Theory of TeV AGNs

(Buckley, Science, 1998)

Amir Levinson, Tel Aviv University

Open questions

• What rapid variability tells us about the central engine?

• Implications for kinematics of the source ?

• Where is the location of the VHE emission zone ?

• Emission mechanisms ?

• Jet composition ?

Basic picture

Opacity: γγ absorption; photo-π (target photons: synchrotron and /or external(

Emission mechanism:

Electromagnetic: synchrotron, IC, pair production

Hadronic: photopion production, nuclear collisions

Emission sites:

BH magnetosphere

inner jet

intermediate scales (eg., HST-1 in M87; other

TeV radio galaxies)

Conditions in the source: central engine, etc

General remarks

Blazar emission is presumably multi-component. The new

class (TeV galaxies) seem to indicate emission from less

beamed regions (BH magnetosphere? Boundary shear layers?)

one thus needs to be cautious in modeling spectra, etc. !

Combination of very rapid variability + VHE emission can

provide some general constraints on basic physics!

In general the structure may be quite involved, as seem to be

indicated by e.g., extreme flares

Variability

Shortest durations: a few minuets (PKS 2155-304; Mrk 501).But duty cycle seems low!

• γ- ray blazars are highly variable

An extreme example :

Central Engine

grd

rg

crt g /var in the rest frame of the BH if a major

fraction of shell energy dissipates.

Timescale :

erg/s 10 28

24

45 MBLBZ Power:

G )/(10 2/18

5 MmB B accretion rate in Eddington units - m

B field strength:

MBH =108 M8 solar

Application to PKS 2155-304

sec 300var t erg/s 1046TeVL

5.08 M

)2/( 2TeVjBZ LLL

G )1.0/)(1.0/(102 2/14 B

• Near Eddington accretion• Low radiative efficiency (ADAF type?)

Estimates of black hole mass from MBH - Lbulge relation:

Mrk 421 –

Mrk 501 –

PKS2155-394 -

scatter ?? Interesting check for a sample

28 M

48 M

208 M

Alternatives:

compact emission region within the jet? Collision with external disturbance?

Jet in a jet?

Other?

Low duty cycle expected!

Variability time may imprint size scale of some external disturbance, e.g., collision with a cloud.

a

but!! at most a fraction of jet power can be

tapped for -ray production, so:

2)/( gra

BZgTeV LraL )/2()/( 22 22BaLTeV

Conditions depend on variability time, not on MBH (Levinson 09)

where is the rest of the energy ?

22 :recall BrL gBZ

Collision with external disturbance

Jet in a jet ? (e.g., Gainos et al. 09)

Dissipation results in internal relativistic motion with respect to rest frame of the shell .

Reconnection?? Relativistic turbulence??

Beaming: f ()-1

PKS 2155: binary system? (Dermer/Finke `08)

TeV jet

109 Msolar

-ray emission: kinematics & location

• BH magnetosphere ?• Inner jet ?• Intermediate scales ? (e.g., boundary shear layers)

• Supercriticality? (photon breeding; converter; etc.)

BH magnetosphere

Internal shocks in inner jet

recollimation shocks ;boundary layers

reflection points

Schematic structure

h

V

• Implies efficient curvature emission at TeV energies (Levinson `00)

,peak 1.53 c/ 5 M91/2(B4/Z)3/4 TeV

• Detectable by current TeV telescopes if normalized to UHECRs

flux (Levinson ‘00)

volt)/(/104.4 294

20grhMaMBV

Potential drop along B field lines:

• Particle acceleration in a vacuum gap of a Kerr BH.

• Proposed originally by Boldt/Gosh ‘99 to explain

UHECRs from dormant AGNs.

TeV from black hole magnetosphere?

• Application to TeV blazars and M87 (Levinson ’00;

Neronov/Aharonian ’07; 08). Implications for jet formation?

Screening Vacuum breakdown will quench emission.

Gap potential is restored intermittently ?

• Compton scattering of ambient radiation:

screens gap if Ls > 1038 M9 (R/Rs) erg/s

- application to M87: requires R>50RsR

• Back reaction (curvature emission + single pair production)

expected if B > 105 M9-2/7 G

e

• opacity: γ-spheric radius increases with increasing energy.

• avoiding γγ absorption requires Γ ~ 30 -100 in TeV blazars!

• why pattern , determined from radio obs., are much smaller

than fluid inferred from TeV emission ?

• what is the origin of rapid TeV flares ?

Inner jet? Dissipation at: r Γ2rg ~ 1016-17 cm

r0

GeV)1(r )( r varr

if dissipation occurs over a wide range of radii then flares should propagate from low to high -ray energies (Blandford/Levinson 95).

250 sec delay between γ at >1.2 TeV and γ at 0.15-0.25 TeV was reported for Mrk 501 (Albert etal. 07). Corresponds to r=2ctdelay 1016

(/30) 2 cm

Will be constrained by Fermi in powerful blazars and MQs

r(cm)

r0

107 109 1011

1014 1017 1019

MQPowerful blazar

GeV)1(r TeV)1(r

Implications for variability in opaque sources

Supercritical processesPhoton breading: Stern + Putanen

Hadron converter: Derishev

Naively expected but seem not to be supported by data. Implications for jet structure and/or environmental conditions?

Exponentiation of seed photons (or hadrons). Efficient converter of bulk energy to radiation. Energy gain in each cycle 2

from Stern & Putanen

Intermediate scales: boundary layers and recollimation shocks

• Interaction with the surrounding medium helps collimation

and produces oblique shocks, shear layers, and

recollimation nozzles.

• A substantial fraction of the bulk energy dissipates in these

regions and can lead to a less beamed (though sometimes

highly variable as in HST-1 knot) emission.

Relevant for radio Galaxies and blazars! (e.g., Marscher, Sikora et al.)

Collimation of a jet by pressure and inertia of an ambient medium Bromberg + Levinson 07,09 (see also simulations by Alloy et al.)

Shoc

ked

laye

r

Shoc

ked

laye

r

unsh

ocke

d fl

ow

Internal shocks at reflection point

Confin

ing m

edium

3 zpConfin

ing m

edium

3 zp

Radiative focusing

no cooling

efficient cooling

M87- HST1 Source of violent activity. Deprojected distance of ~ 120 pc (=30 deg) Resolved in X-rays. Variability implies r ~ 0.02 D pc. Radio: stationary with substructure moving at SL speed M87 has been detected at TeV, with r ~ 0.002 D pc. Related to HST1 ?

From Cheung et al. 2006

M87

• jet power required to get reflection shocks at the location of HST-1 is consistent with other estimates, for the external pressure profile inferred from observations. • The model can account for the rapid X-ray variability but not forthe variable TeV emission

Summary • Rapid TeV flares imply either small mass BH or, alternatively, a compact emission region within the jet (e.g., collision with a small cloud). In any case, near Eddington accretion is required to account for flare luminosity. Look for disk emission during TeV flares.

• Large Doppler factors seem to be implied for TeV blazars by -ray observations. Differ considerably from pattern speed in TeV blazars.

• VHE emission appears to be multi-component. Radio Galaxies reveal less beamed emission zones. Need further studies to better locate those regions.

• Collimation may be an important dissipation channel, e.g., HST-1 in M87; BL Lac (Marscher); 3c 345 (Sikora etal). Also in GRBs? Can this account for rapid variability at relatively large radii?

THE END

Radiative deceleration and Rapid TeV flares

Fluid shells accelerated to Γ0 where dissipation occurs. Radiative drag then leads to deceleration over a short length scale (Georgapoulos/Kazanas 03).

Dissipated energy is converted to TeV photons – no missing energy.

Minimum power of VLBI jet in Mrk 421, Mrk 501 is ~ 1041 erg/s, consistent with this model.

What are the conditions required for effective deceleration and sufficiently small pp opacity that will allow TeV photons to escape?

Γ0 >>1 Γ ~ 4 VLBI jet

(Levinson 2007)

cSTx

Radiative friction

We solved fluid equations:

- If q sufficiently small ( 2 is best) and (Γ0 max ) ~ a few, then..

a background luminosity of about 1041 erg/s is sufficient to

decelerate a fluid shell from 0>>1 to ~ a few, but still be

transparent enough to allow TeV photons to escape the system.

max ;

qe

d

dnEnergy distribution of emitting electrons:

max,000 ;

er

l

rl

l