ICRR 17/9/2001

Gamma-ray emission from AGN

Qinghuan Luo

School of Physics, University of Sydney

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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Mk421, Mk501

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3C273, 3C279

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Rapid variations

Mk501

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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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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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TeV -rays from Mk421, Mk501

(Krennrich et al 1999)

: Mk 501: Mk 421

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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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TeV flares

Mrk 501

-Intraday variability (possibly ~ hrs) requires relativistic beaming!

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Radio IDV

(Kedziora-Chudczer et al. 1997)

PKS 0405-385

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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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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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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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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: Sobs=S0Dp

(e.g. Kollgaard et al 1996)

RBLs

E=log(Pc/Pex)

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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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ERC

Photon energy:

s~222 (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

Incoming photons

Lab frame

e+ e-

Jet frame

Incoming photons

e-

e+

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Compton drag (cont’d)

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The KN effect

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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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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 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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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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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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Bulk Lorentz factor

Terminal Lorentz factor

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Emission from dragged jets

(e.g. Eldar & Levinson 2000)

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SED

(Wagner 1999)

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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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• 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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Equipartition

(e.g. Ghisellini 1999)

Lj=Lk+LB

Lsyn neB’2

LjLsyn/(B’)2

LB (B’)2

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Multifrequency observations

(Wagner 1999)

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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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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.