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Nonthermal Processes in Supernova Remnants

Nonthermal Processes in Supernova Remnants. F.A. Aharonian, DIAS (Dublin) & MPIK (Heidelberg). Challenges in Particle Astrophysics, Chateau Royal de Blois, May 19, 2008. SNRs as Cosmic Ray Accelerators - TeVatrons !.

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Nonthermal Processes in Supernova Remnants

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  1. Nonthermal Processes in Supernova Remnants F.A. Aharonian, DIAS (Dublin) & MPIK (Heidelberg) Challenges in Particle Astrophysics, Chateau Royal de Blois, May 19, 2008

  2. SNRs as Cosmic Ray Accelerators - TeVatrons ! SNRs in our Galaxy: more than 230 (Green et al. 2001) with nonthermal X-ray emission - 10 or so best candidates - young SNRs with nonthermal synchrotron X-rays SN1006 Diffusive source Tycho Kepler CasA 30 arcmin TeV -ray emission synchrotron X-rays - presence of multi-TeV electrons, but only TeV -rays provide mode-independent evidence

  3. Galactic PeVatrons –particle accelerators responsible for Cosmic Rays with energies up to 1015 eV SNRs ? Pulsars/Plerions ? OB, W-R Stars ? Microquasars ? Galactic Center ? . . . Gaisser 2001 * the source population responsible for the bulk of GCRs are PeVatrons ?

  4. SNRs – the most probable factories of GCRs ? (almost) a common belief based on two arguments: • necessary amount of available energy – 1051erg • Diffusive Shock Acceleration (DSA)– >10% efficiency and E-2type spectrum to very high energies but… (i) DSA theory cannot provide robust predictions concerning the conversion efficiency (ii) for acceleration of particles to 1015 eV, the acceleration efficiency should exceed 100%

  5. 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 1 to 10 % of the maximum rate ( h=10-100) 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

  6. Knee Ankle SNRs - sources of CRs up to >1015 eV? Cosmic Rays from 109 to 1020 eV • up to 1015-16 eV (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

  7. SNRs – the most probable factories of GCRs ? straightforward proof: detection and identification of gamma-rays, neutrinos and hard X-rays from p-p interactions (as products of decays of secondary neutral and charged pions) objective: (i) to probe the content of nucleonic component of CRs in SNRs at d < 10kpc at the level 1049 -1050 erg, and (ii) to demonstrate that at least in some SNRs particles are accelerated to 1015 eV realization: sensitivity of detectors - down to 10-13 erg/cm2 s crucial energy domains: : up to 100+ TeV : 1 - 100 TeV X: 10 - 100 keV

  8. Visibility of SNRs in high energy gamma-rays for CR spectrum witha=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) Crab 1o sensitivity Detectability ?compromise between angle q (r/d) and flux Fg (1/d2) typically A: 0.1-0.01 q: 0.1o - 1o 0.1o TeV g-rays – detectable if A > 0.1 po component dominates if A > 0.1 (Sx/10 mJ)(B/10 mG ) -2 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 nucleonic component of CRs - “visible” through TeV (and GeV) gamma-rays !

  9. three basic mechanisms of -ray production in SNRs: characteristic timescales: pp -> 0 -> 2 e-Bremsstrahlung IC (e+2.7K) for comparison • IC is very effective as long as magnetic fieldB < 10 mG • Bremsstrhlung important in dense, n > 102 cm-3 , environments • pp interactions dominate over Bremsstrahlung if the ratio of energy densities of protons to electrons wp/we > 10, and Inverse Compton component if wp/we > 500 (n/1cm-3) -1 (at energies above 10 TeV)

  10. SN 1006 - a good candidate for particle source acceleration H.E.S.S.upper limits - an order of magnitude below the flux reported by CANGAROO a trouble ? not at all … HESS upper limits imply IC : B > 25 mG po : Wp<(0.2-2) x 1050 erg no problem for the hypothesis of SNR origin of Galactic CRs …

  11. Cas A – a proton accelerator (100MeV)/radio => B > 0.1 G IC origin is unlikely TeV gamma-rays of hadronic origin ? yes, but Wp(>1TeV) =1049 erg and very small proton/electron ratio (<10) Wp=2x1049 erg , n=20 cm-3 5-6 sigma detection Cas A is well designed to operate as a PeVatron ? with a “right“ combination of B-field, shock speed and age to accelerate and confine particles up to 1 PeV - a source of >1TeV gamma -rays and neutrinos? • very important target for VERITAS and MAGIC • GLAST should detect GeV gamma-ray emission in any case • no way to detect TeV neutrionos even with km3 scale detectors

  12. recent news from Magic E- exp(-E/Eo) type proton spectrum with (i) =2 and cutoff energy Eo <10 TeV or (ii) =2.3 (or so) and Eo >> 10 TeV implications? (i) protons are not accelerated to > 10 TeV energies (why?) (ii) total energy in protons (down to 1 GeV) close to 1050 erg (as expected) obserations with VERITAS/MAGIC and GLAST will soon tell us a lot about Cas A

  13. Vela Junior RXJ1713.7-3946 TeV images of two young “1Crab“ strength shell type SNRs flux and spectra - similar, morphology – shell type

  14. XMM FOV RX J1713.7-3946 structure of the entire remnant (XMM-Newton) 0.7-2keV, resolution15” Chandra FOV Chandra image

  15. RXJ1713.7-4639 > 30 TeV -rays and shell type morphology: acceleration of protons and/orelectrons in the shell to energies (well) exceeding 100 TeV very good correlation with X-rays 1 almost constant photon index ! 2 for 0-meson decay gamma-rays:

  16. origin of gamma-rays… the key issue: identification of g-ray emission mechanisms: p0 or IC ? 0 hadronic origin of gamma-ray derivation of the energy spectrum and the total energy Wp (with an uncertainty related to the uncertainty in n/d2 ) IC leptonic origin of gamma-rays model-independent derivation of the spatial and spectral distributions of electrons and, in combination with X-ray data - model independent map of the B-field

  17. argument in favor of IC origin of -rays: existence of multi-TeV electrons from synchrotron radiation nice spatial correlation with X-rays argument against hadronic models leptonic model - IC on 2.7 K IC origin ?– very small B-field, B < 10 mG, and very large Emax > 100 TeV two assumptions hardly can co-exists within standard DSA models; bad spectral fit below a few TeV … IC origin of -rays implies that we see distribution of electrons => nice correlation of electron distribution with synchrotron X-ray distribution => homogeneous magnetic field, but distinct spatial variation of electrons

  18. the energy spectrum of electrons at the shock front *) in the regime of Bohm diffusion and under assumpton of dominance of synchrotron cooling the spectra of electrons and synchrotron radiation can be calculated with god accuracy: E0 almost coincides with the value derived from tacc = t synch the spectrum of synchrotron radiation at the shock front Synchrotron cutoff energy approximately 10 times h observations of X-rays above 10 keV are needed to compare with model-predictions V.Zirakashvili & FA 2007

  19. Elctron and synch. rad. spectra at shock front e synch. rad. downstream upstream -functional approx. =4, B1= B2 electrons IC on 2.7 MBR Sy Thompson! downstream upstream upstream V.Zirakashvili, FA 2006

  20. recent broad-band measurements of Suzaku energy spectrum of synchrotron radiation of electrons in the framework of DSA (Zirakashvili&FA 2007) h0=0.55 keV strong support for acceleration in Bohm diffusion regime (h ~ 1) - from postion of synchrotron cutoff given that the shock speed v < 4000 km/s (Chandra) Uciyama et al. 2008 electron spectrum derived from Suzaku data DSA prediction (Zirakashvili&FA07) “standard E-3 exp(-E/E0) type elec. spectrum Tanaka et al. 2008 derived electron spectrum allows model-independent calculations of IC spectrum !

  21. IC calculations based on Suzaku data Solid - total 2. IC on 2.7 3. IC on FIR 4. IR on optical Standard ISM radiation fields B=14G density of Optical field: 140 eV/cm3 second electron component with a cutoff at 10 TeV or should adopt that the source is >104 years old

  22. hadronic origin of TeV gamma-rays acceleration spectrum with power-law the same, except for index =2; B=200 G, age: 1000 years =1.7 Wp=2.7 x 1050 (n/1cm-3)-1 erg Wp=1.6 x 1050 (n/1cm-3)-1 erg We=3.1 x 1046 erg We=6.0 x 1045 erg

  23. challenges for hadronic models: strong X-TeV correlaton in fact this is more natural for hadronic rather than leptonic models - this could an indication that electrons and protons are accelerated relatively weak radio emission different than in other SNRs, but why not, i.e. because of very hard acceleration spectra lack of thermal X-ray emission because of (i) very low density plasma n or (ii) low electron temperature Te ? gas density n cannot be much less than 1 cm-3 because of available energy budget we do not (yet) know the mechanism(s) of electron heating in supernova remnants; Coulomb exchange is not sufficient for heating; most likely this happens through plasma waves. As long as the mechanism is not understood one cannot use the lack of thermal emission as an argument against hadronic model

  24. 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, 100G 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, Takahashi, Tanaka,Maeda,Nature 2007 *)explanation by variation of B-field does’t work as demonstrated for Cas A (Uciyama&FA, 2008)

  25. pp pogg – good spectral fits • pp p±ereliable predictions • protons: • dN/dE: E-aexp[-(E/Ecut)b] • g-rays/neutrinos • dN/dE: 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 Kel’ner et al. 2006

  26. total inelastic pp cross-section and production rates of  and nm for power-law spectrum of protons: nH =1 cm-3 , wp=1 erg/cm-3

  27. 3 channels of informationabout cosmic PeVatrons:10-100 TeV gamma-rays 10-100 TeV m-neutrinos 10 -100 keV hard X-rays SNRs as Cosmic PeVatrons ? sensitivities ? better than 10-12 erg/cm2s • g-rays: difficult, but possible with future“10km2“ areamulti-TeV IACT arrays • neutrinos: difficult, but KM3NeT should be able to see (marginal) signals from SNRs RX 1713.7-3946and Vela Jr • “prompt“ synchrotron X-rays: a very promising channel

  28. Probing PeV protons with X-rays SNRs shocks can accelerate CRs to <100 TeV(e.g. Cesarsky&Lagage 1984) unlessmagnetic field significantly exceeds 10 mG Recent theoretical developments: applification of the B-field up to >100 G is possible through plasma waves generated by CRs (Bell and Lucek 2000) >1015 eV protons >1014 eV gamma-rays and electrons “prompt“ synchrotron X-rays cooling time:t(e) = 1.5 (e/1keV) -1/2 (B/1mG) -3/2 yr << tSNR energy range: typically 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 !

  29. electron injection spectrumQe(Ee)=QoEe-aexp[(-Ee/Ee,o)s ] spectrum of synchrotron radiation of cooled electrons e-(a/2+1)exp[(-e/eo)l]l= s/s+2; b=1 s=0.5,l=1/5 synchrotron spectrum of primary electrons:l= eo : characteristic synchrotron frequency proportionalBE02( prop. to B3 ) * for Eo =1 PeV, B=1 mG X-ray emission extends well beyond 10 keV while the cutoff energy in the synchrotron spectrum from directly accelerated electrons is expected around1 keV simultaneous measurements of p0-decay g-rays and associated synchrotron radiation provide unambiguous estimate of B-field in the acceleration region ! “hadronic“ X-rays versus synchrotron radiation of primary electrons:

  30. protons broad-band GeV-TeV-PeV gs synch. hard X-rays broad-band emision initiated by pp interactiosn : Wp=1050 erg, n=1cm-3

  31. key observations to prove the hadronic origin of gamma-rays • large (>>10G) magnetic field X-ray studies (Chandra) • gamma-rays to and beyond 100 TeV UHE gamma-ray studies (arrays for detection multi-TeV gamma-rays) • neutrinos detection of neutrins (IceCube and KM3NeT) • hard X-rays produced by secondary electrons detection of gamma-rays from 10keV to 100 keV (NuSTAR, Sibol-X, NeXT) key observations to prove the hadronic origin of gamma-rays population studies (CTA, AGIS, and radio and X-ray detectors)

  32. shower ~ 10 km 5 nsec -ray Seff = 1m2 at 1 GeV Crab flux: F(>E)=5x10-4 (E/1TeV)-1.5 ph/m2 hr gamma-ray telescopes -- “fast” detectors 100m Seff >3x104 m2 at 1 TeV in principle also unlimited

  33. Jacques Paul Observables – The Violent Universe – International Winter School – 13 and 14 March 2007 Slide 33 km3 volume neutrino telescopes km3 volume detector A. Kappes effective area: 0.3m2 at 1 TeV 10m2 at 10 TeV => several events from a “1Crab” source per 1year nevertheless km3 volume scale detectors will provide first meaningful probes of SNRs

  34. detection rate of neutrinos with KM3NeT R.White the flux of strongest SNRs A few neutrinos per year at presence of comparable background events

  35. 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, but this should not be a big issue for SNRs

  36. searching for galactic PeVatrons ... TeV gamma–rays from Cas A and RX1713.7-3946, Vela Jr – a proof that SNRs are responsible for the bulk of GCRs ?– not yet the hunt for galactic PeVatrons continues unbiased approach – deep survey of the Galactic Plane – not to miss any recent (or currently active) acceleration site: SNRs, Pulsars/Plerions, Microquasars... not only from accelerators, but also from nearby dense regions

  37. Gamm-rays/X-rays from dense regions surrounding accelerators the existence of a powerful accelerator by itself is not sufficenrt for gamma 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

  38. older source – steeper g-ray spectrum tesc=4x105(E/1 TeV) -1k-1 yr (R=1pc); k=1 – Bohm Difussion Qp / E-2.1 exp(-E/1PeV) Lp=1038(1+t/1kyr) -1 erg/s

  39. 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]

  40. MGRO J1908+06 - a PeVatron? HESS preliminary Milagro

  41. TeV gamma-rays from GMCs in GC diffuse emission along the plane! HESS J1745-303 gamma-rays from GMCs - echo of a particle acceleration in the past ?

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