Theory of TeV AGNs
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Transcript of Theory of TeV AGNs
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Theory of TeV AGNs
(Buckley, Science, 1998)
Amir Levinson, Tel Aviv University
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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 ?
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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
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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
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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 :
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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
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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?)
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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
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Alternatives:
compact emission region within the jet? Collision with external disturbance?
Jet in a jet?
Other?
Low duty cycle expected!
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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
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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
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PKS 2155: binary system? (Dermer/Finke `08)
TeV jet
109 Msolar
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-ray emission: kinematics & location
• BH magnetosphere ?• Inner jet ?• Intermediate scales ? (e.g., boundary shear layers)
• Supercriticality? (photon breeding; converter; etc.)
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BH magnetosphere
Internal shocks in inner jet
recollimation shocks ;boundary layers
reflection points
Schematic structure
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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?
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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
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• 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
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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
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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
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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.)
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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
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Radiative focusing
no cooling
efficient cooling
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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
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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
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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?
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THE END
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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)
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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