curvature

IAU name detected class [c] detected [deg] complexity pc-scales [deg]

3EG J0222+4253 0219+428√

0.444 B 4.28√

8 2 20

3EG J0721+7120 0716+714√

∼0.3 B 1.44√

75 5 16

3EG J0808+4844 0804+499? poss. Id. 1.43 HPQ 5.02√

55 2 32

3EG J0845+7049 0836+710√

2.172 Q 13.96√

7 3 22

3EG J0917+4427 0917+449? poss. Id. 2.180 Q 8.17√

20 4 23

3EG J0952+5501 0954+556√

0.901 HPQ√

95 4 13

3EG J0958+6533 0954+658√

0.368 B 0.16√

85 2 58

3EG J1104+3809 MRK 421√

0.031 B 0.19√

13 4 13

3EG J1635+3813 1633+382√

1.814 LPQ 5.18√

86 4 30

MRK 501√

0.033663 B 0.31√

83 4 56

3EG J1738+5203 1739+522√

1.375 HPQ 11.44√

106 2 44

3EG J2202+4217 BL Lacertae√

0.069 B 2.66√

30 4 34

3EG J2352+3752 2346+385? poss. Id. 1.032 Q 5.62√

66 2 16

3EG J2358+4604 2351+456√

1.992 LPQ 14.91 88 2 39

quasars are on average slightly higher than for the complete sample (9.19c for 7quasars). Three γ-bright CJF-sources (BL Lac objects) even reveal subluminalvelocities (confirmed by Edwards & Piner 2002 for Mrk501 and Piner et al. 1999for Mrk421). Thus, we do find some evidence for larger apparent velocities forthe γ-bright quasars but lower values for the BL Lac objects.The results presented in the literature are quite inhomogeneous. A reason forthis might be the different selection criteria for the samples and hereby intro-duced biases.Kellermann et al. (2004) find similar mean speeds for 19 EGRET detectedsources compared to 39 sources with no EGRET detections from the 2cm sur-vey. Tingay et al. (1996) also find no significant difference in jet speeds ofγ-ray quiet and γ-ray loud radio AGNs. Piner & Kingham (1998) found, basedon apparent superluminal motion studies, no indication that EGRET blazars aremore strongly beamed than non-EGRET blazars.In contrast Jorstad et al. (2001) find much higher apparent superluminal veloc-ities for 33 γ-bright sources, in VLBA observations performed mainly at 43 &22 GHz, than for the general population of bright compact radio sources.All but one (2351+456) of the sources of the γ-bright subsample have beendetected within the ROSAT observations.The average pc-scale curvature of the γ-bright CJF subsample is significantly

56

higher than in the rest of the CJF sources. This is confirmed by Jorstad et al.(2001) who find that most of the jets of γ-bright objects are bent. They find for43% of the sources that the jet bends by more than 20. In our CJF subsamplethis value is even higher: 64 % of the objects reveal bending by more than 20.This strong bending in the γ-bright objects is consistent with amplification ofmodest actual changes in position angle by projection effects. Bending out ofthe line of sight of an initially aligned jet is much more likely than bendingdirectly into the line of sight of an initially misaligned jet.All of the sources of our subsample show extended kpc-scale structure; thesesources on average reveal a more complex VLA structure than the ROSAT de-tected CJF sample in total. The high number of misaligned sources (8 of the14 sources) in this subsample, higher than in the rest of the sample, is espe-cially noticeable. This can easily be explained within the geometrical model ofa helical jet viewed close to the line of sight, in which the helical amplitudesaturates.

7.2.2 Curvature and Paths - Evidence for Binary Black Holes?

Some γ-bright AGN may harbour a binary black hole in the center. In the caseof the γ-bright blazar PKS 0420-014 (not a member of CJF), a binary blackhole model is capable of explaining the structural evolution (Britzen et al. 2001)by a precession of the accretion disk, which is due to the presence of a binarysystem of black holes in the nucleus. The light curve of an optical outburst(simultaneous to a radio and γ-ray outburst) associated with the birth of a newVLBI component can be explained by the motion of the beam-emitting blackhole around the center of gravity of the binary black hole system (Britzen et al.,2001). Several authors discuss the possibility that MRK 501 (also γ-bright) alsohosts a binary black hole (e.g., Villata et al. 1999; Conway & Murphy 1993).A step further is the speculation by Conway & Murphy that the bimodality inthe misalignment distribution in general could easily be explained if helicallydistorted jets occurred in binary black hole sources and straight jets in sourcescontaining a single black hole.

57

”Everything should be made as simple as possible, but not simpler” (AlbertEinstein).

8 Unification of AGN

We observe a broad variety of different AGN phenomena: the so-called AGN’zoo’. Should we be able to untangle evolutionary effects that change the intrin-sic characteristics (luminosity, morphology, and/or number of AGN) with time(redshift), only then we might be able to approach the more physically interest-ing properties of AGN like: the black hole mass, black hole angular momentum,and accretion rate.The light we receive from an AGN is the result of real intrinsic differences inthe AGN and its environment (like luminosity, etc.) and the effects of apparentdifferences which are due to observer-dependent parameters (like orientation).The central prediction of so called “unified” (orientation-dependent) schemesfor AGN is that the viewing angle θ — the angle between the line of sightand the radio jet axis of the source — is the decisive parameter in determiningthe classification of an active galaxy, such as radio galaxies, radio-loud quasars,and blazars (e.g., Scheuer & Readhead 1979; Blandford & Konigl 1979; Orr &Browne 1982; Barthel, 1989; Urry & Padovani 1995).Blandford & Rees (1978) were the first to recognize that blazar phenomena(rapid variability, high brightness temperatures, high polarization) could be ac-counted for by supposing that these were otherwise ’normal’ AGNs which weare viewing along the radio axis and thus the observed flux is dominated by thebeamed component.Fanaroff and Riley (1974) provided an early step towards unification by recog-nizing that radio galaxies separate into two distinct luminosity classes, each withits own morphology.

8.1 Current unification scheme

AGNs with strong radio jets will typically be classified as radio galaxies if the ori-entation of the jet to our line of sight is larger than a critical value, θcrit ' 40,while the same source will be called a quasar if θ < θcrit (e.g., Barthel, 1989;Urry & Padovani 1995). The most successful unification scheme for quasarsproposes that as the viewing angle decreases, a FR II (high power) radio galaxyappears first as an ordinary quasar and then as a blazar-type quasar (Barthel1989). If the jet is very close to our line of sight (θ ' Γ−1) then relativisticeffects strongly enhance the observed fluctuations and polarization. Then thesource might be classified as an Optically Violently Variable quasar or other type

58

of blazar.Turning to the weaker radio sources, there is abundant evidence that FR I radiogalaxies are the parent population for BL Lacertae objects, in the sense that atypical FR I source, if viewed at small θ, would show the properties of a blazar,and that the relative numbers of these classes are nicely understood if this uni-fication holds (e.g., Urry & Padovani 1995).Should these unification scenarios be right, then the isotropic properties ofquasars and FR II Galaxies on the one hand as well as of BL Lac objects andFR I galaxies on the other hand should be comparable, e.g, the radio luminosi-ties of the extended structures, the narrow emission lines, the morphology andmagnitude of radio galaxies and quasar host galaxies, the environments of theunified classes, and the relative numbers and luminosities of each class.Continuity in the properties of blazars along a power sequence has been sug-gested by Maraschi & Rovetti (1994), Sambruna et al. (1996) and Fossati etal. (1997) on the basis of statistical arguments. Whithin this frame, the blazarsequence manifests itself in several observational properties, including the totalsource power, the luminosity in emission lines, the extended radio power, thedominance of γ-rays over the other spectral components and the broad bandshape of the SED.

8.2 Low- and high-redshift BL Lac Objects

The high-redshift members of any flux-limited sample tend to be systematicallymore luminous than the low-redshift members because of the induced correlationof luminosity and redshift. Among BL Lac objects, such differences have beeninterpreted as evidence for a ”true” (type 0) BL Lac class at low redshift and a”quasar-like” (type 1) BL Lac class at high redshift. A particular motivation forthis division (Antonucci 1993) is the fact that broad emission lines, occasionallywith large equivalent widths, have been seen in some high-redshift but not in low-redshift BL Lac objects. Urry & Padovani (1995) in detail discuss two questions:1) whether there is any evidence requiring separate high- and low-redshift BLLac populations, and 2) whether the evidence shows that high-redshift BL Lacsare similar to quasars. They believe, based on presently available data, theanswer to both questions is no.

8.3 CSS and GPS

The place of Compact Steep-Spectrum Sources (CSS) and the possibly relatedGigahertz Peaked-Spectrum Sources (GPS) in the unified scheme for high-powerradio sources is an open question. CSS and GPS include both quasars and radiogalaxies. CSS and GPS could represent an early stage of radio source evolution(Fanti et al. 1990) and if so, probably should be included in unification.

59

8.4 Testing unification theories with superluminal motion statistics

Superluminal motion statistics can potentially be used to test so-called unifica-tion models, which hold that quasars and FR II radio galaxies are similar objectsintrinsically, and that they form a sequence with increasing jet inclination tothe line of sight (e.g., Orr & Browne 1982; Barthel 1989). They predict thatthe highest βapp should be found in core-dominated quasars, with lower valuesin the lobe-dominated quasars, and even lower values in FR II galaxies. It ispossible that the higher luminosity BL Lacertae objects belong into the samesequence and have their jets pointed almost directly at the observer, yieldingpredominantly low βapp. Similar unification models have also been suggested forlower luminosity BL Lac objects and FR I radio galaxies (e.g., Impey, Lawrence,& Tapia 1991; Antonucci, 1993; Urry & Padovani 1995). Based on the resultsfor the CJF sources, we can confirm this prediction of the unification scenario,with quasars revealing highest apparent velocities and galaxies the lowest veloc-ities. Intermediate apparent velocities are found for the BL Lac objects.The CJF data can be used to also test the predictions of the unification scenarioconcerning the inclination to the line of sight for the different classes of objects.In the picture of ballistic motion of a knot, the Lorentz factor Γ and the viewingangle θ can be calculated with the help of δIC and βapp (e.g., Ghisellini et al.1993).

Γ =β2

app + δ2IC + 1

2δIC(3)

tan(θ) =2βapp

β2app + δ2

IC − 1(4)

Fig. 26 shows the relation between βapp and the viewing angle for the differentclasses of objects. From inspection of the figure we find a tendency for thequasars to fill the higher βapp – lower θ part of the plot. The galaxies appearat lower βapp – larger θ and the BL Lac objects occupy the slow βapp – smallθ part of the plot. This is in excellent agreement with the unification scenariosdiscussed in 8.1.The misalignment distribution, although it shows an unexpected bimodal distri-bution of aligned and orthogonally misaligned objects, can be explained withinthe beaming scenario. The fact that more BL Lac objects are misaligned caneasily be explained with a small θ and fits into our unification picture. Whilethe galaxies predominantly harbor aligned jets, a significant fraction of the BLLac objects shows orthogonally misaligned jets.The chapter on the correlations between X-ray and VLBI properties showed thatsignificantly more ROSAT detected objects have large-scale structure. We cur-rently expect that beaming at the large separations of the kpc-jet will explainthe X-ray emission in these objects. However, the observational results are still

60

0 20 40 60 80viewing angle [deg]

0

10

20

30

40

50

60

70

beta

_app

[c]

QSOBLLgalaxies

Figure 26: The figure shows the relation between the viewing angle and thebulk Lorentz factor. The same symbols for the different classes of objects as inFig.22 are used.

lacking for larger samples of such objects.The γ-ray subsample reveals significantly more bending on pc-scales and moreorthogonally misaligned sources than the complete sample. Both effects fit intothe Conway & Wrobel (1995) picture of viewing into a helix cone, supportingthe unification scenario.

8.5 Is the current beaming model sufficiently complex?

Although we confirm the predictions of the unification scenario for different ap-parent superluminal motions and viewing angles among the different classes ofobjects, the current beaming model does not account for all results that havebeen obtained with improved interferometric techniques and increased baselines(VSOP).Jets are complicated structures, more so than usually assumed in statisticaltests (Lind & Blandford 1985; Bridle & Perley 1994). They are most certainlynot well-behaved, showing curvature on all scales, variable bulk Lorentz factors,trailing and recollimation shocks, limb- and edge-brightening, hot spots at localshocks, etc. Jet component velocities almost certainly vary along the jet path,whereas we have assumed for the beaming analysis that they are fixed; observedsuperluminal motion in well-monitored sources is indeed complex (e.g., Zensus1997 and references herein). Finally, there is always the problem of bulk vs.pattern velocity, with only the former causing Doppler boosting. Despite manyyears of sophisticated AGN studies, we are still lacking a quantitative assess-ment of the beaming parameters, i.e., the values of the bulk and/or patternvelocities in the jets. These values are of basic importance both for understand-ing the physics of jets and in view of the so called ’unified’ models of AGNs.

61

Current theoretical progress makes detailed predictions for the bulk and/or pat-tern velocities. High dynamic range observations will soon start to address thisquestion.Unified schemes for radio-loud objects can also be complicated if the beam-ing is wavelength dependent. Although this is probably not the case for BLLacs, it could be important in quasars (Impey et al. 1991; Hough & Read-head 1989). We find evidence for this in the much higher apparent velocitiesfound by Jorstad et al. (2001) for the quasars. The dependence of beaming onwavelength occurs naturally for an obscuring torus because its transparency iswavelength dependent. Additionally, an inhomogeneous jet model with variableLorentz factor along its length will also show wavelength and location dependentbeaming behaviour. The usually adopted beaming model requires some mod-ifications since it is undoutbedly too simple to explain the observations madewith state-of-the-art instruments. On the other hand, our results are based onthree–five epoch data. Many of these sophisticated jet properties can only beseen in dedicated monitoring campaigns. Our observations clearly resolve thebasic jet properties for which the beaming model was invented to explain.As previously discussed, the misalignment distribution shows the expected bi-modal distribution of aligned and orthogonally misaligned objects. While thegalaxies predominantly harbor aligned jets, a significant fraction of the BL Lacobjects shows orthogonally misaligned jets. This can be explained within unifiedschemes if the misalignment is explained within the Conway & Murphy (1993)scenario.

8.6 Grand Unification

The ”Grand Unification” hypothesis assumes that AGN and galaxies are exactlythe same population, with AGN representing a particular phase of normal galaxyevolution. Then a normal galxy could have an AGN phase when episodes ofhigh accretion are triggered, and could be influenced by interactions or mergers.This hypothesis could also explain the observation that more AGN are found athigh redhshift, since the galaxy density is far higher and interactions are morecommon. Testing this Grand Unification scenario is the next step on the wayto unraveling the AGN.

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9 Conclusions

To conclude this thesis, I summarize the basic results derived for the CJF sourcesfrom the analysis of the VLBI studies, the ROSAT soft X-ray observations, thecorrelation analysis between VLBI and soft X-ray properties, and the investiga-tion of the by EGRET detected objects concerning their for the γ-brightnesspossibly decisive radio properties.

• Morphologypc-scale: The CJF comprises a very rich variety of morphological classes of jet struc-

tures, ranging from point-like up to very complex and extended jets. Core-jet structures are most abundant and jet curvatures are commonly ob-served.

kpc-scale: Pronounced extended structure is detected for most of the CJF sources.Roughly one third of the sources is strongly core-dominated and doesnot reveal kpc-scale structure. Misalignments between the preferentialdirection on pc-scales and the direction of the overall structure are morecommon than expected in ”simple” beaming models. Whereas galaxiesdo not show this misalignment, quasars and especially BL Lac objectsshow this effect in a pronounced way. This finding is readily explainedwithin unified schemes to be the consequence of different viewing angleson to a helically expanding jet.

• KinematicsProper motion: No marked differences have been found among the proper motions for

galaxies, quasars, and BL Lac objects. The expansion velocities withinthe individual jets are not uniform, i.e., only in rare cases a constant ex-pansion velocity can be established along the jet. Both accelerations anddeleclerations of the motion are found. The expected decrease of thevalue for the proper motion with redshift is confirmed for the completesample.

βapp:The apparent velocities βapp show differences for the different optical identifica-tion classes with the quasars being the fastest and the galaxies showingthe lowest values.There seems to be a maximum velocity of βapp for the sources in the CJFsample (equivalent to a proper motion of 0.35 mas/year). We find signsfor a decrease of the measured βapp with increasing redshift.

• ROSAT observationsAlmost half of the CJF sources have been detected by ROSAT. The goodcorrelation between the radio- and X-ray luminosities of the CJF sourceson one hand and between the optical- and the X-ray luminosities on the

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other hand support the explanation that a common origin of the radiationcan be derived, whereas differences between galaxies and quasars beyondtheir different luminosities are indicated which can be explained by anadditional cluster emission in the case of the galaxies.The comparison of the inverse Compton Doppler factor with βapp (forthose sources detected by ROSAT) shows that the bulk relativistic mo-tion without any additional pattern speeds can explain the observationsfor quasars and BL Lac objects. For the galaxies there is some evidencethat X-ray radiation processes other than inverse Compton emission con-tribute to the observed X-ray brightness.We find a good agreement for the Doppler factors derived from equipar-tition agruments and from inverse Compton calculations especially in thecase of a continuous jet model.We find no indication for different apparent velocities for the classes ofquasars and galaxies between those sources detected by ROSAT andnot detected by ROSAT. The median βapp values for detected and not-detected values are similar for quasars. The same holds for the galaxies.117 out of 293 sources from the complete CJF show no extended radioemission. For these objects the probability of ROSAT detectable X-rayemission is significantly lower than for the whole sample. Sources detectedby ROSAT show a higher degree in the complexity of their large-scale ra-dio morphologies. ROSAT detected AGN have a greater probability fororthogonal misalignment between pc- and kpc-scale radio structures.

• EGRET detected objectsThe subsample of γ-bright CJF sources (14 objects) reveals apparent ve-locities that are slightly higher for quasars and slightly lower for the BLLac objects. Three BL Lac objects show apparent subluminal motion.The expected exceptionally high velocities in blazars are not found.Besides of one object, all γ-bright objects are detected by ROSAT.All of the γ-bright AGN reveal extended kpc-scale structure with highercomplexity than would be expected based on the results for the ROSAT-detected CJF sample.γ-bright AGN more often show misalignment (8 out of 14 sources).The pc-scale curvature in γ-bright AGN is more pronounced than in theaverage CJF source.

• Unified SchemesAlthough our analysis of the CJF survey is principally consistent withthe current unified scheme(s), the wealth of observational results ask forrefinements and improvements in those models. Especially the role of thesimple concept of beaming phenomena, as applied generally, has to beimproved according to the new observations.

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Acknowledgements

First of all I am very grateful to Prof. Dr. I. Appenzeller who accepted andhosted me as a ”Habilitandin” at his institute, the Landessternwarte in Heidel-berg. His steady interest and support were of great importance for me and theprogress of this work.

Also I am deeply indebted to the Claussen-Simon Stiftung for awarding me aHabilitandenstipendium during the preparation of this Habilitationsschrift. Alsotheir interest in the progress of my work - especially evident during the unfor-gettable days in 2002 at the reunion in Hamburg - is very much appreciated.

I also acknowledge the steady interest and cooperation of my colleagues atthe Landessternwarte, especially I wish to thank Prof. Dr. S. Wagner, Frau M.Darr, and Prof. Dr. M. Camenzind. The lively and pleasant atmosphere at theinstitute had great importance for me.

Special thanks are also due to the collaborators within the CERES/CJF net-work and the numerous discussions with Dr. R. Vermeulen and Profs. A.G. deBruyn, I. Browne, P. Wilkinson are gratefully acknowledged.

Also I am indebted to my colleagues at the Max-Planck-Institut fur Radioas-tronomie with whom I collaborated on several projects. Besides Drs. A. Witzel,T.P. Krichbaum, E. Ros, and A. Kraus, I want to thank Dr. A. Zensus, thedirector of the VLBI-group of the MPIfR, for his hospitality and for assigningme the status of a scientific guest at the institute.

I am also greately indebted to Prof. Dr. W. Brinkmann, whose great in-volvement in X-ray astronomy and especially the ROSAT mission and researchwas so important to our collaborative efforts.

Dr. J. Roland’s patience and expertise in discussing theoretical questions andartistic concepts is gratefully acknowledged.

Last – but not at all least – I want to thank Dr. R.M. Campbell for his collabo-ration during many stages and aspects of this work including his careful readingof the manuscript.

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List of Figures

1 Artists impression of the new unified model; Schematic diagramof radio-loud AGN . . . . . . . . . . . . . . . . . . . . . . . . . 9

2 Schematic diagram of the inner jet and disk . . . . . . . . . . . 10

3 Overview selected pc-scale CJF jets . . . . . . . . . . . . . . . 20

4 The CJF misalignment distribution . . . . . . . . . . . . . . . . 22

5 The misalignment distributions of different classes of AGN . . . 22

6 A misaligned AGN: S5 1803+784 on different angular scales . . 23

7 Examples for jet component motion in the quasar 0711+356 andthe galaxy 1106+380 . . . . . . . . . . . . . . . . . . . . . . . 27

8 Example for jet component motion in the unclassified AGN 2138+389 28

9 Jet-component identification and motion in the xy-plane . . . . 29

10 Jet component position in x and y as function of time; radialand azimuthal proper motion as function of the core separation(0711+356) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

11 Jet component position in x and y as function of time; radialand azimuthal proper motion as function of the core separation(1106+380) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

12 Jet component position in x and y as function of time; radialand azimuthal proper motion as function of the core separation(2138+389) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

13 Proper motion histograms for the three classes of objects (com-plete sample and most reliable subsamples . . . . . . . . . . . . 33

14 Proper motion/redshift relation of a subsample of most reliablejet components . . . . . . . . . . . . . . . . . . . . . . . . . . 34

15 Histograms of the apparent superluminal motion in the threedifferent classes of objects . . . . . . . . . . . . . . . . . . . . . 36

16 Azimuthal βapp distribution in quasars and BL Lac objects, βapp-z relation for the quality 1 subsample . . . . . . . . . . . . . . . 37

17 Redshift distribution of the CJF sources detected and of thosenot detected by ROSAT . . . . . . . . . . . . . . . . . . . . . 44

18 Redshift distributions of the three classes of objects . . . . . . . 45

19 Monochromatic X-ray luminosity as function of the radio lumi-nosity and the optical luminosity . . . . . . . . . . . . . . . . . 46

20 X-ray loudness as function of the optical luminosity (for quasars) 47

21 Inverse Compton factor as function of βapp for spherical andcontinuous jets . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

22 Relation between the Equipartition and Inverse Compton Dopplerfactor for spherical and continuous jets . . . . . . . . . . . . . . 50

75

23 βapp-z relation for the detected and not detected CJF sources;Histograms showing the βapp distribution of the (ROSAT de-tected and not detected) jet components in the three classes ofobjects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

24 Misalignment angle for the ROSAT detected and not detectedCJF sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

25 Relation between the large-scale structure complexity and themisalingment angle for the ROSAT detected and not-detectedCJF sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

26 Bulk Lorentz factor as function of the viewing angle for thedifferent classes of CJF sources . . . . . . . . . . . . . . . . . . 61

List of Tables

1 Median proper motion values, based on quality 1 sources. . . . . 34

2 Median βapp-values, based on the total proper motion values forthe quality 1 sources . . . . . . . . . . . . . . . . . . . . . . . . 36

3 Median δIC- and βapp-values for the quality 1 sources . . . . . . 49

4 Distribution of the complexity factors for the with ROSAT de-tected and not-detected CJF sources . . . . . . . . . . . . . . . 54

5 The EGRET detected CJF subsample . . . . . . . . . . . . . . 56

76