APP UK 2008 meeting, Oxford, June 20, 2008 Theoretical aspects of VHE -ray astronomy: Exploring Nature’s Extreme Accelerators Felix Aharonian Dublin Institute for Advanced Studies, Dublin Max-Planck-Institut f. Kernphysik, Heidelberg
Astroparticle 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 Nature’s 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
Knee Ankle Cosmic Rays from 109 to 1020eV Extragalactic? • 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 Accelerators SNRs ? T. Gaisser
Particles 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
FA, Belyanin et al. 2002, Phys Rev D, 66, id. 023005 acceleration sites of 1020 eV CRs ? confinement • signatures of extreme accelerators? • synchrotron self-regulated cutoff: energy losses FA 2000, New astronomy, 5, 377 • 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 confinement energy losses
VHE gamma-ray and neutrino astronomies: two key research areas of High Energy Astrophysics/Astroparticle Physics VHE- 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 ﬁelds, 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
VHE gamma-ray astronomy - a success story 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... 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 RXJ 1713.7-3946 PSR 1826-1334 PKS 2155-309 28th July 2006 multi-functional tools:spectrometrytemporal studiesmorphology • extended sources:from SNRs to Clusters of Galaxies • transient phenomenaQSOs, AGN, GRBs, ... Galactic Astronomy | Extragalactic Astronomy | Observational Cosmology TeV image and energy spectrum of a SNR enrgy dependent image of a pulsar wind nebula variability 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 %) resolutions broad energy interval - from 100 (10) GeV to 100 (1000) TeV nice sensitivity (minimum detectable flux): 10-13 (10-14) erg/cm2 s
VHE gamma-ray observations: “Universe is full of extreme accelerators on all astronomical scales” TeV gamma-ray source populations 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 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 holes • cosmological issues - Dark Matter, Extragalactic Background Light (EBL) …..
Extragalactic Sources Galactic Sources GeV GeV GeV GeV GeV GeV GRBs AGN GLX CLUST IGM ISM SFRs SNRs Pulsars Binaries Potential Gamma Ray Sources Blazars Radiogalaxies Normal Starburst GMCs Microquasars Cold Wind Magnetosphere Binary Pulsars Pulsar Nebula EBL Major Scientific Topics G-CRs Compact Objects EXG-CRs Relativistic Outflows Cosmology
unique carriers of astrophysical and cosmological information about non-thermal phenomena in many galactic and extragalactic sources TeV neutrinos -- a complementary channel 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
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 GRBs
what should we do if hadronic gamma-rays and neutrinos appear at “wrong” energies ? detect radiation of secondary electrons ! Bethe-Heitler electrons photomeson electrons 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 Kelner and FA, 2008, Phys Rev D
probing hadrons with secondary X-rays with sub-arcmin resolution! Simbol-X new 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 energies” disadvantage - ambiguity of origin of X-rays
exploring Nature’s Extreme Particle Accelerators with neutrinos, gamma-rays, and hard X-rays
Galactic TeVatrons and PeVatrons - particle accelerators responsible for cosmic rays up to the “knee” around 1 PeV SNRs ? Pulsars/Plerions ? OB, W-R Stars ? Microquasars ? Galactic Center ? . . . Gaisser 2001 * the source population responsible for the bulk of GCRs are PeVatrons ?
Visibility of SNRs in high energy gamma-rays for CR spectrum with =2 Fg(>E)=10-11 A (E/1TeV)-1 ph/cm2s A=(Wcr/1050erg)(n/1cm-3 )(d/1kpc) -2 p0 –decay (A=1) Inverse Compton 1000 yr old SNRs (in Sedov phase) Detectability ?compromise between angle q (r/d) and flux Fg (1/d2) typically A: 0.1-0.01 q: 0.1o - 1o TeV g-rays – detectable if A > 0.1 if electron spectrum >> 10 TeV synchrotron X-rays and IC TeV g’s main target photon field 2.7 K: Fg,IC/Fx,sinch=0.1 (B/10mG)-2 po component dominates if A > 0.1 (Sx/10 mJ)(B/10 mG ) -2 nucleonic component of CRs - “visible” through TeV (and GeV) gamma-rays !
RXJ1713.7-4639 TeV-rays and shell type morphology: acceleration of p or e in the shell to energies exceeding 100TeV can be explained by -rays from pp ->o ->2 and with just ”right” energetics Wp=1050 (n/1cm-3)-1 erg/cm3 2003-2005 data but IC canot be immediately excluded…
leptonic versus hadronic 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 all IC origin ?– very small B-field, B < 10 mG, and very large E, Emax > 100 TeV two assumptions hardly can co-exists within standard DSAmodels, bad fit of gamma-ray spectrum below a few TeV, nevertheless …
Variability of X-rays on year timescales - witnessing particle acceleration in real time flux increase - particle acceleration flux decrease - synchrotron cooling *) both require B-field of order 1 mG in hot spots and, most likely, 100G outside 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) Uchiyama, FA, Tanaka, Maeda, Takahashi, Nature 2007 *)explanation by variation of B-field does’t work as demonstrated for Cas A (Uciyama&FA, 2008)
acceleration in Bohm diffusion regime energy spectrum of synchrotron radiation of electrons in the framework of DSA (Zirakashvili&FA 2007) (Tanaka et al. 2008) with h=0.67 +/- 0.02keV Strong support for Bohm diffusion - from the synchrotron cutoff given the upper limit on the shock speed of order of 4000 km/s ! B=100 G + Bohm diffusion - acceleration of particles to 1 PeV
RXJ 1713.7-3946 • protons: • dN/dE=K E-aexp[-(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)2 neutrinos: marginally detectable by KM3NeT
Probing PeV protons with X-rays SNRs shocks can accelerate CRs to <100 TeV unlessmagnetic field significantly exceeds 10 mG recent theoretical developments: amplification of the B-field up 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 << tSNR typically in the range between 1 and 100 keV with the ratio Lx/Lg larger than 20% (for E-2 type spectra) “hadronic“ hard X-rays and (multi)TeV g-rays – similar morphologies !
10-100 TeV m-neutrinos 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 - don’t expect spectrometry, morphology; uniqueness - unambiguous signatute! • “prompt“ synchrotron X-rays: smooth spectrum a very promising channel - quality!(NexT, NuSTAR, SIMBOL-X)
broad-band emision initiated by pp interactiosn : Wp=1050 erg, n=1cm-3 protons broad-band GeV-TeV-PeV gs synch. hard X-rays no competing X-ray radiation mechanisms above 30 keV
Searching for Galactic PeVatrons the existence of a powerful accelerator is not yet sufficenrt for -radiation; an additional component –a dense gas target- is required gamma-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
older source – steeper g-ray spectrum tesc=4x105(E/1 TeV) -1k-1 yr (R=1pc); k=1 – Bohm Difussion Qp = k E-2.1 exp(-E/1PeV) Lp=1038(1+t/1kyr) -1 erg/s
Gamma-rays and neutrinos inside and outside of SNRs 1 - 400yr, 2 - 2000yr, 3 - 8000yr, 4 - 32,000 yr gamma-rays neutrinos SNR: W51=n1=u9=1 GMC: M=104 Mo d=100pc d=1 kpc ISM: D(E)=3x1028(E/10TeV)1/2 cm2/s [S. Gabici, FA 2007]
MGRO J1908+06 - a PeVatron? HESS preliminary Milagro
gamma-ray emitting clouds in GC region diffuse emission along the plane! HESS J1745-303 indirect discovery of the site of particle acceleration measurements of the CR diffusion coefficient
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* TeV gamma-rays from GC many of them are potential g-ray emitters -Shell Type SNRs Plerions, Giant Molecular Clouds Sgr A * itself, Dark Matter … HESS: FoV=5o 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 << 1 arcmin
Crab Nebula – a perfect PeVatron of electrons (and protons ?) 1-10MeV Standard MHD theory cold 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 nebula Inverse Compton => high energy gamma-ray nebula MAGIC (?) . 100TeV HEGRA Crab Nebula – a very powerful W=Lrot=5x1038 erg/s and extreme accelerator: Ee > 1000 TeV Emax=60 (B/1G) -1/2h-1/2 TeV and hncut=(0.7-2) af-1mc2h-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 * for comparison, in shell type SNRs DSA theory gives h=10(c/v)2=104-105
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 oflarge spin-down flux gamma-ray flux << “spin-down flux“ because of large magnetic field but the strength of B-field also depends on less powerful pulsar weaker magnetic field higher gamma-ray efficiency detectable gamma-ray fluxes from other plerions HESS confirms this prediction ! – many famous PWNe are already detected in TeV gamma-rays - MSH 15-52, PSR 1825, Vela X, ...
HESS J1825 (PSR J1826-1334) energy-dependent image - electrons! red – below 0.8 TeV yellow – 0.8TeV -2.5 TeV blue – above 2.5 TeV Luminosities: spin-down: Lrot= 3 x 1036 erg/s X: 1-10 keV Lx=3 x 1033 erg/s (< 5 arcmin) g: 0.2-40TeV Lg=3 x 1035 erg/s (< 1 degree) Pulsar‘s period: 110 ms, age: 21.4 kyr, distance: 3.9 +/- 0.4 kpc 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.
Gamma-ray Binaries Mirabel 2006
PSR1259-63 - 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 termination the same 3 components - Pulsar/Pulsar Wind/Synch.Nebula - as in plerions but with characteristics radiation and dynamical timescales less than hours both 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
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 source 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
Probing the wind Lorentz factor with comptonizied radiation Khangulyan et al. 2008 HESS GLAST Loretz factors exceeding 106 are excluded the effect is not negligible, but not sufficient to explain the lightcurve
TeV Gamma Rays From microquasars? HESS, 2005 MAGIC, 2006 microqusars or binary pulsars? independent of the answer – particle acceleration is linked to (sub) relativistic outflows
LS5039 and LS I +61 303 as TeV gamma-ray emitters 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 …
LS 5039 as a perfect TeV clock and an extreme TeVatron close to inferior conjuction - maximum close 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)
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 future key observations
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
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...