High energy radiation from extragalactic jets and prospects for GLAST

Greg Madejski

Stanford Linear Accelerator Center and Kavli Institute for Particle Astrophysics and Cosmology

In collaboration with: Jun Kataoka, Tad Takahashi, Marek Sikora, Lukasz Stawarz, Yuri Kovalev, Stefan Wagner, …

• Context: blazars as active galaxies with dominant relativistic jet

• Main questions: structure, acceleration, collimation, and content of the jet, -> all inferred from radiative processes via broad-band spectra

• Recent Suzaku observations of blazars – signature of “bulk-Compton” process?

• Future: GLAST

Context: Broad-band spectra and time variability (example: archetypal GeV blazar 3C279, 1996 flare)

* GeV emission dominates the observed flux

* Correlated variability on day time scales is common

* Variability in X-ray and -ray bands puts constraints on the minimum relativistic boost (j) of the innermost region (via absorption to e+/e- pair production)

* Lorentz factors j of jets inferred from VLBI (multi-parsec scales) can be compared against j inferred from variability (sub-parsec scales)

-> jet as a function of distance from the black hole? - > constraints on acceleration process of the jet

(Data from Wehrle et al. 1998)

EGRET All Sky Map (>100 MeV)CygnusRegion

3C279

Geminga

Vela

Cosmic RayInteractions

With ISM

LMC

PKS 0528+134

PKS 0208-512

Crab

PSR B1706-44

“Working picture” and approach• Radiation processes probably are

adequately understood (synchrotron for LE peak, inverse Compton for the HE peak) – but hadronic processes are not completely ruled out

• Pressing questions: • How are the jets accelerated and

collimated? • What is the process of energy

dissipation (=acceleration of radiating particles) in the jet?

• What is the content of the jet as a function of distance from the BH?

Approach for all three: study the time-resolved spectra, infer the radiative processes and thus jet structure, connection to the accretion processand the black hole ->

MULTI-BAND MONITORING IS REQUIRED

Focus here: jet contentDiagram from Padovani and Urry

The “blazar sequence”

* Work by G. Fossati, G. Ghisellini, L. Maraschi, others (1998 and on)

* Multi-frequency data on blazars reveals a “progression” –

* As the radio luminosity increases:

- Location of the first and second peaks moves to lower frequencies

- Ratio of the luminosities between the high and low frequency components increases

- Strength of emission lines increases

High-luminosity, MeV - GeV-peaking objects (FR-IIs beamed towards us)Lower-luminosity, TeV-peaking objects (FR-Is beamed towards us)

Suzaku observations of blazars

Suzaku XIS + HXD PIN spectrum of 0836+714 Synchrotron + Compton model by Tavecchio et al. 2000 (old BeppoSAX data)

•Suzaku observed a number of blazars - the list of EGRET blazars includes PKS 1510-089, BL Lacertae, SWIFT0746+25, 0836+714 (covered below), but also many TeV blazars

•Generally, the X-ray spectra of EGRET blazars are very hard power laws (photon indices often < 0.5), extending into the HXD PIN energy range (contrast to TeV blazars)* Very difficult to explain such hard spectra!

Suzaku observations of PKS 1510-089: hard

power law + “soft excess” • Suzaku observed PKS 1510-

089, a blazar at z ~ 0.3 for 120 ks (joint observation of Jun Kataoka & GM; Kataoka+ 2007)

• Spectrum is a hard power law (energy index ~ 0.2), but with a soft X-ray “excess” below ~ 1.5 keV, best described as a steep power law or a thermal component with kT ~ 0.2 keV

• Explanation as a “tail” of the synchrotron emission unlikely – extrapolation does not work

• Too hot to be the tail of the “blue bump”

Suzaku obervations of PKS 1510-089: possible

evidence for “bulk-Compton” bump? • This soft excess might be the tentative evidence for the “Sikora bump” – arising by

the inverse Compton scattering of BEL light by the cold electrons comoving in the relativistic jet (Ebump ~ E(Ly) x 2 ~ 1 keV)

• Even if it is not “bulk-Compton”…

• From its isotropic luminosity of LBC < 3 x 1044 erg/s - we can set a limit on the energy flux Le,cold carried by the cold electrons and the e+/e- pair content of the jet:

-since LBC = (4T/3mec2) UBEL rBLR j3 Le,cold

we have Le,cold < 2.7x1043 (rBLR/0.1pc) (j/10)-3 (LBEL/1045erg/s)-1 erg/s

- Significantly less than the required kinetic luminosity of the jet

- Now the total power delivered by the jet must be 8x1044 erg/s

-With more realistic parameters, ne/np in the jet is < 5--> Jet contains more pairs than protons, but cannot be

dynamically dominated by e+/e- pairs-For details, see Kataoka+ 07 (-> AstroPh in a week)

GLAST LAT has much higher sensitivity to weak sources, with much better angular resolution

Planned launch: December 2007

GLAST EGRET

GLAST Large Area Telescope – principles of operation

-rays interact with the hi-z material in the foils, pair-produce, and are tracked with silicon strip detectors

* Energy range is ~ 30 MeV – 300 GeV, with the peak effective area of ~ 10,000 cm2

* This allows an overlap with TeV observatories

* The instrument “looks” simultaneously into ~ 2 steradians of the sky

Schematic of operationof GLAST LAT

GLAST observing strategy & performance: angular resolution, broad-band sensitivity (1 year)

(for more details, see poster by Paneque)

100 s

1 orbit

1 day

LAT 1 yr

2.3 10-9 cm-2 s-1

3EG limit

0.01

0.001

Sensitivity of GLAST to measure flux & variability of -ray sources

• GLAST can measure flux of 3C279 in outburst to better than 10% in a day, can determine the index to ~ 0.1

(assumed = 2)

• Expected light curve of 3C279 from GLAST LAT

(flux history is assumed to be the same as that measured by EGRET for 3C279 in 1996)

SUMMARYSUZAKU HAS PROVEN TO BE AN EXCELLENT INSTRUMENT TO STUDY BLAZARS,

BUT THE MOST IMPORTANT INFERENCES WILL BE VIA MULTI-BAND MONITORING OF THOSE HIGHLY VARIABLE SOURCES

MOST CRUCIAL WILL BE JOINT OBSERVATIONS WITH GLAST, TOGETHER WITH OPTICAL OBSERVATIONS

WE HAVE A GOOD IDEA ABOUT RADIATIVE PROCESSES IN BLAZARS - HIGHLY ENERGETIC PARTICLES RADIATING IN A RELATIVISTIC JET – BUT MANY QUESTIONS REMAIN:

• WHAT IS THE CONNECTION OF THE JET TO THE BLACK HOLE?

• WHAT ACCELERATES AND COLLIMATES THE JET?

• HOW IS BULK KINETIC ENERGY OF THE JET CONVERTED TO ENERGY OF RADIATING PARTICLES?

• WHAT IS THE RELATIONSHIP OF BLAZARS TO RADIO GALAXIES?

GLAST AND SUZAKU WIILL OPEN NEW ERA FOR STUDIES OF ASTROPHYSICAL JETS, ALLOWING NEW

INSIGHTS INTO THE STRUCTURE OF THOSE EXTREME PARTICLE ACCELERATORS

Recent outburst of 3C454.3

Example of an object where ERC may

dominate: 3C279(data from Wehrle et al. 1998)

Example of an object where SSC may

dominate: Mkn 421(data from Macomb et al. 1995)

SSC or ERC?

Modelling of radiative processes in blazars• In the context of the synchrotron models, emitted photon frequency is

s = 1.3 x 106 B x el2 Hz

where B is the magnetic field in Gauss and el is the electron Lorentz factor

• The best models have B ~ 1 Gauss, and el for electrons radiating at the peak of the synchrotron spectral component of ~ 103 – 106,

depending on the particular source• Degeneracy between B and el is “broken” by spectral variability

+ spectral curvature, at least for HBLs (Perlman et al. 2005)

• The high energy (Compton) component is produced by the same electrons as the synchrotron peak and compton = seed x el

2 Hz

• Still, the jet Lorentz factor j is ~ 10, while Lorentz factors of radiating electrons are el ~ 103 – 106

• Thus, one of the central questions in blazar research is:

HOW ARE THE RADIATING PARTICLES ACCELERATED?

Interpretation of the observational data for blazars

PARTICLE ACCELERATION

* The most popular models invoke the Fermi acceleration process in shocks forming via collision of inhomogeneities or distinct plasma clouds in the jet (“internal shock” model, also invoked for GRBs)

* This can work reasonably well: the acceleration time scale acc to get electron up to a Lorentz factor el can be as short as ~ 10-6 el B-1 seconds, while the cooling time (due to synchrotron losses) is ~ 5 x 108 el

-1 B-2 seconds, perhaps up to 10 times faster for Compton cooling, so accelerating electrons to el up to ~ 106 via this process is viable (but by no means unique!)

INTERNAL SHOCK SCENARIO MODEL

* This model assumes that the central source produces multiple clouds of plasma and ejects them with various relativistic speeds: those clouds collide with each other, and the collision results in shock formation which leads to particle acceleration

* A simple "toy model" that reproduces observations well assumes two clouds of equal masses, with Lorentz factors 1 and 2 with 1 < 2 (1 and 2 >> 1)

* From 2 and 1 one can infer the efficiency (fraction of kinetic power available for particle acceleration)* Recent simulations reproducing well the X-ray light curves of Mkn 421 (and applicable to other

objects) (Tanihata et al. 2003) imply that the dispersion of cannot be too large * However, the small dispersion of implies a low efficiency – (< 0.1%) so there might be a problem

- as huge kinetic luminosities of particles are required…

* MY OWN PREJUDICE IS THAT THE JETS ARE LAUNCHED AS MHD OUTFLOWS, AND ARE INITIALLY DOMINATED BY POYNTING FLUX

* WE NEED TO UNDERSTAND DISSIPATION/PARTICLE ACCELERATION AS WELL AS THE DISK – JET CONNECTION

Content of the jet• Are blazar jets dominated by kinetic energy of particles

from the start, or are they initially dominated by magnetic field (Poynting flux)? (Blandford, Vlahakis, Wiita, Meier, Hardee, …)

• There is a critical test of this hypothesis, at least for quasar-type (“EGRET”) blazars:

• If the kinetic energy is carried by particles, the radiation environment of the AGN should be bulk-Compton-upscattered to X-ray energies by the bulk motion of the jet

• If jet = 10, the ~10 eV, the H Ly photons should appear bulk-upscattered to 102 x 10 eV ~ 1 keV

• X-ray flare should precede the -ray flare (“precursor”)

• X-ray monitoring concurrent with GLAST observations is crucial to settle this

• A lack of X-ray precursors would imply that the jet is “particle-poor” and may be dominated by Poynting flux

GLAST LAT Science Performance Requirements Summary

Parameter RequirementPeak Effective Area (in range 1-10 GeV) >8000 cm2

Energy Resolution 100 MeV on-axis <10%

Energy Resolution 10 GeV on-axis <10%

Energy Resolution 10-300 GeV on-axis <20%

Energy Resolution 10-300 GeV off-axis (>60º) <6%

PSF 68% 100 MeV on-axis <3.5°

PSF 68% 10 GeV on-axis <0.15°

PSF 95/68 ratio <3

PSF 55º/normal ratio <1.7

Field of View >2sr

Background rejection (E>100 MeV) <10% diffuse

Point Source Sensitivity(>100MeV) <6x10-9 cm-2s-1

Source Location Determination <0.5 arcmin

Sensitivity of GLAST LAT

Recent outburst of 3C454.3

• The points represent the energy flux for which will get 2 sigmas that that specific energy range (1/4 of a decade). Since we show 15 points, that implies an overall signal significance of ~ sqrt(15)*2 ~ 8 sigmas.

• 1 GeV = 1.6 e-3 erg

• GLAST performance: angular resolution, effective area as a function of energy and off-axis angle

100 s

1 orbit

1 day

LAT 1 yr2.3 10-9 cm-2 s-1

3EG limit

0.01

0.001

Radio, optical and X-ray images of the jet in M 87

* Jets are common in AGN – and radiate in radio, optical and X-ray wavelengths* Blazars are the objects where jet is pointing close to the line of sight* In many (but not all) blazars, the jet emission dominates the observed spectrum

Suzaku observations of blazars

Suzaku XIS + PI spectrum of PKS 1510-089 Synchrotron + Compton model by Tavecchio et al. 2000(see poster by Kohmura) (old BeppoSAX data)

* Suzaku observed a number of blazars in the SWG and GO phases* The list of EGRET blazars includes PKS 1510-089

0836+714, BL Lacertae, and SWIFT0746+25* Generally, the X-ray spectra of EGRET blazars are very hard power laws ( < 1.5), extending

into the HXD PIN energy range* This is in contrast to the TeV blazars, showing softer power laws ( > 2), gradually

steepening towards higher energy ( ~ 0.3 / decade) – Costamante’s talk

Example of recent Suzaku data for blazars: SWIFT0746

* Simultaneous multi-wavelength observations are very important, but not all the ground-based data for recent campaigns are “in” yet

* The hard X-ray spectra measured by Suzaku (here: photon index 1.2)already pose challenges to hadronic models sometimes invoked to explain the “high energy” peaks of blazars

From Sikora, Begelman, and Rees 1994

• Most viable models are “leptonic” – synchrotron emission for the low-energy peak, Compton emission for the high energy peak

• Source of the “seed” photons for inverse Compton scattering can depend on the environment - internal to the jet (the “Synchrotron self-Compton”) or external to the jet (“External Radiation Compton”)

• The latter (ERC) is probably applicable to blazars hosted in quasars such as 3C279

• Modelling is pretty robust, gives B ~ 1 Gauss, max of radiating electrons ~ 105

• Most likely the jets are initially dominated by magnetic energy (Poynting flux); Sikora, GM, Begelman, Lasota (2005)

MODELING OF BLAZAR EMISSION

Suzaku 50 ks observation of 1510-089BeppoSAX + multi- 0836 data from Tavecchio et al. (2000)

Suzaku and EGRET blazars* 0836+714 (=4C71.07) is one of most distant EGRET blazars, z = 2.172 shows a power law with = 1.4

* The X-rays are likely produced by Compton upscattering of external, broad emission line photons or IR

* Hard X-ray band probes the „bulk” of the radiating particles (low radio – contaminated by extended comp.)

* Suzaku-inferred power law index index = 1.4 implies power law index of radiating particles p ~ 1.8, even for such a hard spectrum

* Very important towards the determination of the jet content, and – because of the radiation environment of the host galaxy – the distance from the black hole where the jet forms

XISHXD PIN

Another example of Suzaku-observed blazar: BL Lacertae

BL Lacertae shows complex X-ray spectrum: Hard power law above ~ 2 keV (onset of the

Compton component, probably SSC) + “soft excess,” probably the “tail” of the

synchrotron component

Light curve (1 day): Modest variability (10%)

Suzaku spectrum: Absorption less than Galactic(plotted as the green model)-> 2 component spectrum (soft E<2 keV + hard E>2 keV)

Synchrotron

ssc

ERC

SSC+ERC model of simultaneous data from 1998 (Madejski et al. 1999)

Example of VLBI superluminal expansion of a (potential) GLAST blazar: 0716+714

• All known EGRET blazars show powerful radio jets

• 0716+714 is just one example, - most bright EGRET blazars are monitored with VLBI (Jorstad et al. 2001)

• Lorentz factors of jets inferred from VLBI (multi-parsec scales) can be compared against inferred from variability (sub-parsec scales)

• -> jet as a function of distance from the black hole? - > constraints on acceleration process of the jet

Diagram for the internal shock scenario – colliding shells model: 2 > 1, shell 2 collides with shell 1

Accretion disk and black hole

Broad line regionproviding the ambient UV

Time ->

Toy model: particle acceleration via internal shocks