1 / 28

Theory of TeV AGNs

(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 ?

ike
Download Presentation

Theory of TeV AGNs

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. (Buckley, Science, 1998) Theory of TeV AGNs Amir Levinson, Tel Aviv University

  2. 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 ?

  3. 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(

  4. 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

  5. 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!

  6. in the rest frame of the BH if a major fraction of shell energy dissipates. Timescale: Power: accretion rate in Eddington units B field strength: Central Engine MBH =108 M8 solar rg

  7. Application to PKS 2155-304  • Near Eddington accretion • Low radiative efficiency (ADAF type?)

  8. Estimates of black hole mass from MBH - Lbulge relation: Mrk 421 – Mrk 501 – PKS2155-394 - scatter ??Interesting check for a sample

  9. Alternatives: compact emission region within the jet ? Collision with external disturbance ? Jet in a jet ? Low duty cycle expected ! Other ?

  10. but!! at most a fraction of jet power can be tapped for g-ray production, so: Conditions depend on variability time, not on MBH(Levinson 09) where is the rest of the energy ? Collision with external disturbance Variability time may imprint size scale of some external disturbance, e.g., collision with a cloud.

  11.  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

  12. PKS 2155: binary system? (Dermer/Finke `08) 109 Msolar TeV jet

  13. -ray emission: kinematics & location • BH magnetosphere ? • Inner jet ? • Intermediate scales ? (e.g., boundary shear layers) • Supercriticality? (photon breeding; converter; etc.)

  14. BH magnetosphere recollimation shocks; boundary layers reflection points Internal shocks in inner jet Schematic structure

  15. Particle acceleration in a vacuum gap of a Kerr BH. Potential drop along B field lines: TeV from black hole magnetosphere ? • Proposed originally by Boldt/Gosh ‘99 to explain UHECRs from dormant AGNs. • Implies efficient curvature emission at TeV energies (Levinson `00) • ,peak  1.53 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?

  16. Back reaction (curvature emission + single pair production) expected if B > 105 M9-2/7 G  • Compton scattering of ambient radiation: • screens gap if Ls > 1038 M9 (R/Rs) erg/s • - application to M87: requires R>50Rs e R Screening Vacuum breakdown will quench emission. Gap potential is restored intermittently ?

  17. r0 Inner jet ? Dissipation at: r Γ2rg ~ 1016-17 cm • opacity: γ-spheric radius increases with increasing energy. • avoiding γγ absorption requiresΓ ~ 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 ?

  18. r0 MQ r(cm) 1011 107 109 Powerful blazar 1019 1014 1017 Implications for variability in opaque sources • 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

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

  20. 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.)

  21. 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 Shocked layer Shocked layer unshocked flow Confining medium Confining medium

  22. Radiative focusing no cooling efficient cooling

  23. From Cheung et al. 2006 M87- HST1 • Source of violent activity. Deprojected distance of ~ 120 pc (q=30 deg) • Resolved in X-rays. Variability implies Dr ~ 0.02GD pc. • Radio: stationary with substructure moving at SL speed • M87 has been detected at TeV, with Dr ~ 0.002GD pc. Related to HST1 ?

  24. 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

  25. 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?

  26. THE END

  27. VLBI jet Γ0 >>1 Γ ~ 4 Radiative deceleration and Rapid TeV flares (Levinson 2007) • 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?

  28. We solved fluid equations: Radiative friction Energy distribution of emitting electrons: • 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.

More Related