Implications of VHE Emission in
Gamma-Ray AGN
Amir Levinson, Tel Aviv University
Outline
• Constraints on source parameters from VHE g-ray obs. Rapid TeV
flares  D>>1 ? Inconsistent with SL motion and unification. Stationary
radio features in blazars are common (Jorstad et al. 2001) . Reasons?
•Radiative deceleration of Γ>>1 shells, and rapid TeV variability.
• Size and location of emission region ? HST1 in M87 and TeV blazars.
•Collimation and dissipation in recollimation shocks.
•TeV emission from BH magnetosphere?
• Constraints on production of VHE neutrinos in jets from g-ray
observations . GLAST can be exploited to identify best candidates
for upcoming km^3 n detectors.
Opacity sources (gg and photo-π)
blazar
MQ
 Target photons: synchrotron and /or external
 Electromagnetic: synchrotron, IC, pair production
 Hadronic: photopion production, nuclear collisions
Challenges
LBZ  10 45 B42 M 82 erg/s
LEdd  10 46 M 8 erg/s
rg
c
LTeV  10 46 erg/s
t var  300 sec
 500 M 8 sec
rdiss   2 rg
Estimates of black hole mass from M -  relation:
Mrk 421 - M 8  2
Mrk 501 - M 8  4
M8 
M BH
B
;
B

4
108 M 
10 4 G
Pair production opacity
γ  γ  e  e
g-spheric radius versus energy
external

(Levinson 2006)
synchrotron
10 2
r d gg
1
g
10 3
log (rγ /rg )
10 4
10 5
rd
GLAST
log ( g / 1TeV)
relevant to powerful
blazars (e.g., 3C279)
relevant to
TeV blazrs
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 GLAST in powerful blazars and MQs
Constraints on Doppler factor from variability
t var - variabili ty time of g - ray emission at  g (measured)
sn - synchrotro n flux density (measured or computed)
z - redshift (measured)
D - Doppler factor (unknown)
Dct var
Size of emission region in Lab frame : r  rvar 
1 z
r0
rg (1GeV)
rg ( g )
rvar
Assuming that rγsyn ( g )  rvar or more generally that the
synchrotron emission originates from the same region
emitting TeV rays implies a lower limit on Doppler
factor of emitting fluid:
D  Dmin (z,sν ,εγ ,t var )
In TeV blazars : D ~ 30 150
Consistent with estimates based on fits of SED to SSC model
Other requirements
Cooling time:
tcool  t var
Synchrotron cooling
gB 2  105.5 (t var / 300 s)
SSC in Poynting dominated blob (Begelman et al. 2008):
  50...
  40...
opacity
cooling
PKS 2155
No momentum loss → Why radio jet much slower?
IC cooling: gu s  10 4 (t var / 300 s) (cgs)
9
-2
8
-3
ERC: uext  10 (tvar / 300 s) L j 46 erg/cm
Jet decelerates?
Photon breading ? Next talk
• Large Γ really necessary ?
• If they are then why pattern  are much smaller than
fluid  ?
• What is the origin of rapid TeV flares ?
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.
Emission region located at a large radius?
 gg opacity may decrease with radius. Large Doppler
factor may not be required if rem >> r.
• but!! only a fraction (r/R)2 of jet energy can be tapped

are still required if synchrotron emission
forLarge
g-rayΓproduction.
originates
from theto
same
region
emitting
the TeV
photons
• May be difficult
explain
large
amplitude
flares!
(can be constrained by variability of IR emission)
Is there a way to channel the bulk energy into a small region?
g
R

rem
r
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
Can HST1 knot be the site of TeV emission?
L TeV (observed) ~ L X (observed)  10 40.5 erg/s
(compared with 10 44.5 erg/s in Mrk 421, 501, PKS 2155)
L j ~ 10 44 erg/s ? (based on radio lobes;
Bicknell  Begelman 96 )
if true then it implies a small radiative efficiency
Requires jet opening angle  0.01 rad.
Reconfinement can help (Stawarz et al. 06 ). Can also explain th e
presence of stationary radio features in other blazars (Komissaro v  Falle 97)
See also Sikora et al. 2008 for 3C454.3
May be an important dissipation channel also in blazars, MQs, and GRBs
Collimation of a jet by pressure and inertia of an ambient medium
Bromberg + Levinson 2007 (see also simulations by Alloy et al.)
Internal shocks at
reflection point
pz
3
p  z 3
Effect of cooling (preliminary results)
Internal shocks at
reflection point
Cold jet
p  z 2
p  z 2
Confinement by a supersonic wind
TeV from black hole ?
• Particle acceleration in vacuum gap of a Kerr BH.
• Proposed originally by Boldt/Gosh ‘99 to explain UHECRs from
dormant AGNs
• Implies efficient curvature emission at TeV energies; detectable by
current TeV telescopes (Levinson ‘00)
• Application to M87 (Neronov/Aharonian ‘07)
Vacuum gap
Potential drop along B field lines
2
 a  h 
 B  M
   volt
V  4.4  10  4  9
 10 G  10 M solar  M  Rs 
20
V
h
But! Vacuum breakdown will quench emission.
Expected if luminosity of ambient emission
exceeds roughly 1040 erg/s
Gap potential is restored intermittently ?
Hadronic processes
Inelastic nuclear collisions
Photopion production
Inelastic nuclear collisions
p + n  p + p + -  - + n  e- + ne + n + n
p + n  n + n + +  + + n  e+ + ne + n + n
 pp ~ 50 mbn
p + n  p + n +0  g + g
 pp 
 pp nb r

 p j
 0.1 2 3
 
 pp  1 in AGNs, Microquasars (except perhaps for HMXBs where
stellar wind may contribute)
May be important in GRBs
Photo- production
p + g  + + n  + + n  e+ + ne + n + n
 ~ 0.1 mbn
p + g  0 + p  g + g
π - sphere
mBH  108 ;   10
log rπ /rg
mBH  3;   3
ext
sync
ε p,max
dissipation
radius
ε p,max
Relations between photo-  production and gγ pair-production
 Same target photons for both processes.

 pg ( p ) ns ( p ,th )  pg
p
 2.5 

 10
 1.2 106 
 gg ( g ) ns ( g ,th )  gg
g






for ns ( )  ε  α
Opacity ratio (target: synchrotron photons)
(blazar)
r/rg  105
r/rg  102
Mrk 421
εp,max
ξ B j /m B  101
(MQ)
Consequences
• Regions of significant photo- opacity are opaque to emission
of VHE gamma rays.
• Highly variable VHE g-ray sources, in particular TeV blazars
are not good candidates for km3 neutrino detectors.
• In regions of high photo- opacity, g- rays produced through
π0 decay will be quickly degraded to GeV energies (in blazars
and lower in MQs). Correlation between GeV and neutrino
emissions is expected. (Also temporal changes in the g-ray spectrum in
the GLAST band during intense neutrino emission.)
Neutrino yields (in a km3 detector)
 TeV BLLac: < 0.03 event per year for Mrk 421, 501 during
intense flares
 Blazars: ~ 1 event per year at Z~1 for the most powerful
sources (e.g, 3c279). May be constrained further by GLAST.
 MQ: a few events from a powerful flare like the 1994 event
seen in GRS 1915
 GRBs: a few events from a nearby source or cumulative
detection
Summary
• Large Doppler factors are implied by g-ray obs. in blazars.
Seem to differ considerably from pattern speed.
• Location of emission region may far from the BH, as HST1 in
M87. Suggests that collimation may be an important dissipation
channel. Also in GRBs? Can this account for rapid variability at
relatively large radii?
• Extreme flares in TeV blazars may be produced by radiative
deceleration of fast fluid shells on small scales? Consistent with
the small superluminal speeds on VLBA scales and minimum jet power
estimates.
• Forthcoming obs. of VHE emission can be used to identify
potential neutrino sources
THE END