relativistic jet-motion in the core of the radio-loud quasar j1101+7225 · 2004. 11. 30. · 2...

Astronomy & Astrophysics manuscript no. 10˙J1101paper November 18, 2004(DOI: will be inserted by hand later)

Relativistic jet-motion in the core of the radio-loud quasarJ1101+7225

J.-U. Pott1,2, A. Eckart1, M. Krips1,3, T.P. Krichbaum4 , S. Britzen4, W. Alef4, and J.A. Zensus4

1 I Physikalisches Institut, University of Cologne, Zulpicher Strasse 77, 50939 Koln, Germany2 European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching b. Munchen, Germany3 Institut de Radio-Astronomie Millimetrique (IRAM), 300 rue de la piscine, 38406 Saint Martin d’Heres, France4 Max-Planck-Institut fur Radioastronomie, Auf dem Hugel 69, D-53121 Bonn, Germany

Abstract. Multi-epoch GHz-VLBI data of the radio-loud quasar J1101+7225 were analyzed to estimate the proper motion ofthe extended optically thin jet components. Two components separated from the core could be mapped at 1.66 GHz, consistentlywith earlier observations. In one case we even found evidence for high apparent superluminal motion (βapp = 22.5± 4) at large(deprojected) distances to the core (22 mas ∼ 4 kpc, at z = 1.46). Typically in other quasars such high separation velocitiesare only found much closer to the core component. Furthermore the Doppler factor, the magnetic field strength and the angularsource size of the unresolved optically thick core were derived involving published X-ray data. The analysis of 5 GHz VLBIdata reveal the existence of further jet components within the central 5 mas. Additionally the so far published data of the GHz-spectrum were discussed at all angular resolutions. J1101+7225 turns out to be a standard quasar to study different aspects ofradio jet kinematics out to kpc-scales.

Key words. active galactic nuclei (AGN) – radio-loud quasar – radio jet kinematics – apparent superluminal motion –J1101+7225

1. Introduction

The radio-loud quasar J1101+7225 is a source of the VLBACalibrator Survey VCS1 for phase-referencing observations(Beasley et al. 2002, in the following B02). We observed thesource within larger experiments, which were focused on imag-ing faint Seyfert Galaxies. It turned out that J1101+7225 showsa complex source structure, which makes it physically inter-esting, but on the other hand also less suitable as a calibrator.Since so far only little information is available for individualsources of the VCS1, we present our results in this article. Theanalysis of the (relativistic) kinematics of non-thermal radiosources gives important physical insights into the inmost re-gions of an AGN. The ejection of radio jet components and the

Send offprint requests to: J.-U. Pott, e-mail:[email protected]

property value ref.IAU a / other name: J1101+7225 / [HB89] 1058+726R.A. (J2000) 11h 01m (48.8054 ± 0.0001)s [B02]Decl. (J2000) +72◦25’(37.1183 ± 0.6 · 10−3)” [B02]redshift 1.46 [J91]abs. magnitude -27.3 mag≈ 1046.4 erg

s [V01]1.4GHz radio cont (1451 ± 30) mJy [W92]

Table 1. General properties of J1101+7225. a: InternationalAstronomical Union

jet kinematics are most probably related to the accretion pro-cess onto the nucleus itself. Quantitative estimates about phys-ical properties such as magnetic fields and source sizes can bederived, too. Our analysis is based on the comparison of our1.66 GHz VLBI-map, which was obtained in 2002, with theavailable maps of past observations (Thakkar et al. 1995 (T95)at 1.66 GHz and B02 at 2.3 GHz). Further the data of a multi-epoch study at 5 GHz of the central 5 mas of J1101+7225 byBritzen (2002; Britzen et al.in prep.) were used to determinethe source structure of the central region, unresolved at lowerobserving frequencies.

In Table 1 the general properties of J1101+7225 are sum-marized to introduce the quasar. B02 estimated the positionof J1101+7225 with (sub)mas-accuracy (Table 1) which mini-mizes errors in the mapping process of the interferometric data.This is particularly important for a study of kinematics of theradio-jet components as presented in this article. In Sec. 2 de-tails are given concerning the VLBI observations which are thebasis for this article. Flux densities, the resultant maps and akinematical analysis of the mapped structure are presented inthe following Sec. 3. Finally the results are discussed and sum-marized in Sec. 4.

2 J.-U. Pott et al.: Relativistic jet-motion in the core of the radio-loud quasar J1101+7225

date of observation obs. frequency bit rate & type of samp.13. Feb. 2002 1630.49MHz 256 Mbps & 2 bit

No. & bandwidth of IF polarization system and correlator:4 DSB-IFs & 16 MHz LCP MKIV at MPIfR, Bonn

Table 2. Details of the observing schedule of the EVN observationwith nine antennae of J1101+7225.

2. Observations

The quasar J1101+7225 was observed as a calibrator sourcewithin a larger EVN1 experiment. The observations were con-ducted in 2002 during the period February 13-14, 21h-09h UT,while the scans of J1101+7225 typically lasted for three min-utes. We observed at 18 cm with the 100m antenna of theMPIfR at Effelsberg (GER), the 76m antenna at Jodrell Bank(UK), the 32m antenna at Medicina (I), the 25m antenna atOnsala (SWE), the 25m antennae at Shanghai and Urumqi(CH), the 32m antenna at Torun (POL) and the 14x25m an-tenna array at Westerbork (NL). Details concerning the observ-ing mode are given in Table 2. After the observations, the datawere correlated at the VLBI correlator of the MPIfR in Bonn,Germany, and imported into AIPS via MK4IN (Alef & Graham2002). The data were fringe fitted and calibrated in a standardmanner with the AIPS package and imaged using the DIFMAPVLBI package (Pearson et al. 1994).

Further we present 5 GHz VLBI maps of J1101+7225,obtained in VLBA and global VLBI observations as partof a multi-epoch VLB study (the Caltech-Jodrell Bank flat-spectrum sample CJF; Taylor et al. 1996; Britzen 2002). Theseobservations aim at a statistical investigation of the kinematicsof a complete sample of AGN (Britzen 2002; Britzen et al. inprep.). The sources were observed in 5.5 minute snapshot ob-servations to determine the position and motion of jet compo-nents in the central 5 mas. The data were recorded over 32 MHztotal bandwidth broken up into 4 baseband channels, with 1-bitsampling. The recorded data were correlated in Socorro, USA.

In Sec. 3.2 we comprise the earlier published VLBI-mapsof J1101+7225 to analyze the evolution of the found structure.T95 observed the quasar with global VLBI at 1.66 GHz as amember of the CJ1 survey which became later on a part ofthe CJF sample. The other map at 2.3 GHz was reduced fromVLBA data (B02) via automatic imaging using the CaltechDIFMAP package.

3. Radio properties of the quasar J1101+7225

3.1. Flux densities

The flux density of J1101+7225 has been measured occasion-ally during the last two decades. In Table 3 we summarize themeasurements at the various frequencies. Based on these datawe calculated the spectrum, shown in Fig. 1. The single-dishobservations show a steep spectral index up to 5 GHz (α1.4;5

single ∼

−0.4). The same shape was observed with the higher angu-lar resolution of local interferometry (α1.4;8.4

locIF ∼ −0.4). Above5 GHz the single-dish spectrum flattens significantly (α5;22

single ∼

1 European VLBI Network

obs.type freq. [GHz] flux dens. [mJy] ref.single-dish 1.4 (1451 ± 30) [W92]single-dish 2.7 (1070 ± 35) [K81]single-dish 5 (858±76] [G96]single-dish 22 (820 ± 100) [T01]local interf. 1.4 (748 ± 40) [X95]local interf. 8.4 (349 ± 20) [P92]VLBI/peak 1.66 (407±20) -EVN Feb.2002-VLBI/peak 2.3 (520 ± 25) [B02]VLBI/peak 5 (97 ± 5) - 1991.4 -VLBI/peak 5 (209 ± 10) - 1993.4 -VLBI/peak 5 (314 ± 15) - 1996.6 -VLBI/peak 5 (417 ± 20) - 1999.9 -VLBI/peak 8.4 (383 ± 20) [B02]

Table 3. The table shows measured GHz-fluxes of J1101+7225, ob-tained with single-dish telescopes, local interferometers and VLBI.The values are published by different authors and are plotted as spec-tra in Fig. 1. For the interferometric observations the peak fluxes and5% flux errors are given. The beamsizes are ∼ 200 mas and ∼ 5 masfor the local interferometers respectively the VLB interferometer.

Fig. 1. The plot shows the GHz-spectrum of J1101+7225 at differentangular resolutions, corresponding to Tab. 3. All VLBI peak fluxes areshown with the observing date. An interpretation is given in Sec. 3.1.

S 5GHz[mJy] date of observation author ann.:778±4 between feb.1977 & mar.1978 [K81] (1)788±31 nov.1986 [G96] (2)953±73 oct.1987 [G91] (2)623±56 1992.5 = jul.1992 [R00] (3)

Table 4. The published single-dish flux densities at 5 GHz are shownto investigate the source variability. (1) observed at 4.9 GHz and cor-rected to 5GHz via a spectral index α10.7GHz

2.7GHz ; (2) at 4.85GHz; (3) at4.75GHz

0). This flattening already indicates a VLBI sub-structure inthe central 5 mas, which is still nearly unresolved at the ob-serving frequencies of 1.66 GHz and 2.3 GHz, respectively.K onigl (1981) calculated such flat GHz-spectra of compactsynchrotron sources, involving optical thickness for the syn-chrotron radiation due to synchrotron-selfabsorption. Indeedwe measured with the EVN a peak-flux of (407 ± 20) mJy at1.66 GHz. This confirms in combination with the VLBI peak-

J.-U. Pott et al.: Relativistic jet-motion in the core of the radio-loud quasar J1101+7225 3

PeakFlux location size(mJy) r (mas) ϑ (deg) a (mas) Axial ratio

Core 396 0 0 2.88 [email protected]◦

C 149 3.3 14.1 1.83 [email protected]◦

A 54 23.3 21.0 3.62 1B1 16 47.4 17.7 6.22 1B2 7 62.8 18.5 4.21 1

Table 5. The best fit Gaussians as mentioned in the text. a labels themajor axis of elliptical Gaussians. A, B1 and B2 were only fitted by cir-cular Gaussians, because the locations of lower flux components canbe fitted more reliable with less degrees of freedom. The flux errors are∼ 5%, the uncertainties of the location and size of the Gaussians areranging between 0.2-0.5 of the beamsize depending of the respectiveflux.

fluxes, obtained at other frequencies, a flat spectral shape2 atGHz-frequencies (Fig. 1). Thus most of the central radio emis-sion of J1101+7225 at these frequencies is emitted by a com-pact, unresolved core.

A significant fraction (up to 50%) of the single-dish fluxof the whole galaxy is radiated by the inmost region, stillunresolved by interferometric observations. Relativistic beam-ing models explain the extraordinary luminosity of radio-loud cores with unresolved radio-jet components approach-ing nearly along the line of sight at relativistic velocities (e.g.Blandford & K onigl 1979 and Sec. 3.4).

A consequence of these models is the straightforward ex-planation of the often observed variability of the core-flux bya small variations of the jet-orientation with respect to the ob-server. This variability is still observable with single-dish ob-servations in case of core-dominant GHz emission. Table 4presents single-dish radio flux densities, measured at 5GHz atdifferent epochs. The values show a flux variation up to 20%between 1986 and 1987. This variability has to be taken into ac-count and leads to significant changes of short-range3 spectralindices in the GHz domain from different observing epochs.The underlying variability of the VLBI core flux was observedat 5 GHz (Table 3) and shows the same minimum around1992. Therefore only the later measurements were taken inFig. 1 to estimate the spectral index of the VLBI core. The ob-served flux-density variability strongly supports the concept ofbeamed radio emission which could be confirmed in Sec. 3.4.

3.2. High apparent superluminal motion outside thecentral 10 mas

In Fig. 2 we present the map, which was reduced from the dataof the EVN observation in Feb. 2002. Clearly two extended re-gions are visible beside the core with more than 1.2% and 4.8%of the core peak flux. They are labeled with A and B, respec-tively. More information about the underlying source structurecan be retrieved by fitting circular and elliptical Gaussians tothe visibility data and optimizing the fit to the amplitudes and

2 within the scatter due to different observations and epochs3 Due to the power-law relation between flux density and frequency,

the influence of the flux variability on the spectral index decreaseswith increasing frequency interval.

Fig. 2. Cleaned map of the EVN observation of J1101+7225 at1.66 GHz. The map shows a peak flux of 407 mJy/beam, the con-tour levels are -0.15, 0.15, 0.3, 0.6, 1.2, 2.4, 4.8, 9.6, 19.2, 38.4,76.8% of the peak flux and the beam size is 6.22x5.28 mas. In theupper right corner the Gaussian model fit representation of the VLBIimage is given (with the same contour levels, but now with respectto a 399 mJy/beam peak flux), obtained by fitting Gaussian compo-nents to the visibility data. By this technique the extended structureat ∼ 50 mas distance to the core could be clearly resolved into twocomponents.

closure phases (cf. Pearson 1995). Our best model of the sourcestructure is given in Tab. 5 and the corresponding map is in-serted in Fig. 2.

This analysis shows, that the extended B component fromthe cleaned map is a blend of at least two components (B1, B2).Further within the central 5 mas a significant fraction of the fluxis radiated by an extended source beside the dominating unre-solved core at a distance of a few mas to the core (see Sec. 3.3).This region close to the limit of resolution of the 1.66 GHz ob-servations is studied in more detail in the next section (3.3) athigher frequencies.

T95 detected both A and B, too. A detailed comparison ofour data with the Gaussian component model of T95 revealsa high separation velocity of the A-component at an unusuallylarge deprojected distance to the core. In contrast to Tab. 5,they could fit three elliptical Gaussian components to the re-gion around A and one to the B region. Because the angularextension of both regions (A and B) are close to the limit ofresolution, we prefer the blend interpretation of this situation:


epoch & observ. freq. [GHz]: 25.9.1991 at 1.66 [T95] 13.2.2002 at 1.66 mean app. transv. velocity 19.4.1995 at 2.3 [B02]components: Distances in Fig. 3 [mas] µ[ mas

yr ] βapp;h=0.71(b) Dist. in Fig. 3 [mas]

Core-A (19.6 ± 0.5) (23.3 ± 0.5) (0.36 ± 0.07) (22.5 ± 4) (21.5 ± 3)Core-B (53.7 ± 1) (52.1 ± 1) 0 (a) 0 (52 ± 5)

Table 6. The angular distances between the core and the separated fitted Gaussian components (or their flux weighted mean; cf. text). The errorsreflect uncertainties in the fitting process and increase with decreasing component flux. The derivation of the apparent velocities is described inthe text. The position angles of the components did not change significantly durig the observing epochs and were omitted. In the last columnthe read out values from a map by B02 are given for comparison. These values cannot be included in the calculations due to lack of a Gaussianmodel fit.(a): No significant propagation of the B component was found with respect to the errors.(b): For the calculations we used H0 = 71+4

−3kms

1Mpc as recently found by WMAP (cf. the webpage http://map.gsfc.nasa.gov/index.html). Further

a deceleration parameter q0 = 0.1 was applied.

Fig. 3. The cleaned maps are aligned vertically (rotation about -20◦),which is justified by the fact, that no significant change of the positionangle of the jet axis was observed. The apparent linear velocities ofthe fitted Gaussian components (cf. Tab. 6) are given. The first map byT95 at T(1,1.66 GHz) shows a peak flux of 476 mJy/beam, the contourlevels are -2, -1, 1, 2, 4, 8, 16, 32, 63, 127, 253 mJy/beam and the beamsize is 4.2x3.2 mas. The second map by B02 at T(2,2.3 GHz) shows apeak flux of 520 mJy/beam, the contour levels are -3, 3, 6, 12, 24, 48,96, 192, 384 mJy/beam and the beam size is 3.3x6.7 mas.

Our A component does not correspond to one of the three fit-ted components of T95 but does correspond to a flux-weightedmean of all three components. And vice versa the mean of ourB1 and B2 components corresponds to the one component ofthat region, fitted by T95 to their visibility data4.

Therefore we determined the apparent motion of the radiojet structures A and B with respect to the core (Tab. 6), wherein case of several fit components the labels refer to the fluxweighted mean of these. The angular velocities µ are presentedin Tab. 6 on the right side. They are confirmed by the snap-shot VLBA map at 2.3 GHz of B02 (last column in Tab. 6). We

4 Although the FWHM of the circular Gaussians B1 and B2 (cf.Tab. 5) are not overlapping, the modeling process showed, that dueto their low flux densities the sizes of the B components are insecureup to a factor of two. It is also possible that a third component exists,slightly to weak to be fitted. Thus the used flux weighted mean appearsto be the most adequate presentation.

did not include directly these data into the velocity calculation.The introduced uncertainties due to the different observing fre-quency and the lack of a Gaussian modelfit would annihilatethe increased calculation accuracy. To demonstrate the appar-ent angular motion we show in Fig. 3 the three VLBI maps inone image on the same angular scale and rotated about −20 ◦.

The dimensionless linear equivalents βapp are calculatedvia

βapp = µz

H0(1 + z)

1 +√

1 + 2q0z + z

1 +√

1 + 2q0z + q0z

(cf. Pearson & Zensus 1987). The measured apparent superlu-minal transverse motion can be transformed via special relativ-ity5 into a minimal intrinsic velocity, expressed as:

βAmin ≈ 0.9990+0.0003

−0.0005 at ϑAmin ≈ 2.54◦ +0.55

−0.38 (1)

The 22 mas distance to the core at ϑ = 2.5◦ can be deprojectedto a large linear distance of about 4 kpc (at z = 1.46, q0 = 0.1).It turns out that the uncertainties in the estimation of βapp andthe Hubble parameter are affecting βA

min by less than 0.1%. Themore distant B-component was found to be stationary withinthe errors at a mean angular distance of 52.9 mas.

3.3. Resolved structure in the central 10 mas

The higher resolved maps of the 5 GHz snapshots of Britzenet al. (in prep.) are presented in Fig. 4. They show explicitlythat the Core region of the 1.66 GHz map is not totally com-pact on the 1 mas scale of the 5 GHz observations. ExtendedGaussian components could be fitted to the data beside thedominant6 central source. This was already suggested by theC-component, fitted to the 1.66 GHz data (Tab. 5) and confirmsthe power of the model fits in analysing the data.

In Table 7 the fitted Gaussian components are shown. A lin-ear fit of the core-distance with respect to the observing epochs

5 If ϑ is the angle enclosed by the line of sight and the direction ofmotion and β is the intrinsic velocity, one finds βapp =

β sinϑ1−β cosϑ . The

minimal intrinsic velocity βmin =

√

(β2app)/(1 + β2

app) implies an angle

ϑmin fulfilling: cot(ϑmin) = βapp.6 e.g. in the last epoch the extended component show a peak flux of

35% of the central flux density


Fig. 4. The 5 GHz maps of the central 5 mas. The details are givenin the form: peak flux; contour levels in %; beam size. First epoch:0.097 mJy/beam; levels are -8, 8, 12, 18, 27, 40.5, 60.8 %; beam:1.08x0.79 mas. Second epoch: 0.209 mJy/beam; levels are 6, 9,13.5, 20.3, 30.4, 45.6, 68.3 %; beam: 0.96x0.90 mas. Third epoch:0.314 mJy/beam; levels are 1.6, 2.56, 4.1, 6.55, 10.5, 16.8, 26.8, 42.9,68.7, %; beam: 1.95x1.64 mas. Fourth epoch: 0.417 mJy/beam; levelsare -0.9, 0.9, 1.44, 2.3, 3.69, 5.9, 9.44, 15.1, 24.2, 38.7, 61.8 %; beam:2.11x1.64 mas. The crosses indicate the positions of circular Gaussianmodelfit components. The detailed model parameters are presented inTable 7.

(Fig. 5) demonstrates the apparently superluminal motion ofboth components and shows mean separation velocities of

β(C1) ≈ 2.5 and β(C2) ≈ 3.7 (2)

Because a significant intrinsic acceleration of the jet mate-rial from the 4 mas regime of the inner C-components towardthe 20 mas regime far away from the central engine is veryunlikely, we adopt the βA

min ≈ 0.999 from Equ. 1 as intrinsicseparation velocity also for the inner jet components. From theobserved (with respect to A) lower apparent separation veloci-ties of C1 and C2 one calculates with the equations of specialrelativity two possible jet orientation angles to the line-of-sight(with βA

min = 0.999+0.0003−0.0005):

epoch PeakFlux location size(mJy) r (mas) ϑ (deg) diam (mas)

1991.4 Core 139 0 0 0.57C1 132 1.81 0.2 1.01C2 133 3.38 11.3 0.85

1993.4 Core 282 0 0 0.51C1 113 2.47 4.9 0.57C2 61 3.76 10.7 0.20

1996.6 Core 337 0 0 0.40C1 97 2.32 6.0 1.13C2 80 3.9 12.0 0.94

1999.9 Core 439 0 0 0.41C1 78 2.07 7.8 1.00C2 84 3.83 11.1 1.18

Table 7. The best fit circular Gaussians of the 5 Ghz data over fourepochs. The flux errors are ∼ 5%, the uncertainties of the locationand size of the Gaussians are ranging between 0.2-0.5 of the beamsizedepending of the respective flux.

Fig. 5. The plot shows the evolving core-distance (in mas) of the bestfit Gaussians, taken from Table 7, over the years. Furthermore a least-squares fit of a linear time-dependance is shown with respect to theerrorbars. The standard deviation of the resulting apparent separationvelocities is σ ≈ 0.04mas/yr.

β(C1) ≈ 2.5 : ϑC1s ≈ (0.16 ± 0.06)◦; ϑC1

l ≈ (43.4 ± 0.1)◦

β(C2) ≈ 3.7 : ϑC2s ≈ (0.24 ± 0.08)◦; ϑC1

l ≈ (30 ± 0.1)◦(3)

Both differ significantly from ϑAmin ≈ (2.54+0.55

−0.38)◦ of Equ. 1.A closer look at the modelfits in Tab. 7 reveals some trends

over the observing epochs: The separation motion seems todecrease at later epochs in correlation with a decrease in therespective component brightnesses and an increasing positionangle of C1 from 0.2◦ to 7.8◦ over the four epochs. Withthe given, relatively poor time-resolution of four observationswithin eight years it is not possible to fit these trends reliably.Nevertheless in combination with the different calculated an-gles in Equ. 1&3, they can be interpreted as indications fora spatially curved jet structure. Different line-of-sight orien-tations account for different brightnesses, position angles andapparent separation velocities.


(νm; S m) α(1keV;5keV)X

a) S Xobs(1keV) a)

(8.4GHz; 383mJy) (−0.5 ± 0.2) 0.1µJyθcirc

8.4GHz Doppler factor magnetic field B0

(0.25+0.8−0.1)mas δ = (2.6+3.2

−1.1) (3 · 10−(2±1))G

Table 8. The values are discussed in Sec. 3.4. The upper panel givesthe direct estimates whereas in the lower panel the derived values arepresented. a) From Fiore et al. (2001; 5 keV) and Brinkmann et al.(1997; 1 keV).

The core flux has strongly increased over the four epochs.This suggests that over the later epochs either a new jet com-ponent has emerged from the compact core region or that theunresolved jet has changed its orientation with respect to theline of sight. During the fitting process we found indications,that indeed a third component in sub-mas distance to the coreis hidden, but probably due to its proximity to the core, thisnew component could not be fitted separately without doubts.

On the other hand the increased brightness of the core in thelater epochs may affect uncertainties in the modelfitting processof the close C components. The standard values of one fith ofthe beam size, which are given in Tab. 7 and used for the linearfit in Fig. 5, may slightly underestimate the real errors due toincreased overblending of the C components by the close coreflux. Because this effect is not quantifiable with our data, wekept using the standard errors.

The idea of a jet curvature over the observed core-distancesdoes not rule out a ballistic situation at the origin of the jet,as was described recently by Stirling et al. (2003). They foundas origin of the radio jet in BL Lacertae a ’precessing nozzle’which ejects the single components along straight trajectories.But outside 2 mas (corresponding to 0.4 mas at the redshift ofJ1101+7225) these trajectories became curved as well.

Summarizing the motion of all detected jet components in-cluding A and B, the situation resembles the different mea-sured apparent component speeds of the radio jet7 of the S5quasar 0836+710, where also no systematic correlation be-tween the component speed and its distance to the core seemsto be present (Otterbein et al. 1998 and references therein).

3.4. Indirect estimation of relativistic bulk motion in thecore

Our VLBI-data of J1101+7225 allow (in combination withpublished data at other wavelengths) a straightforward analysisof the observed VLBI core flux density applying a relativisticbeaming model. The crucial idea of such models is, that due torelativistic bulk motion of the sources the radiation is amplifiedsignificantly in the rest-frame of the observer, if the direction ofmotion is close to the line of sight. Of course the bulk motioncannot be observed directly due to the lack of spatial resolution.But we share the common assumption that the X-ray emissionof the core consists mainly of synchrotron photons, scattered toshorter wavelengths via the inverse Compton effect. Marscher(1983) derived quantitatively the connection between radio and

7 which extends over more than 150 mas at z ∼ 2.17 (Hummel et al.1992)

X-ray fluxes, their spectral indices and the Doppler-factor8 δ

for a homogeneous source of spherical shape. This implies, thatδ can be estimated from the other measured values.

In Tab. 8 the ingoing values of the calculations are givenwhere the derivation of the turn-over point (νm; S m) in the ra-dio spectrum9 is described by Eckart et al. (1986). The turn-over frequency is not 2.3 GHz as may be suggested by Fig. 1.The 5 GHz snapshots show clearly, that the central region, stillunresolved at the lower frequencies, includes both extendedsteep spectrum jet components and a central compact com-ponent. Therefore we believe, that the highest observed VLBIfrequency is a more realistic though probably still slightly un-derestimated turn-over frequency10 of the spectrum of the op-tically thick central source.

The calculations lead to δ = (2.6+3.2−1.1) and a magnetic field

strength of B0=(3 · 10−(2±1)) G of the unresolved synchrotron-self absorbed source (cf. table 8). If other processes beside theinverse Compton scattering were indeed contributing signifi-cantly to the X-ray spectrum in contrast to our assumption, thehere calculated δ and B0 were too small.

The angular source size in Tab. 8 refers to the adopted ho-mogeneous and spherical shape and therefore should be un-derstood as an order of magnitude estimate11. The limits ofthe source size are given by the range of brightness tempera-ture in which the simultaneously observed synchrotron emis-sion and the inverse Compton-scattering are both effectiveenough to fulfill the here adopted inverse Compton scenario.The measured synchrotron radiation cannot exceed the equiv-alent brightness temperature of Tmax ∼ 1012K, because thecooling due to the inverse Compton-effect would be too ef-fective and reduce permanently the density of radio photons(Krauss 1986). Below Tmin ∼ 2 · 1011K the effectivity of the in-verse Compton scattering would be so low (Bloom & Marscher1991), that the synchrotron and the X-ray spectrum should beuncorrelated or no X-ray spectrum should be observable.

With the derived source size range, we also calculated theequipartition Doppler-factor δequ following Readhead (1994)and Guijosa & Daly (1996), which implies that the radiatingparticles have the same energy as the penetrating magneticfield. The results12:

δequ(θ(Tmin) = 0.33 mas) = 2.3 (4)

δequ(θ(Tmax) = 0.15 mas) = 15.5 (5)

show in comparison to the inverse-Compton Doppler-factor,that the source is only in energy equipartition, if the bright-ness temperature is close to Tmin. Readhead (1994) concluded

8 With the terms of footnote 5 it is δ = (γ (1 − β cosϑ))−1 withγ = 1/

√

1 − β2

9 which gives the lower energy end of the pure optically thin syn-chrotron spectrum

10 with an underestimated turn-over frequency the calculatedDoppler-factor would be overestimated

11 If much more (spectral and multi-epoch) data are available as e.g.in the case of the quasar 3C 345, also inhomogeneities in the parti-cle number density can be estimated. Lobanov & Zensus (1999) usedsuch more detailed analyzes to explain quantitatively the observed fluxdensity variations with time of 3C 345.

12 assuming as above h = 0.71


that many powerful, non-thermal extragalactic radio sourcesare close to the energy equipartition. And the brightness tem-peratures of the respective sources were found to be far belowTmax.

As in the previous section, we adopt now that the intrinsicvelocities along the jet are equal to the highest minimal intrin-sic velocity, given by the apparent separation speeds. Then theangle between the direction of motion and the line of sight canbe estimated from the Doppler-factor:

δcore = 2.6+3.2−1.1; βA

min ≈ 0.999+0.0003−0.0005 → ϑcore ≈ (10.7 ± 4.5)◦ (6)

Thus ϑcore is well between each of the solutions ϑC1,2s,l of

Equ. 3, if the same intrinsic velocity βAmin is present along the

jet. This result supports the idea of a spatially curved radio jet(Sec.3.3).

4. Discussion of the results

For the first time the kinematics of the non-thermal radiojet components of J1101+7225 were estimated. We calcu-lated from VLBI-measurements the apparent motions of thetwo extended components, observable at low GHz-frequencies.We found an apparent superluminal separation speed ofβapp(Core − A) = (22.5 ± 4) for the A-component over thepast decade, at an exceptionally large deprojected distance tothe core (22 mas ∼ 4 kpc at z = 1.46, q0 = 0.1). The evenmore distant B-component was found to be stationary withinthe errors at a core-distance of ∼ 53 mas.

Further a Doppler-factor δ = (2.6±3.21.1) and an angular

source size θ = (0.25+0.8−0.1) mas was estimated for the un-

resolved optically thick13 synchrotron radiation of the coreof J1101+7225. The strength of the magnetic field, neces-sary for the synchrotron process, was estimated to be B0 =

(3 · 10−(2±1)) G. The estimated size of the unresolved opti-cally thick nuclear component is not much smaller than thesynthesized beam of an interferometric observation at cm-wavelengths. This suggests in combination with the estimatedDoppler-factor, that in the future additional VLBI-componentscould appear outside the core.

More observations of these nuclear components could re-veal, if a deceleration of the A-component will appear whileA is approaching the B-component. This would be similarto the well known case of 4C 39.25 (Alberdi et al. 1993a).Deceleration at these distances to the core can confirm an in-teraction of the jet component with the circumnuclear matter ofthe host galaxy as described by Taylor et al. (1989) for Seyfertnuclei. Such an interaction is strongly supported by the com-parison of the here presented VLBI-maps with the jet geome-try as observed with the VLA (Xu et al. 1995). In their 1.4 GHzmap two corresponding radio jets appear, extending along ∼ 5′′

from the core. While the south-western jet extends to the oppo-site direction of the VLBI jet, the north-eastern VLA-jet doesnot coincide with the VLBI structure. Rather it appears to bebent towards a north-western direction, perhaps induced by rampressure of the surrounding material.

13 at the observing frequencies

Apparent superluminal velocities are explained by a motiontowards the observer at relativistic velocity. The additionallypresented 5 GHz maps and Gaussian modelfits of the central5 mas show further jet components with significantly smallerapparent velocities. Thus the separation velocities of the differ-ent VLBI radio jet components of J1101+7225 show no sim-ple correlation with their distance to the core. But this doesnot rule out a constant intrinsic velocity, because already smallvariations of the angle between jet and the line of sight can in-troduce variations of the apparent speeds of the observed orderof magnitude. In the case of such a constant intrinsic velocityalong the whole jet, the very high velocity, derived from theA-component, has to be chosen. Such high intrinsic velocitiesexplain both the large extension of the radio jet over severalarcsec as estimated with the VLA and the high luminosity ofthe core.

We believe that our findings may indicate a helical bendingof the jet, where the fast components are moving on a sectionwhich is curved towards the observer. This is supported by theestimated differing jet orientations with respect to the line ofsight. Zensus et al. (1995) could explain the acceleration of a jetcomponent of 3C 345 by a curved jet of constant intrinsic bulkvelocity. It has been shown that helical jet patterns are resultingfrom Kelvin-Helmholtz and current-driven jet instabilities inrelativistic flows (Birkinshaw 1991, Istomin & Pariev 1996).

Beside this interpretation of the observed motion as an in-trinsic bulk motion of the radiating plasma, shock waves maytravel along the jet (Alberdi et al. 1993b). Different componentvelocities may also be observed if the slower components aretrailing in the wake of faster ones. This hydrodynamical expla-nation could successfully be adopted to the complex compo-nent motion in the radio jet of the radio galaxy 3C 120 (Gomezet al. 2001). Hardee et al. (2001) found mechanisms to producedifferentially moving and stationary features in a jet by analyz-ing the relativistic hydrodynamic equations.

Thus the nuclear region of J1101+7225 provides the rarepossibility to observe the total range of jet kinematics includ-ing apparent superluminal separation velocities even far out ofthe central parsec-region. The here presented results can giveobservational constraints far from the jet origin for numericaljet models.

Acknowledgements. We are grateful to the correlator team of theMax-Planck Institut fur Radioastronomie in Bonn for their assistance.The European VLBI Network is a joint facility of European, Chinese,South African and other radio astronomy institutes funded by theirnational research councils. This work was supported in part by theDeutsche Forschungsgemeinschaft (DFG) via grant SFB 494.

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