Gamma-ray emission from AGN
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Transcript of Gamma-ray emission from AGN
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ICRR 17/9/2001
Gamma-ray emission from AGN
Qinghuan Luo
School of Physics, University of Sydney
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ICRR 17/9/2001
Blazars
• EGRET sources:
Most of them are AGN
• Diffuse -ray background:
- Unresolved blazars or
- Exotic processes
e.g. annihilation lines from supersymmetric particle dark matter or
unstable particle relics?
Third EGRET Catalog
(Hartman et al 1999)
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ICRR 17/9/2001
Mk421, Mk501
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ICRR 17/9/2001
3C273, 3C279
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ICRR 17/9/2001
Rapid variations
Mk501
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ICRR 17/9/2001
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?
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ICRR 17/9/2001
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
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ICRR 17/9/2001
TeV -rays from Mk421, Mk501
(Krennrich et al 1999)
: Mk 501: Mk 421
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ICRR 17/9/2001
Escape of TeV -rays
• Absorption of TeV -rays via +e++e-.
• The maximum photon energy:
ph~Dmaxmec2 in the KN regime;ph ~15TeV requires D ~30 for =106.
A large is needed to explain IDV in -ray emissionfrom Mk 501
Photon number density nph≈F d2/(c3t2varD4)
(Protheroe 1998)
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ICRR 17/9/2001
TeV flares
Mrk 501
-Intraday variability (possibly ~ hrs) requires relativistic beaming!
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ICRR 17/9/2001
Radio IDV
(Kedziora-Chudczer et al. 1997)
PKS 0405-385
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ICRR 17/9/2001
The brightness temperature problem
-VLBI measurement:
-Variability brightness temperature:
Tvar= Sd2/ 22t2var
In the jet frame T’var~Tvar/D3
•Space-based VLBI survey: the highest Tb=1.81012 K (0133+476) (Lister et al 2001; Tingay et al 2001).
•The intrinsic brightness temperature: T’b=Tb(1+z)/D, D=[(bcos)]-1
e.g. for PKS 0405-385, Tvar= 1021 K! (Kedziora-Chudczer et al. 1997)
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ICRR 17/9/2001
Constraints on Tb
• Synchrotron self-absorption: Tb≤ mec2/kB
• 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
• Inverse Compton scattering (Kellermann & Pauliny-Toth 1969)
-Coherent processes is not favoured
• Equipartition (Readhead 1994)
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ICRR 17/9/2001
Interpretation of radio IDV
•Various models
- Intrinsic: Coherent emission; Geometric effects (Spada et al 1998)
- Extrinsic: Interstellar scintillation
•Relativistic bulk beaming with >10 needed?
Synchrotron radiation by protons (Kardashev 2000)
Non-stationary models (Slysh 1992)
IDV may be due to both intrinsic effects and scintillation.
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ICRR 17/9/2001
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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ICRR 17/9/2001
Superluminal motions- Measured obs gives only the minimum .
-D from beaming models: Sobs=S0Dp
(e.g. Kollgaard et al 1996)
RBLs
E=log(Pc/Pex)
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ICRR 17/9/2001
Formation of jets
•Radiation drag: - Radiation fields from the disk and jet’s surroundings decelerate the jet
•Acceleration mechanisms: no widely accepted model.
- Acc. by tangled magnetic fields: Heinz & Begelman (2000)
- “Twin exhaust’’ model: Blandford & Rees (1974) - Radiation acc.: O’Dell (1981)
Phinney (1982, 1987): ~ eq < 10.
Sikora et al. (1996)
- The unipolar model: Blandford & Znajek (1977), Macdonald & Thorne (1982)
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Emission mechanisms: SSC vs ERC
•Synchrotron self-Compton (SSC):
•External radiation Compton (ERC):
(e.g. Konigl 1981; Marscher & Gear 1985; Ghisellini & Maraschi 1989)Synchrotron photons are both produced and Comptonized by the same Population of electrons.
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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ICRR 17/9/2001
ERC
Photon energy:
s~222 (Thomson scattering)
s~mec2 (KN scattering)
Luminosity:
LIC=(4/2)∫ Ajdr dEe/dt ne
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ICRR 17/9/2001
Radiation drag by external photon fields
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ICRR 17/9/2001
Compton drag
Incoming photons
Lab frame
e+ e-
Jet frame
Incoming photons
e-
e+
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ICRR 17/9/2001
Compton drag (cont’d)
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ICRR 17/9/2001
The KN effect
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ICRR 17/9/2001
Equilibrium bulk
• < eq: radiation forces accelerate a jet
• > eq: radiation forces decelerate a jet
• When acceleration is dominant, is determined by acceleration
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ICRR 17/9/2001
Photon fields from a disk
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ICRR 17/9/2001
Electron-proton jets
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ICRR 17/9/2001
Extended disks•Drag due to radiation fields from an extended disk
- A plasma blob at z=100Rg, 102 Rg and 3103 Rg with =100. Pairs have a power-law, isotropic distribution in the jet frame.
-An extended disk reprocesses radiation from the inner disk.
-Terminal depends on the initial distance and jet content
- KN scattering important only for >100
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ICRR 17/9/2001
Dust torus
• Drag due to radiation fields from disk + torus
- Blazars with a dusty molecular torus?
- Pier & Krolik (1992) model
• Deceleration region extended
• 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)
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ICRR 17/9/2001
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)
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ICRR 17/9/2001
Bulk Lorentz factor
Terminal Lorentz factor
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ICRR 17/9/2001
Emission from dragged jets
(e.g. Eldar & Levinson 2000)
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ICRR 17/9/2001
SED
(Wagner 1999)
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ICRR 17/9/2001
LIC vs Lk
=Lk/LB
Lj=Lk+LB=1046 erg s-1.
LIC=(4/2)∫ Ajdr dEe/dt ne
LIC is the received power from IC:
0=20, 50,100Ld=1046erg s-1
Lj=1046erg s-1
Z0=103Rg
<>=5.
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ICRR 17/9/2001
• Equapartition but a small Lj<1046erg s-1
• Or Lj =1046 erg s-1 but LB>Lk
Poynting flux dominated jets?
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ICRR 17/9/2001
Equipartition
(e.g. Ghisellini 1999)
Lj=Lk+LB
Lsyn neB’2
LjLsyn/(B’)2
LB (B’)2
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ICRR 17/9/2001
Multifrequency observations
(Wagner 1999)
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ICRR 17/9/2001
Radio emission
- Photosphere: the radius self-ab<1
- Doppler boosted Tb decreases
decreases- Frequency dependence of Tb
- Tb changes with t ?
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ICRR 17/9/2001
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=1.51013 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.