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High energy radiation from extragalactic jets and prospects for GLAST

High energy radiation from extragalactic jets and prospects for GLAST. Greg Madejski Stanford Linear Accelerator Center and Kavli Institute for Particle Astrophysics and Cosmology.

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High energy radiation from extragalactic jets and prospects for GLAST

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  1. 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

  2. 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 g-ray bands puts constraints on the minimum relativistic boost (Gj) of the innermost region (via g-g absorption to e+/e- pair production) * Lorentz factors Gj of jets inferred from VLBI (multi-parsec scales) can be compared against Gj inferred from variability (sub-parsec scales) -> Gjet as a function of distance from the black hole? - > constraints on acceleration process of the jet (Data from Wehrle et al. 1998)

  3. EGRET All Sky Map (>100 MeV) 3C279 Cygnus Region Vela Geminga Crab PKS 0528+134 LMC Cosmic Ray Interactions With ISM PSR B1706-44 PKS 0208-512

  4. “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 process and the black hole -> MULTI-BAND MONITORING IS REQUIRED Focus here: jet content Diagram from Padovani and Urry

  5. 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)

  6. Suzaku observations of blazars • 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 • a often < 0.5), extending into the HXD PIN energy range (contrast to TeV blazars) • * Very difficult to explain such hard spectra! Suzaku XIS + HXD PIN spectrum of 0836+714 Synchrotron + Compton model by Tavecchio et al. 2000 (old BeppoSAX data)

  7. 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 a ~ 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”

  8. 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(Lya) x G2 ~ 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 = (4sT/3mec2) UBEL rBLRGj3 Le,cold • we have • Le,cold • < 2.7x1043 (rBLR/0.1pc) (Gj/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)

  9. GLAST LAT has much higher sensitivity to weak sources, with much better angular resolution Planned launch: December 2007 EGRET GLAST

  10. GLAST Large Area Telescope – principles of operation * g-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 operation of GLAST LAT

  11. 100 s 1 orbit 1 day 3EG  limit 0.01  0.001 LAT 1 yr 2.3 10-9 cm-2 s-1 GLAST observing strategy & performance: angular resolution, broad-band sensitivity (1 year) (for more details, see poster by Paneque)

  12. Sensitivity of GLAST to measure flux & variability of g-ray sources • GLAST can measure flux of 3C279 in outburst to better than 10% in a day, can determine the index to DG ~ 0.1 (assumed G = 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)

  13. SUMMARY SUZAKU 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

  14. Recent outburst of 3C454.3

  15. SSC or ERC? 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)

  16. Modelling of radiative processes in blazars • In the context of the synchrotron models, emitted photon frequency is ns = 1.3 x 106 B x gel2 Hz where B is the magnetic field in Gauss and gel is the electron Lorentz factor • The best models have B ~ 1 Gauss, and gel for electrons radiating at the peak of the synchrotron spectral component of ~ 103 – 106, depending on the particular source • Degeneracy between B and gel 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 ncompton = nseed x gel2 Hz • Still, the jet Lorentz factor Gj is ~ 10, while Lorentz factors of radiating electrons are gel ~ 103 – 106 • Thus, one of the central questions in blazar research is: HOW ARE THE RADIATING PARTICLES ACCELERATED?

  17. 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 tacc to get electron up to a Lorentz factor gel can be as short as ~ 10-6gelB-1 seconds, while the cooling time (due to synchrotron losses) is ~ 5 x 108gel-1B-2 seconds, perhaps up to 10 times faster for Compton cooling, so accelerating electrons to gel 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 G1 and G2 with G1 < G2 (G1 and G2 >> 1) * From G2 and G1 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 G cannot be too large * However, the small dispersion of G 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

  18. 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 Gjet = 10, the ~10 eV, the H Lya photons should appear bulk-upscattered to 102 x 10 eV ~ 1 keV • X-ray flare should precede the g-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

  19. GLAST LAT Science Performance Requirements Summary

  20. Sensitivity of GLAST LAT

  21. Recent outburst of 3C454.3

  22. 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

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

  24. 100 s 1 orbit 1 day 3EG  limit 0.01  0.001 LAT 1 yr 2.3 10-9 cm-2 s-1

  25. 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

  26. Suzaku observations of blazars * 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 (G < 1.5), extending into the HXD PIN energy range * This is in contrast to the TeV blazars, showing softer power laws (G > 2), gradually steepening towards higher energy (DG ~ 0.3 / decade) – Costamante’s talk Suzaku XIS + PI spectrum of PKS 1510-089 Synchrotron + Compton model by Tavecchio et al. 2000 (see poster by Kohmura) (old BeppoSAX data)

  27. 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

  28. MODELING OF BLAZAR EMISSION 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, gmax of radiating electrons ~ 105 • Most likely the jets are initially dominated by magnetic energy (Poynting flux); Sikora, GM, Begelman, Lasota (2005)

  29. Suzaku and EGRET blazars Suzaku 50 ks observation of 1510-089 • * 0836+714 (=4C71.07) is one of most distant EGRET blazars, z = 2.172 shows a power law with G = 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 n radio – contaminated by extended comp.) • * Suzaku-inferred power law index index G = 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 BeppoSAX + multi-l0836 data from Tavecchio et al. (2000) XIS HXD PIN

  30. 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%) Synchrotron ssc ERC Suzaku spectrum: Absorption less than Galactic (plotted as the green model) -> 2 component spectrum (soft E<2 keV + hard E>2 keV) SSC+ERC model of simultaneous data from 1998 (Madejski et al. 1999)

  31. 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 G of jets inferred from VLBI (multi-parsec scales) can be compared against G inferred from variability (sub-parsec scales) • -> Gjet as a function of distance from the black hole? - > constraints on acceleration process of the jet

  32. Toy model: particle acceleration via internal shocks Diagram for the internal shock scenario – colliding shells model: G2 > G1, shell 2 collides with shell 1 Broad line region providing the ambient UV Accretion disk and black hole Time ->

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