Theory of TeV AGNs
Amir Levinson, Tel Aviv University
(Buckley, Science, 1998)
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
Conditions in the source: central engine, etc
Emission sites:
 BH magnetosphere
 inner jet
 intermediate scales (eg., HST-1 in M87; other
TeV radio galaxies)
Emission mechanism:
 Electromagnetic: synchrotron, IC, pair production
 Hadronic: photopion production, nuclear collisions
Opacity:
 γγ absorption; photo-π (target photons:
synchrotron and /or external(
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
• γ- ray blazars are highly variable
An extreme example:
Shortest durations: a few minuets (PKS 2155-304; Mrk 501).
But duty cycle seems low!
Central Engine
MBH =108 M8 solar

rg
d  rg
Timescale: t var  rg / c in the rest frame of the BH if a major
fraction of shell energy dissipates.
Power:
LBZ  10 45 B42 M 82 erg/s
B field strength: B  105 ( B m
 / M 8 )1/ 2 G
 - accretion rate in Eddington units
m
Application to PKS 2155-304
LTeV  10 46 erg/s
t var  300 sec
M 8  0.5
LBZ  L j  ( / 2) LTeV

2
B  2 104 ( / 0.1)( / 0.1)1/ 2 G

• Near Eddington accretion
• Low radiative efficiency (ADAF type?)
Estimates of black hole mass from MBH - Lbulge relation:
Mrk 421 – M 8  2
Mrk 501 – M 8  4
PKS2155-394 - M 8  20
scatter ?? Interesting check for a sample
Alternatives:
compact emission region within the jet ?
Collision with external disturbance ?
Jet in a jet ?
Low duty cycle expected !
Other ?
Collision with external disturbance
Variability time may imprint size scale of some external disturbance,
e.g., collision with a cloud.
a
but!! at most a fraction (a / rg )2 of jet power can be
tapped for g-ray production, so: LTeV  (a / rg )2 (2 /  2 ) LBZ
2 2
L

a
B
recall : TeVLBZ  rg B
2
2
Conditions depend on variability time, not on MBH (Levinson 09)
where is the rest of the energy ?
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  (g)-1
g

PKS 2155: binary system?
(Dermer/Finke `08)
109 Msolar
TeV jet
g-ray emission: kinematics & location
• BH magnetosphere ?
• Inner jet ?
• Intermediate scales ? (e.g., boundary shear layers)
• Supercriticality? (photon breeding;
converter; etc.)
Schematic structure
BH
magnetosphere
recollimation shocks;
boundary layers
reflection points
Internal shocks
in inner jet
TeV from black hole magnetosphere ?
• Proposed originally by Boldt/Gosh ‘99 to explain UHECRs
from dormant AGNs.
V
h
• Particle acceleration in a vacuum gap of a Kerr BH.
Potential drop along B field lines:
V  4.4 1020 B4 M 9 a / M (h / rg )2 volt
• Implies efficient curvature emission at TeV energies (Levinson `00)
g,peak  1.5g3 c/ 5 M91/2(B4/Z)3/4 TeV
• Detectable by current TeV telescopes if normalized to UHECRs
flux (Levinson ‘00)
• Application to TeV blazars and M87 (Levinson ’00;
Neronov/Aharonian ’07; 08). Implications for jet formation?
Screening
Vacuum breakdown will quench emission.
• Back reaction (curvature emission + single pair production)
g
expected if B > 105 M9-2/7 G
e
• Compton scattering of ambient radiation:
screens gap if Ls > 1038 M9 (R/Rs) erg/s
- application to M87: requires R>50Rs
Gap potential is restored intermittently ?
R
Inner jet ?
Dissipation at: r  Γ2rg ~ 1016-17 cm
• 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 ?
r0
rg (1GeV)
rg ( g )
rvar
Implications for variability in opaque sources
r0
107
1014
rg (1GeV)
109
1017
rg (1TeV)
1011
1019
MQ r(cm)
Powerful blazar
 if dissipation occurs over a wide range of radii then flares should
propagate from low to high g-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
Supercritical processes
Photon breading: Stern + Putanen
Hadron converter: Derishev
Exponentiation of seed photons (or hadrons). Efficient
converter of bulk energy to radiation. Energy gain in
each cycle  2
from Stern & Putanen
Naively expected but seem not to be supported by data.
Implications for jet structure and/or environmental conditions?
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.)
Internal shocks at
reflection point
p  z 3
p  z 3
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 for
the 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 g-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
(Levinson 2007)
Γ0 >>1  Γ ~ 4
VLBI jet
 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?
We solved fluid equations:



T

S
c
x
Radiative friction
Energy distribution of emitting electrons:
l
  0
;
l  r0
dne
 g q ; g  g max
dg
l
 g

r0  gg  e,max
- If q sufficiently small ( 2 is best) and gg(Γ0 gmax ) ~ 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.