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Gamma-ray emission from AGN

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Title: 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

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 ???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.
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
Superluminal motions
- Measured ?obs gives only the minimum ?.
  • D from beaming models SobsS0Dp
  • (e.g. Kollgaard et al 1996)

Elog(Pc/Pex)
17
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

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
?s2?2?2? (Thomson scattering)
?s??mec2 (KN scattering)
Luminosity
LIC(4/??2)? Ajdr ?dEe/dt? ne
20
Radiation drag by external photon fields
21
Compton drag
Lab frame
?
e-
e
Incoming photons
22
Compton drag (contd)
23
The KN effect
24
Equilibrium bulk ?
  • ? lt ?eq radiation forces ? accelerate a jet
  • ? gt ?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 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

28
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 (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)

30
Terminal Lorentz factor
Bulk Lorentz factor
31
Emission from dragged jets
(e.g. Eldar Levinson 2000)
32
SED
(Wagner 1999)
33
LIC vs Lk
?020, 50,100 Ld1046erg s-1 Lj1046erg
s-1 Z0103Rg lt?gt5.
?Lk/LB LjLkLB1046 erg s-1.
34
Poynting flux dominated jets?
  • Equapartition but a small Ljlt1046erg s-1
  • Or Lj 1046 erg s-1 but LBgtLk

35
Equipartition
LjLkLB
Lsyn? neB2
Lj?Lsyn/(?B)2
LB? (?B)2
(e.g. Ghisellini 1999)
36
Multifrequency observations
(Wagner 1999)
37
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.
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