Theoretical aspects of VHE  -ray astronomy: Exploring Nature’s Extreme Accelerators

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APP UK 2008 meeting, Oxford, June 20, 2008. Theoretical aspects of VHE -ray astronomy: Exploring Natures Extreme Accelerators. Felix Aharonian Dublin Institute for Advanced Studies, Dublin Max-Planck-Institut f. Kernphysik, Heidelberg. Astroparticle Physics. - PowerPoint PPT Presentation

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Theoretical aspects of VHE -ray astronomy: Exploring Natures Extreme AcceleratorsFelix AharonianDublin Institute for Advanced Studies, DublinMax-Planck-Institut f. Kernphysik, HeidelbergAPP UK 2008 meeting, Oxford, June 20, 2008Astroparticle Physics a modern interdisciplinary research field at the interface of astronomy, physics and cosmology one of the major objectives: study of nonthermal phenomena in their most energetic and extreme forms in the Universe (the High Energy Astrophysics branch of Astroparticle Physics) all topics of this research area are related, in one way or another, to exploration of Natures perfectly designed machines Extreme Particle Accelerators Extreme Accelerators [TeVatrons, PeVatrons, EeVatrons] machines where acceleration proceeds with efficiency close to 100% efficiency ? (i) fraction of available energy converted to nonthermal particles in PWNe and perhaps also in SNRs, can be as large as 50 % (ii) maximum (theoretically) possible energy achieved by individual particles acceleration rate close to the maximum (theoretically) possible rate * sometimes efficiency can exceed 100% (!) e.g. at CR acceleration in SNRs in Bohm diffusion regime with amplification of B-field by CRs (Emax= ~ B (v/c)2 ) this effect provides the extension of the spectrum of Galactic CRs to at least 1 PeV > 100% efficiency because of nonlinear effects: acceleration of particles creates better conditions for their further acceleration Extragalactic?T. GaisserSNRs ?up to 1015-16 (knee) - Galactic SNRs: Emax ~ vshock Z x B x Rshock for a standard SNR: Ep,max ~ 100 TeV solution? amplification of B-field by CRs 1016 eV to 1018 eV: a few special sources? Reacceleration? above 1018 eV (ankle) - Extragalactic 1020 eV particles? : two options top-down (non- acceleration) origin or Extreme AcceleratorsCosmic Rays from 109 to 1020 eVParticles in CRs with energy 1020 eV difficult to understand unless we assume extreme accelerators the Hillas condition - l > RL - an obvious but not sufficient condition (i) maximum acceleration rate allowed by classical electrodynamics t-1=qBc ( x (v/c)2 in shock acceleration scenarios) with ~ 1 (ii) B-field cannot be arbitrarily increased - the synchrotron and curvature radiation losses become a serious limiting factor, unless we assume perfect linear accelerators only a few options survive from the original Hillas (l-B) plot: >109 Mo BH magnetospheres, small and large-scale AGN jets, GRBs acceleration sites of 1020 eV CRs ?FA, Belyanin et al. 2002, Phys Rev D, 66, id. 023005confinementconfinementenergy lossesenergy losses signatures of extreme accelerators? synchrotron self-regulated cutoff: neutrinos (through converter mechanism)production of neutrons (through p interactions) which travel without losses and at large distan- ces convert again to protons => 2 energy gain ! Derishev, FA et al. 2003, Phys Rev D 68 043003 observable off-axis radiation radiation pattern can be much broader than 1/ Derishev, FA et al. 2007, ApJ, 655, 980 FA 2000, New astronomy, 5, 377 VHE gamma-ray and neutrino astronomies: two key research areas of High Energy Astrophysics/Astroparticle PhysicsVHE- and - astronomies address diversity of topics related to the nonthermal Universe: acceleration, propagation and radiation of ultrarelativistic protons/nuclei and electrons generally under extreme physical conditions in environments characterized with huge gravitational, magnetic and electric elds, highly excited media, shock waves and very often associated with relativistic bulk motions linked, in particular, to jets in black holes (AGN, Microquasars, GRBs) and cold ultrarelativistic pulsar winds over last several years HESS has revolutionized the field before astronomy with several sources and advanced branch of Particle Astrophysics now a new astronomical discipline with all characteristic astronomical key words: energy spectra, images, lightcurves, surveys... VHE gamma-ray astronomy - a success story major factors which make possible this success ? effective acceleration of Tev/PeV particles almost everywhere in Universe the potental of the detection technique (stereoscopic IACT arrays) good performance => high quality data => solid basis for theoretical studies 28th July 2006TeV image and energy spectrum of a SNRenrgy dependent image of a pulsar wind nebulavariability of TeV flux of a blazar on minute timescales huge detection area+effective rejection of different backgrounds good angular (a few arcminutes) and energy (15 %) resolutionsbroad energy interval - from 100 (10) GeV to 100 (1000) TeV nice sensitivity (minimum detectable flux): 10-13 (10-14) erg/cm2 s multi-functional tools: spectrometry temporal studies morphology extended sources: from SNRs to Clusters of Galaxies transient phenomena QSOs, AGN, GRBs, ... Galactic Astronomy | Extragalactic Astronomy | Observational CosmologyRXJ 1713.7-3946PSR 1826-1334PKS 2155-309 VHE gamma-ray observations: Universe is full of extreme accelerators on all astronomical scales Extended Galactic Objects Shell Type SNRs Giant Molecular Clouds Star formation regions Pulsar Wind Nebulae Compact Galactic Sources Binary pulsar PRB 1259-63 LS5039, LSI 61 303 microquasars? Cyg X-1 ! - a BH candidate Galactic Center Extragalactic objects M87 - a radiogalaxy TeV Blazars with redshift from 0.03 to 0.18 and a large number of yet unidentified TeV sources VHE gamma-ray source populationsTeV gamma-ray source populations highlight topics particle acceleration by strong shocks in SNR physics and astrophysics of relativistic outflows (jets and winds) probing processes close to the event horizon of black holescosmological issues - Dark Matter, Extragalactic Background Light (EBL) ..Potential Gamma Ray SourcesMajor Scientific Topics G-CRs Relativistic Outflows Compact Objects Cosmology ISM SNRsSFRsPulsars Binaries Galactic SourcesExtragalactic SourcesGRBsAGN GLXCLUST IGMGMCsMagnetosphereMicroquasars Cold WindPulsar Nebula Binary PulsarsRadiogalaxies Blazars Normal StarburstEXG-CRsEBLGeVGeVGeVGeVGeVGeV unique carriers of astrophysical and cosmological information about non-thermal phenomena in many galactic and extragalactic sources why TeV neutrinos ? like gamma-rays, are effectively produced, but only in hadronic interactions (important - provides unambiguous unformation) unlike gamma-rays do not interact with matter, radiation and B-fields (1) energy spectra and fluxes without internal/external absorption (2) hidden accelerators ! but unlike gamma-rays, cannot be effectively detected even 1km3 volume class detectors have limited performance: minimum detectable flux approximately equivalent to 1 Crab gamma-ray flux TeV neutrinos -- a complementary channel detection rate of neutrinos with KM3NeT R.Whitesensitivity of km3 volume neutrino detectors1 Crab after several years of observations effective energy range around 10 TeV so far only four galactic gamma-ray sources are detected with a TeV gamma-ray flux at the level of 1 Crab 10 Crab for less than 1 month (background free); effective energy range 1-10 TeV blazars? quite possible if TeV gamma-rays are of hadronic origin burst-like events: fluence: t x FE > 10-5 erg/cm2 GRBs, SGRs/Magnetars, SN events, ets. some remarks concerning the neutrino/gamma ratio: typically > 1, but synchrotron radiation of protons - pure electromagnetic process interaction of hadrons without production of neutrinos generally in hadronic neutrinos and gamma-rays are produced with same rates, but in high density environments (n > 1018 cm-3 and/or B>106 G) production of TeV neutrinos is suppressed because charged mesons are cooled before they decay on the other hand, in compact objects muons and charged pions can be accelerated and thus significantly increase the energy and the flux of neutrinos, e.g. from GRBssynchrotron 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) what should we do if hadronic gamma-rays and neutrinos appear at wrong energies ?photomesonelectronsBethe-HeitlerelectronsKelner and FA, 2008, Phys Rev D detect radiation of secondary electrons !E* = 3x1020 eVprobing hadrons with secondary X-rays with sub-arcmin resolution! Simbol-Xnew technology focusing telescopes NuSTAR (USA), Simbol-X (France-Italy), NeXT (Japan) will provide X-ray imaging and spectroscopy in the 0.5-100 keV band with angular resolution 10-20 arcsec and sensitivity as good as 10-14 erg/cm2s! complementary to gamma-ray and neutrino telescopes advantage - (a) better performance, deeper probes (b) compensates lack of neutrinos and gamma-rays at right energiesdisadvantage - ambiguity of origin of X-rays exploring Natures Extreme Particle Accelerators with neutrinos, gamma-rays, and hard X-rays Microquasars ?Pulsars/Plerions ?SNRs ? Galactic Center ?. . .Gaisser 2001OB, W-R Stars ? * the source population responsible for the bulk of GCRs are PeVatrons ?Galactic TeVatrons and PeVatrons - particle acceleratorsresponsible for cosmic rays up to the knee around 1 PeV Visibility of SNRs in high energy gamma-raysFg(>E)=10-11 A (E/1TeV)-1 ph/cm2sA=(Wcr/1050erg)(n/1cm-3 )(d/1kpc) -2 for CR spectrum with =2if electron spectrum >> 10 TeV synchrotron X-rays and IC TeV gs main target photon field 2.7 K: Fg,IC/Fx,sinch=0.1 (B/10mG)-2 Detectability ? compromise between angle q (r/d) and flux Fg (1/d2) typically A: 0.1-0.01 q: 0.1o - 1o 1000 yr old SNRs (in Sedov phase) po component dominates if A > 0.1 (Sx/10 mJ)(B/10 mG ) -2 TeV g-rays detectable if A > 0.1nucleonic component of CRs - visible through TeV (and GeV) gamma-rays !Inverse Comptonp0 decay (A=1)TeV -rays and shell type morphology: acceleration of p or e in the shell toenergies exceeding 100TeV2003-2005 datacan be explained by -rays from pp ->o ->2 but IC canot be immediately excluded RXJ1713.7-4639and with just right energetics Wp=1050 (n/1cm-3)-1 erg/cm3 leptonic versus hadronicIC origin ? very small B-field, B < 10 mG, and very large E, Emax > 100 TeV two assumptions hardly can co-exists within standard DSA models, bad fit of gamma-ray spectrum below a few TeV, nevertheless arguments against hadronic models:nice X-TeV correlaton well, in fact this is more natural for hadronic rather than leptonic models relatively weak radio emission problems are exaggerated lack of thermal X-ray emission => very low density plasma or low Te ? we do not (yet) know the mechanism(s) of electron heating in supernova remnants so comparison with other SNRs is not justified at allSuzaku measurements => electron spectrum 10 to 100 TeVVariability of X-rays on year timescales - witnessing particle acceleration in real time flux increase - particle accelerationflux decrease - synchrotron cooling *)both require B-field of order 1 mG in hot spots and, most likely, 100G outsideUchiyama, FA, Tanaka, Maeda, Takahashi, Nature 2007*) explanation by variation of B-field doest work as demonstrated for Cas A (Uciyama&FA, 2008) strong support of the idea of amplification of B-field by in strong nonlinear shocks through non-resonant streaming instability of charged energetic particles (T. Bell; see also recent detailed theoretical treatment of the problem by Zirakashvili et al. 2007) acceleration in Bohm diffusion regime Strong support for Bohm diffusion - from the synchrotron cutoffgiven the upper limit on the shock speed of order of 4000 km/s ! with h=0.67 +/- 0.02keVenergy spectrum of synchrotron radiation of electrons in the framework of DSA (Zirakashvili&FA 2007) B=100 G + Bohm diffusion - acceleration of particles to 1 PeV(Tanaka et al. 2008)protons:dN/dE=K E-a exp[-(E/Ecut)b]-rays:dN/dE v E-G exp[-(E/E0)bg]=a+da, da 0.1, bg=b/2, E0 = Ecut/20 Wp(>1 TeV) ~ 0.5x1050 (n/1cm-3)-1 (d/1kpc)2RXJ 1713.7-3946neutrinos: marginally detectable by KM3NeTProbing PeV protons with X-rays SNRs shocks can accelerate CRs to 100 mG is possible through plasma waves generated by CRs >1015 eV protons result in >1014 eV gamma-rays and electrons prompt synchrotron X-rays t(e) = 1.5 (e/1keV) -1/2 (B/1mG) -3/2 yr three channels of information about cosmic PeVatrons:10-1000 TeV gamma-rays 10-1000 TeV neutrinos 10 -100 keV hard X-rays g-rays: difficult, but possible with future 10km2 area multi-TeV IACT arrays neutrinos: marginally detectable by IceCube, Km3NeT - dont expect spectrometry, morphology; uniqueness - unambiguous signatute! prompt synchrotron X-rays: smooth spectrum a very promising channel - quality! (NexT, NuSTAR, SIMBOL-X)10-100 TeV m-neutrinosprotonsbroad-bandGeV-TeV-PeV gssynch. hard X-raysbroad-band emision initiated by pp interactiosn : Wp=1050 erg, n=1cm-3no competing X-ray radiation mechanisms above 30 keVSearching for Galactic PeVatronsgamma-rays from surrounding regions add much to our knowledge about highest energy protons which quickly escape the accelerator and therefotr do not signifi-cantly contribute to gamma-ray production inside the proton accelerator-PeVatron the existence of a powerful accelerator is not yet sufficenrt for -radiation; an additional component a dense gas target - is requiredolder source steeper g-ray spectrum tesc=4x105(E/1 TeV) -1 k-1 yr (R=1pc); k=1 Bohm DifussionQp = k E-2.1 exp(-E/1PeV) Lp=1038(1+t/1kyr) -1 erg/sGamma-rays and neutrinos inside and outside of SNRs neutrinosgamma-raysSNR: W51=n1=u9=1ISM: D(E)=3x1028(E/10TeV)1/2 cm2/sGMC: M=104 Mo d=100pcd=1 kpc1 - 400yr, 2 - 2000yr, 3 - 8000yr, 4 - 32,000 yr [S. Gabici, FA 2007]MGRO J1908+06 - a PeVatron?HESSpreliminaryMilagro gamma-ray emitting clouds in GC region diffuse emission along the plane!HESS J1745-303 indirect discovery of the site of particle accelerationmeasurements of the CR diffusion coefficient HESS: FoV=5o GC a unique site that harbors many interesting sources packed with un- usually high density around the most remarkable object 3x106 Mo SBH Sgr A* many of them are potential g-rayemitters - Shell Type SNRsPlerions, Giant Molecular CloudsSgr A * itself, Dark Matter all of them are in the FoV an IACT,and can be simultaneously probed down to a flux level 10-13 erg/cm2s and localized within Pulsar Winds and Pulsar Wind Nebulae (Plerions) Crab Nebula a perfect PeVatron of electrons (and protons ?)Crab Nebula a very powerful W=Lrot=5x1038 erg/s and extreme accelerator: Ee > 1000 TeV Emax=60 (B/1G) -1/2 h-1/2 TeV and hncut=(0.7-2) af-1mc2 h-1 = 50-150 h-1 MeV h=1 minimum value allowed by classical electrodynamics Crab: hncut= 10MeV: acceleration at ~10 % of the maximum rate ( h10) maximum energy of electrons: Eg=100 TeV => Ee > 100 (1000) TeV B=0.1-1 mG very close the value independently derived from the MHD treatment of the wind 1-10MeV100TeV* for comparison, in shell type SNRs DSA theory gives h=10(c/v)2=104-105Standard MHD theorycold ultrarelativistc pulsar wind terminates by a reverse shock resulting in acceleration with an unprecedented rate: tacc=hrL/c, h < 100 *) synchrotron radiation => nonthermal optical/X-ray nebulaInverse Compton => high energy gamma-ray nebula.MAGIC (?)HEGRA TeV gamm-rays from other Plerions (Pulsar Wind Nebulae) Crab Nebula is a very effective accelerator but not an effective IC -ray emitter we see TeV gamma-rays from the Crab Nebula because of large spin-down flux gamma-ray flux HESS J1825 (PSR J1826-1334)Luminosities: spin-down: Lrot= 3 x 1036 erg/sX: 1-10 keV Lx=3 x 1033 erg/s (< 5 arcmin) g: 0.2-40TeV Lg=3 x 1035 erg/s (< 1 degree)the g-ray luminosity is comparable to the TeV luminosity of the Crab Nebula, while the spindown luminosity is two orders of magnitude less ! Implications ? (a) magnetic field should be significantly less than 10mG. but even for Le=Lrot this condition alone is not sufficient to achieve 10 % g-ray production efficiency (Comton cooling time of electrons on 2.7K CMBR exceeds the age of the source) (b) the spin-down luminosity in the past was much higher. red below 0.8 TeVyellow 0.8TeV -2.5 TeVblue above 2.5 TeVPulsars period: 110 ms, age: 21.4 kyr, distance: 3.9 +/- 0.4 kpcenergy-dependent image - electrons!Gamma-ray BinariesMirabel 2006PSR1259-63 - a unique high energy laboratory binary pulsars - a special case with strong effects associated with the optical star on both the dynamics of the pulsar wind and the radiation before and after its terminationthe same 3 components - Pulsar/Pulsar Wind/Synch.Nebula - as in plerionsbut with characteristics radiation and dynamical timescales less than hoursboth the cold ultrarelativistic wind and shocke-accelerated electrons are illuminated by optical radiation from the companion star => detectable IC gamma-ray emission on-line watch of creation/termination of the pulsar wind accompanied with formation of a shock and effective acceleration of electrons time evolution of fluxes and energy spectra of X- and gamm-rays contain unique information about the shock dynamics, electron acceleration, B(r), plus a unique probe of the Lorentz factor of the cold pulsar wind HESS: detection of TeV gamma-rays from PSR1259-63 several days before the periastron and 3 weeks after the peristron the target photon field is function of time, thus the only unknown parameter is B-field? Easily/robustly predictable X and gamma-ray fluxes ?unfortunately more unknown parameters - adiabatic losses, Doppler boosting, etc. One needs deep theoretical (especially MHD) studies to understand this sourceProbing the wind Lorentz factor with comptonizied radiation Loretz factors exceeding 106 are excludedthe effect is not negligible, but notsufficient to explain the lightcurve GLASTHESSKhangulyan et al. 2008 TeV Gamma Rays From microquasars?HESS, 2005MAGIC, 2006microqusars or binary pulsars? independent of the answer particle acceleration is linked to (sub) relativistic outflows scenarios? -ray production region within and outside the binary system cannot be excluded periodicity expected? yes because of periodic variation of the geometry (interaction angle) and density of optical photons as target photons for IC scattering and absorption, as a regulator of the electron cut-off energy; also because of variation of the B-field, density of the ambient plasma (stellar wind), ... periodicity detected ! is everything OK ?may be OK, but a lot of problems and puzzles with interpretation of the data LS5039 and LS I +61 303 as TeV gamma-ray emittersLS 5039 as a perfect TeV clock and an extreme TeVatron close to inferior conjuction - maximumclose to superior conjuction minimum one needs a factor of 3 or better sensitivity compared to HESS to detect signals within different phase of width 0.1 and measure energy spectra (phase dependent!)can electrons be accelerated to > 20 TeV in presence of radiation? yes, but accelerator should not be located deep inside the binary system, and even at the edge of the system < 10 does this excludes the model of binary pulsar yes, unless the interaction of the pulsar and stellar winds create a relativistic bulk motion of the shocked material (it is quite possible)can we explain the energy dependent modulation by absorption ? yes, taking into account the anysotropic character of IC scattering ? can the gamma-ray producton region be located very deep inside the system no, unless magnetic field is less than 10(R/R*)-1 G (or perhaps not at all) future key observations TeV observations with a sensitivity a factor of 3 (or so) better than HESS, to measure, in particular, the fluxes and spectra within narrow phases , very import are both 10 TeV (maximum electron energy and no absorption) and 0.1 TeV regions (maximum absorption, maximum anysotropy effect, etc.) GeV observation (GLAST) to measure the cascade component X-ray observations - synchrotron radiation of primary and secondary electrons neutrinos - if -ray are of hadronic origin, and less than several percent of the original flux escapes the source, one may expect neutrino flux marginally detectable by km3 volume detectors (current limit from X-ray observations), could be higher If GLAST detects high (cascade) fluxes Blazars and EBL 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 1995Sikora 1994g-rays from >100 Mpc sources - detectable because of the Doppler boostingLarge 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 > 10 otherwise gamma-rays Cannot escape the source due to severe internal photon-photon pair productionCombined 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... TeV emission from Blazars 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 initiated huge interest - especially in AGN and EBL communities todaydetection 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 Hadronic vs. Electronic models of TeV Blazars SSC or external Compton currently most favoured models:easy to accelerate electrons to TeV energieseasy to produce synchrotron and IC gamma-rays recent results require more sophisticated leptonic models Hadronic Models:protons interacting with ambient plasma neutrinos very slow process: protons interacting with photon fields neutrinos low efficiency + severe absorption of TeV g-rays proton synchrotron no 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 :PKS 2155-304 G = 3.32 +/- 0.06 +/- 0.1PKS 2155-304a standard phrase in Whipple, HESS, MAGIC papers SED can be explained within both electronic and hadronic models ... PKS 2155-3042003-2005 HESS observations:leptonic and hadronic cooling and acceleration times of protons Ecut=90 (B/100G)(Ep/1019 eV)2 GeVtsynch=4.5x104(B/100G) -2 (E/1019 eV)-1 s (relatively) comfortable numberstacc=1.1x104 (E/1019) (B/100G) -1 s synchrotron radiation of protons: a viable radiation mechanism Emax =300 -1 j GeV requires extreme accelerators: ~ 1FA 2004Synchrotron radiation of an extreme proton acceleratorrisetime: 173 28 sCrab FluxHESS28th July 2006several min (200s) variabiliry timescale => R=c tvar j=101410 cmfor a 109Mo BH with 3Rg = 1015 cm => j > 100, i.e. close to the accretion disk (the base of the jet), the bulk motion > 100rise time gamma-rays of IC origin? synchrotron peak of PKS2155-409 is located at > 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? integalactic absorption of gamma-raysnew blazars detected at large z: HESS/MAGIC at z> 0.15 !1 ES 1101G = 2.90.2 !H 2356 (x 0.1)G = 3.10.2HESScondition: corrected for IG absorption g-ray spectrumnot harder than E-G (G=1.5) upper limit on EBLHESS upper limits on EBL at O/NIR: direct measurements upperlimitslower limits fromgalaxy counts G=1.5EBL (almost) resolved at NIR ?two options: claim that EBL is detected between O/NIR and MIR or propose extreme hypotheses, e.g. violation of Lorentz invariance, non-cosmological origin of z ... or propose less dramatic (more reasonable) ideas, e.g. very specific spectrum of electrons nFn v Eg1.33 TeV emission from blazars due to comptonization of cold relativistic winds with bulk Lorentz factor G > 106 internal gamma-ray absorption internal gamma-gamma absorptioncan make the intrinsic spectrum arbitrary hard without any real problem from the pointof view of energetics, given that it can be compensated by large Doppler factor, j > 30 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 Gamma Rays from a cold ultrarelativistic wind ?in fact not a very exotic scenario ... M 87 evidence for production of TeV gamma-rays close to BH Distance: ~16 Mpc central BH: 3109 MOJet angle: ~30 not a blazar!discovery (>4s) of TeV g-rays by HEGRA (1998) confirmed by HESS (2003) M87: light curve and variabiliyX-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 ~ 5x1015 dj cm => production of gamma-rays very close to the event horizon of BH?one needs a factor of few better sensitivity at TeV energies to probe fluctuations of the TeV signal on Pair Halos (Aharonian, Coppi, Voelk, 2004)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 how it works ? energy of primary gamma-ray mean free path of parent photons information about EBL flux atgamma-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 twovariables Pair Halos as Cosmological Candles informationabout EBL density at fixed cosmological epochs given by the redshift of the central source unique ! estimate of the total energy release of AGN during the active phase relic sources objects with jets at large angles - many more g-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 EBL at different z and corresponding mean freepaths1. z=0.0342. z=0.1293. z=14. z=21. z=0.0342. z=0.1293. z=14. z=2 SEDs for different z within 0.1o and 1oEBL model Primack et al. 2000Lo=1045 erg/s Brightness distributions of Pair Halosz=0.129z=0.129E=10 GeVA. Eungwanichayapant, PhD thesis, Heidelberg, 2003synchrotron 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) synchrotron -ray halo around a UHECR accelerator in strongly magneized region of IGM (e.g. an AGN within a galaxy cluster) Kelner and FA, PRD, 2008 gamma-radiation of secondary electrons !E* = 3x1020 eVExtragalactic sources of UHECRnon-variable but point-like gamma-rays source !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 Gabici and FA, PRL 2005Futureaim? sensitivity: FE => 10-14 erg/cm2 s (around 1TeV)realization ? 1 to 10 km2 scale 10m+ aperture IACT arrays timescales short (years) - no technological challenges price no cheap anymore: 100+ MEuro expectations guaranteed success - great results/discoveries first priority? classical 100 (30) GeV - 30 (100 ) TeV IACT arrays next step (or parallel?): two possible designs of IACT arrays =>the slide shown first time in Padova in 1995 at the 4th Towards a major Workshop but published 2years later, in: Aharonian 1997, LP97 (Hamburg)HESS Phase 1>3500m asl1500-2000m aslDetector : several 20 to 30m diameter IACTs to study the sky at energies from several GeV to several 100 GeV with unprecedented photon and source statistics Potential: can detect standard EGRET sources with spectra extending beyond 5 GeV for exposure time from 1 sec to 10 minutes Targets: Gamma Ray Timing Explorer for study of nonthermal phenomena AGN jets, Microquasars, GeV counterparts of GRBs, Pulsars ... 5@5 is complementary to FERMI, in fact due to small FoV needs very much FERMI and ... FERMI certainly needs a 5@5 type instrument (1) The Magic detection of 25 GeV gamma-rays from the Crab pulsar demonstrated that a sub-10GeV threshold IACT array can be realized with advanced Cherenkov detectors (2) GLAST detects >10 GeV gamma-rays from pulsars, AGN, GRBs a sub-10GeV threshold IACT array can be realized during the liftime of FERMI5@5 - a GeV timing explorerUnfortunately I cannot tell you much about the detzails results are not yet public available, but very soon we will release the resultsAlso we have still to analyse zthe data, the signal intensity is going down (on average) soIt will be not easy to see signal every nightBut one may expect strong variability, the coolling time of electrons is around 1 hour, so we have a unique chance to monitor the creation and termination of the wind. Also since the IC proceeds in the K-N limit, and the ratio of B-field and star light density is a function of position of pulsare, we expect very unusual spaectra variation in X and gamma rays in very rich combinations !Unfortunately I cannot tell you much about the detzails results are not yet public available, but very soon we will release the resultsAlso we have still to analyse zthe data, the signal intensity is going down (on average) soIt will be not easy to see signal every nightBut one may expect strong variability, the coolling time of electrons is around 1 hour, so we have a unique chance to monitor the creation and termination of the wind. Also since the IC proceeds in the K-N limit, and the ratio of B-field and star light density is a function of position of pulsare, we expect very unusual spaectra variation in X and gamma rays in very rich combinations !m

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