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Gamma-ray emission from AGN

Gamma-ray emission from AGN. Qinghuan Luo School of Physics, University of Sydney. Blazars. EGRET sources: Most of them are AGN. Third EGRET Catalog. Diffuse -ray background: - Unresolved blazars or - Exotic processes

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Gamma-ray emission from AGN

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  1. Gamma-ray emission from AGN Qinghuan Luo School of Physics, University of Sydney

  2. Blazars • EGRET sources: Most of them are AGN Third EGRET Catalog • Diffuse -ray background: - Unresolved blazars or - Exotic processes e.g. annihilation lines from supersymmetric particle dark matter or unstable particle relics? (Hartman et al 1999)

  3. Mk421, Mk501

  4. 3C273, 3C279

  5. Rapid variations Mk501

  6. Overview • Blazars (BL Lac, FSQ): Relativistic jets directed at a small angle to the line of sight. • Intraday variability (IDV): small scales; large . • Relativistic jets, contents, acceleration/deceleration. • Emission mechanisms: SSC vs ERC? • Emission from decelerating/accelerating jets?

  7. High energy spectra of blazars • At least two components: IR-UV (perhaps up to X-rays) and above hard X-rays • High energy range is power-law, =-∂lnL /∂lnE≈0.6-1.6 for EGRET blazars • TeV -rays; No evidence for -ray absorption due to pair production

  8. TeV -rays from Mk421, Mk501 : Mk 501 : Mk 421 (Krennrich et al 1999)

  9. Escape of TeV -rays A large  is needed to explain IDV in -ray emission from Mk 501 • Absorption of TeV -rays via +e++e-. Photon number density nph≈F d2/(c3t2varD4) (Protheroe 1998) • The maximum photon energy: ph~Dmaxmec2 in the KN regime; ph ~15TeV requires D ~30 for =106.

  10. TeV flares • Intraday variability (possibly ~ hrs) requires • relativistic beaming! Mrk 501

  11. Radio IDV PKS 0405-385 (Kedziora-Chudczer et al. 1997)

  12. The brightness temperature problem -VLBI measurement: • Space-based VLBI survey: the highest Tb=1.81012 K (0133+476) • (Lister et al 2001; Tingay et al 2001). • The intrinsic brightness temperature: • T’b=Tb(1+z)/D, D=[(bcos)]-1 • -Variability brightness temperature: Tvar= Sd2/ 22t2var In the jet frame T’var~Tvar/D3 e.g. for PKS 0405-385, Tvar= 1021 K! (Kedziora-Chudczer et al. 1997)

  13. Constraints on Tb • Synchrotron self-absorption: Tb≤ mec2/kB • Inverse Compton scattering (Kellermann & Pauliny-Toth 1969) • Equipartition (Readhead 1994) • Induced scattering: -Induced Compton scattering (kTb/mec2)T≤ 1 (e.g. Coppi, Blandford, Rees 1993; Sincell & Krolik 1994) -Induced Raman scattering and possibly other processes -Coherent processes is not favoured

  14. Interpretation of radio IDV • Various models - Extrinsic: Interstellar scintillation - Intrinsic: Coherent emission; Geometric effects (Spada et al 1998) Synchrotron radiation by protons (Kardashev 2000) Non-stationary models (Slysh 1992) • Relativistic bulk beaming with >10 needed? IDV may be due to both intrinsic effects and scintillation.

  15. Relativistic bulk motions • Rapid variability, high brightness temperature require relativistic bulk motion with a higher . • Continuous jets or blobs? • Observations of -ray flares, IDV appear to suggest the source region being close to the central region. • Both acceleration and deceleration of the jet can occur in the central region. • VLBI observations: ≤ 10. The limit of VLBI or acceleration mechanisms or radiation drag (e.g. Phinney 1987)?

  16. RBLs Superluminal motions - Measured obs gives only the minimum . • D from beaming models: Sobs=S0Dp • (e.g. Kollgaard et al 1996) E=log(Pc/Pex)

  17. - “Twin exhaust’’ model: Blandford & Rees (1974) - Radiation acc.: O’Dell (1981) - Acc. by tangled magnetic fields: Heinz & Begelman (2000) Phinney (1982, 1987): ~ eq < 10. Sikora et al. (1996) Formation of jets • Acceleration mechanisms: no widely accepted model. - The unipolar model: Blandford & Znajek (1977), Macdonald & Thorne (1982) • Radiation drag: • - Radiation fields from the disk and jet’s surroundings decelerate the jet

  18. Emission mechanisms: SSC vs ERC • Synchrotron self-Compton (SSC): (e.g. Konigl 1981; Marscher & Gear 1985; Ghisellini & Maraschi 1989) Synchrotron photons are both produced and Comptonized by the same Population of electrons. • External radiation Compton (ERC): The seed photons are from external sources such as disks, BR, turi, etc. (e.g. Begelman & Sikora 1987; Melia & Konigl 1989; Dermer et al. 1992) • Both SSC and ERC operate

  19. ERC Photon energy: s~222 (Thomson scattering) s~mec2 (KN scattering) Luminosity: LIC=(4/2)∫ Ajdr dEe/dt ne

  20. Radiation drag by external photon fields

  21. Incoming photons e- e+ Jet frame Compton drag Lab frame  e- e+ Incoming photons

  22. Compton drag (cont’d)

  23. The KN effect

  24. Equilibrium bulk  •  < eq: radiation forces  accelerate a jet •  > eq: radiation forces  decelerate a jet • When acceleration is dominant,  is determined by • acceleration

  25. Photon fields from a disk

  26. Electron-proton jets

  27. Extended disks • Drag due to radiation fields from • an extended disk - A plasma blob at z=100Rg, 102 Rg and 3103Rg with =100. Pairs have a power-law, isotropic distribution in the jet frame. • An extended disk reprocesses • radiation from the inner disk. - KN scattering important only for >100 • Terminal  depends on the initial • distance and jet content

  28. The unified scheme, e.g. Barthel (1989) • -ray models for blazars (e.g Protheroe 1996) • Strong correlation between gamma-ray and near-IR • luminosities for a sample of blazars (Xie et al. 1997) Dust torus • Drag due to radiation fields from disk + torus - Blazars with a dusty molecular torus? - Pier & Krolik (1992) model • Deceleration region extended

  29. Compton drag (cont’d) • Acceleration fast enough in < 0.2pc • Pair plasma in the blob relativistic • The terminal f < 20 • Acceleration occurs over a larger range • f > 20 possible (determined by the acc. • mechanism)

  30. Terminal Lorentz factor Bulk Lorentz factor

  31. Emission from dragged jets (e.g. Eldar & Levinson 2000)

  32. SED (Wagner 1999)

  33. LIC is the received power from IC: LIC=(4/2)∫ Ajdr dEe/dt ne LIC vs Lk 0=20, 50,100 Ld=1046erg s-1 Lj=1046erg s-1 Z0=103Rg <>=5. =Lk/LB Lj=Lk+LB=1046 erg s-1.

  34. Poynting flux dominated jets? • Equapartition but a small Lj<1046erg s-1 • Or Lj =1046 erg s-1 but LB>Lk

  35. Equipartition Lj=Lk+LB Lsyn neB’2 LjLsyn/(B’)2 LB (B’)2 (e.g. Ghisellini 1999)

  36. Multifrequency observations (Wagner 1999)

  37. Radio emission - Photosphere: the radius self-ab<1 - Doppler boosted Tb decreases - Frequency dependence of Tb  decreases - Tb changes with t ?

  38. Summary • Compton drag important and should be taken into account in modeling of blazars. • Radiation drag limit to the bulk  in the central region up to 0.1-0.2 pc (for Rg=1.51013 cm). • The terminal  is not well defined; It depends on acc. mechanisms, jet content (protons, cold electrons). A very large  is not favoured. • Emission from the drag constrains jet models; multifrequency obs of IDV provide a test. • For radio IDV, when the emission region is decelerating, change in  change frequency dependence of Tb.

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