theory of tev agns
DESCRIPTION
(Buckley, Science, 1998). Theory of TeV AGNs. 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 ? - PowerPoint PPT PresentationTRANSCRIPT
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 M48 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
hV
• 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>50Rs
R
• 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
r0GeV)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 + PutanenHadron 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 l
ayer
Shoc
ked
laye
r
unsh
ocke
d flo
w
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
ddn
Energy distribution of emitting electrons:
max,000 ;
erl
rll