Gamma-ray emission from AGN

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/decelera
  tion.
• 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? /?lnE0.6-1.6
• for EGRET blazars
• TeV ?-rays No evidence for ?-ray absorption
• due to pair production

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TeV ?-rays from Mk421, Mk501
? Mk 501 ? Mk 421
(Krennrich et al 1999)
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Escape of TeV ?-rays
A large ? is needed to explain IDV in ?-ray
emission from Mk 501

• Absorption of TeV ?-rays via ???ee-.

Photon number density nphF? d2/(c3t2varD4)
(Protheroe 1998)

• The maximum photon energy

?phD?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 Tb1.8?1012
  K (0133476)
• (Lister et al 2001 Tingay et al 2001).

• The intrinsic brightness temperature
• TbTb(1z)/D, D?(???bcos?)-1

-Variability brightness temperature
Tvar S?d2/ 2?2t2var
In the jet frame TvarTvar/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 ?gt10 needed?

IDV may be due to both intrinsic effects and
scintillation.
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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)?

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Superluminal motions
- Measured ?obs gives only the minimum ?.

• D from beaming models SobsS0Dp
• (e.g. Kollgaard et al 1996)

Elog(Pc/Pex)
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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 jets
  surroundings decelerate the jet

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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

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ERC
Photon energy
?s2?2?2? (Thomson scattering)
?s??mec2 (KN scattering)
Luminosity
LIC(4/??2)? Ajdr ?dEe/dt? ne
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Radiation drag by external photon fields
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Compton drag
Lab frame
?
e-
e
Incoming photons
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Compton drag (contd)
23
The KN effect
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Equilibrium bulk ?

• ? lt ?eq radiation forces ? accelerate a jet

• ? gt ?eq radiation forces ? decelerate a jet

• When acceleration is dominant, ? is determined
  by
• acceleration

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Photon fields from a disk
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Electron-proton jets
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Extended disks

• Drag due to radiation fields from
• an extended disk

- A plasma blob at z100Rg, 102 Rg and 3?103
Rg 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 ?gt100

• Terminal ? depends on the initial
• distance and jet content

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Dust torus

• Drag due to radiation fields from disk torus

- Blazars with a dusty molecular torus?
- Pier Krolik (1992) model

• Deceleration region extended

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Compton drag (contd)

• Acceleration fast enough in lt 0.2pc

• Pair plasma in the blob relativistic

• The terminal ?f lt 20

• Acceleration occurs over a larger range
• ?f gt 20 possible (determined by the acc.
• mechanism)

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Terminal Lorentz factor
Bulk Lorentz factor
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Emission from dragged jets
(e.g. Eldar Levinson 2000)
32
SED
(Wagner 1999)
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LIC vs Lk
?020, 50,100 Ld1046erg s-1 Lj1046erg
s-1 Z0103Rg lt?gt5.
?Lk/LB LjLkLB1046 erg s-1.
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Poynting flux dominated jets?

• Equapartition but a small Ljlt1046erg s-1

• Or Lj 1046 erg s-1 but LBgtLk

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Equipartition
LjLkLB
Lsyn? neB2
Lj?Lsyn/(?B)2
LB? (?B)2
(e.g. Ghisellini 1999)
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Multifrequency observations
(Wagner 1999)
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Radio emission
- Photosphere the radius ?self-ablt1
- 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 Rg1.5?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.