Title: Gamma-ray emission from AGN
1Gamma-ray emission from AGN
- Qinghuan Luo
- School of Physics, University of Sydney
2Blazars
- 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)
3Mk421, Mk501
43C273, 3C279
5Rapid variations
Mk501
6Overview
- 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?
7High 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
8TeV ?-rays from Mk421, Mk501
? Mk 501 ? Mk 421
(Krennrich et al 1999)
9Escape 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.
10TeV flares
- Intraday variability (possibly hrs) requires
- relativistic beaming!
Mrk 501
11Radio IDV
PKS 0405-385
(Kedziora-Chudczer et al. 1997)
12The 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)
13Constraints on Tb
- Synchrotron self-absorption Tb mec2?/kB
- Inverse Compton scattering (Kellermann
Pauliny-Toth 1969)
- Equipartition (Readhead 1994)
-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
14Interpretation of radio IDV
- 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.
15Relativistic 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)?
16Superluminal motions
- Measured ?obs gives only the minimum ?.
- D from beaming models SobsS0Dp
- (e.g. Kollgaard et al 1996)
Elog(Pc/Pex)
17Formation 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
18Emission 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)
19ERC
Photon energy
?s2?2?2? (Thomson scattering)
?s??mec2 (KN scattering)
Luminosity
LIC(4/??2)? Ajdr ?dEe/dt? ne
20Radiation drag by external photon fields
21Compton drag
Lab frame
?
e-
e
Incoming photons
22Compton drag (contd)
23The KN effect
24Equilibrium bulk ?
- ? lt ?eq radiation forces ? accelerate a jet
- ? gt ?eq radiation forces ? decelerate a jet
- When acceleration is dominant, ? is determined
by - acceleration
25Photon fields from a disk
26Electron-proton jets
27Extended 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
28Dust torus
- Drag due to radiation fields from disk torus
- Blazars with a dusty molecular torus?
- Pier Krolik (1992) model
- Deceleration region extended
29Compton drag (contd)
- Acceleration fast enough in lt 0.2pc
- Pair plasma in the blob relativistic
- Acceleration occurs over a larger range
- ?f gt 20 possible (determined by the acc.
- mechanism)
30Terminal Lorentz factor
Bulk Lorentz factor
31Emission from dragged jets
(e.g. Eldar Levinson 2000)
32SED
(Wagner 1999)
33LIC vs Lk
?020, 50,100 Ld1046erg s-1 Lj1046erg
s-1 Z0103Rg lt?gt5.
?Lk/LB LjLkLB1046 erg s-1.
34Poynting flux dominated jets?
- Equapartition but a small Ljlt1046erg s-1
- Or Lj 1046 erg s-1 but LBgtLk
35Equipartition
LjLkLB
Lsyn? neB2
Lj?Lsyn/(?B)2
LB? (?B)2
(e.g. Ghisellini 1999)
36Multifrequency observations
(Wagner 1999)
37Radio emission
- Photosphere the radius ?self-ablt1
- Doppler boosted Tb decreases
- Frequency dependence of Tb
?
decreases
- Tb changes with t ?
38Summary
- 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.