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Extragalactic Source Populations of VHE Gamma Rays

Lecture 5. Extragalactic Source Populations of VHE Gamma Rays. Felix Aharonian Dublin Institute for Advanced Studies, Dublin Max-Planck-Institut f. Kernphysik, Heidelberg. Nagoya Winter School, 2009. Blazars and EBL.

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Extragalactic Source Populations of VHE Gamma Rays

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  1. Lecture 5 Extragalactic Source Populations of VHE Gamma Rays Felix Aharonian Dublin Institute for Advanced Studies, Dublin Max-Planck-Institut f. Kernphysik, Heidelberg Nagoya Winter School, 2009

  2. Blazars and EBL

  3. Blazars -sub-class of AGN dominated by nonthermal/variable broad band (from R to g) adiation produced in relativistic jets close to the line of sight, with massive Black Holes as central engines Urry&Padovani 1995 Sikora 1994 g-rays from >100 Mpc sources - detectable because of the Doppler boosting

  4. TeV emission from Blazars Large Doppler factors:make more comfortable the interpretation of variability timescales (larger source size, and longer acceleration and radiation times), reduces (by orders of magnitude) the energy requirements, allow escape of GeV and TeV g-rays (tgg ~ dj6) Uniqueness:Only TeV radiation tells us unambigiously that particles are accelerated to high energies(one needs at least a TeV electron to produce a TeV photon)in the jets with Doppler factors > 10otherwise gamma-rays cannot escape the source due to severe internal photon-photon pair production Combined with X-rays: derivation of several basic parameters like B-field, total energy budget in accelerated particles, thus to develope a quanititative theory of MHD, particle acceleration and radiation in rela- tivistic jets, although yet with many conditions, assumptions, caveats...

  5. important results before 2004 • detection of 6 TeV Blazars, extraordinary outbursts of Mkn 501 in 1999, Mkn 421 in 2001, and 1ES 1959+650 in 2002 with overall average flux at > 1 Crab level; variations on <1h timescales; good spectrometry; first simultaneous X/TeV observations => initiated huge interest - especially in AGN and EBL communities today • detection of >20 TeV blazars, most importantly -rays from distant blazars; remarkable flares of PKS2155-305 - detection of variability on min timescales => strong impact on both blazar physics and on the Diffuse Extragalactic Background (EBL) models

  6. Hadronic vs. Electronic models of TeV Blazars SSC or external Compton – currently most favoured models: • easy to accelerate electrons to TeV energies • easy to produce synchrotron and IC gamma-rays recent results require more sophisticated leptonic models Hadronic Models: • protons interacting with ambient plasmaneutrinos very slow process: • protons interacting with photon fieldsneutrinos low efficiency + severe absorption of TeV g-rays • proton synchrotronno neutrinos very large magnetic field B=100 G + accelaration rate c/rg “extreme accelerator“ (of EHE CRs) Poynting flux dominated flow variability can be explained by nonradiative losses in expense of increase of total energetics, but as long as Doppler factors can be very large (up to 100), this is not a dramatic issue :

  7. leptonic and hadronic PKS 2155-304 2003-2005 HESS observations: PKS 2155-304 PKS 2155-304 G = 3.32 +/- 0.06 +/- 0.1 a standard phrase in Whipple, HESS, MAGIC papers “SED can be explained within both electronic and hadronic models“ ...

  8. Synchrotron radiation of an extreme proton accelerator cooling and acceleration times of protons FA 2004 synchrotron radiation of protons: a viable radiation mechanism Emax =300 -1j GeV requires extreme accelerators:  ~ 1 Ecut=90 (B/100G)(Ep/1019 eV)2 GeV tsynch=4.5x104(B/100G) -2 (E/1019 eV)-1 s (relatively) comfortable numbers tacc=1.1x104 (E/1019) (B/100G) -1 s

  9. several min (200s) variabiliry timescale => R=c tvarj=101410 cm for a 109Mo BH with 3Rg = 1015 cm => j > 100, i.e. close to the accretion disk (the base of the jet), the bulk motion  > 100 risetime: 173 ± 28 s HESS 28th July 2006 Crab Flux rise time <200s

  10. gamma-rays of IC origin? synchrotron peak of PKS2155-409 is located at <100 eV; comparison with hcut=100 -1j MeV =>  > 106j - quite a large number, i.e. very low efficieny of acceleration ... acceleration rate of TeV electrons (needed to produce the IC peak in the SED at energies 10GeV or so (for large Doppler factors, 10-100): tacc= RL/c = 105j (B/1G)-1 sec Since B < 1 G one cannot explain the TeV variability (rise time) in the frame of the jet tvar=200 j sec conclusion: hadronic origin of TeV gamma-rays?

  11. integalactic absorption of gamma-rays

  12. new blazars detected at large z:HESS/MAGIC at z> 0.15 ! HESS 1 ES 1101 G = 2.9±0.2 ! condition: corrected for IG absorption g-ray spectrum not harder than E-G (G=1.5) upper limit on EBL H 2356 (x 0.1) G = 3.1±0.2

  13. favored EBL – before HESS HESS upper limits HESS – upper limits on EBL at O/NIR: EBL (almost) resolved at NIR ? “direct measurements” upper limits G=1.5 lower limits from galaxy counts

  14. two options: claim thatEBLis“detected“between O/NIR and MIR or proposeextremehypotheses, e.g. violation of Lorentz invariance, non-cosmological origin of z ... or proposeless dramatic (more reasonable) ideas, e.g. • very specific spectrum of electrons  nFnv Eg1.33 • TeV emission from blazars due to comptonization of cold relativistic winds with bulk Lorentz factor G> 106 • internal gamma-ray absorption

  15. internal gamma-gamma absorption can make the intrinsic spectrum arbitrary hard without any real problem from the point of view of energetics, given that it can be compensated by large Doppler factor,j > 30

  16. TeV gamma-rays and neutrinos (?) and secondary X-rays 2-3 orders of magnitude suppression of TeV gamma-rays ! if gamma-rays are of hadronic origin => neutrino flux >10 Crab should be detected by cubic-kilometer scale neutrino detectors

  17. Gamma Rays from a cold ultrarelativistic wind ? in fact not a very exotic scenario ...

  18. M 87 – evidence for production of TeV gamma-rays close to BH • Distance: ~16 Mpc • central BH: 3109 MO • Jet angle: ~30° not a blazar! • discovery (>4s) of TeV g-rays by HEGRA (1998) confirmed by HESS (2003)

  19. X-ray (Chandra) nucleus knot A HST-1 one needs a factor of few better sensitivity at TeV energies to probe fluctuations of the TeV signal on <1 day timescales M87: light curve and variabiliy X-ray emission: • knot HST-1[Harris et al. (2005),ApJ, 640, 211] • nucleus(D.Harris private communication) I>730 GeV [cm-2 s-1] short-term variability within 2005 (>4s)  emission region R ~ 5x1015dj cm => production of gamma-rays very close to the ‘event horizon’ of BH?

  20. Pair Halos when a gamma-ray is absorbed its energy is not lost ! absorption in EBL leads to E-M cascades suppoorted by • Inverse Compton scattering on 2.7 K CMBR photons • photon-photon pair production on EBL photons if the intergalactic field is sufficiently strong, B > 10-11 G, the cascade e+e- pairs are promptly isotropised formation of extended structures –Pair Halos

  21. energy of primary gamma-ray mean free path of parent photons information about EBL flux at gamma-radiation of pair halos can be recognized by its distinct variation in spectrum and intensity with angle , and depends rather weakly (!) on the features of the central VHE source two observables – angular and energy distributions allow to disentangle two variables how it works ?

  22. Pair Halos as Cosmological Candles • informationabout EBL density at fixed cosmological epochs given by the redshift of the central sourceunique ! • estimate of the total energy release of AGN during the active phase relic sources • objects with jets at large angles - many moreg-ray emitting AGN but the “large Lorents factor advantage“ of blazars disapeares: beam isotropic source therefore very powerful central objects needed QSOs and Radiogalaxies (sources of EHE CRS ?) as better candidates for Pair Halos this requires low-energy threshold detectors

  23. EBL at different z and corresponding mean freepaths 1. z=0.034 2. z=0.129 3. z=1 4. z=2 1. z=0.034 2. z=0.129 3. z=1 4. z=2

  24. SEDs for different z within 0.1o and 1o EBL model – Primack et al. 2000 Lo=1045 erg/s

  25. Brightness distributions of Pair Halos z=0.129 z=0.129 E=10 GeV A. Eungwanichayapant, PhD thesis, Heidelberg, 2003

  26. Extragalactic sources of UHECR synchrotron -ray “halo” around a UHECR accelerator in strongly magneized region of IGM (e.g. an AGN within a galaxy cluster) gamma-radiation of secondary electrons ! Kelner and FA, 2008 synchrotron radiation of secondary electrons from Bethe-Heitler and photomeson production at interaction of CRs with 2.7K MBR in a medium with B=1 G (e.g. Galaxy Clusters) E* = 3x1020 eV

  27. photon spectra for a source at a distance of 1 Gpc in a 20 Mpc region of the IGM: power of UHECR source is 1046 erg/s (proton spectral index  = 2) top: Ecut = 1021 eV, (1) B=0.5 nG, (2) 5 nG , (3) 50 nG bottom: Ecut = 5 x 1020 , 1021, 5 x 1021 eV and B=1nG dotted lines - intrinsic spectra, solid lines - absorption in EBL secondary synchrotron gamma-rays produced wthin > 10 Mpc region of IGM around a UHECR accelerator non-variable but point-like gamma-rays source ! Gabici and FA, 2005

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