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Gamma-rays from SNRs and cosmic rays

Gamma-rays from SNRs and cosmic rays. V.N.Zirakashvili. Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radiowave Propagation, Russian Academy of Sciences (IZMIRAN), 142190 Troitsk, Moscow Region, Russia. Outline. Acceleration of particles at forward and reverse shocks in SNRs

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Gamma-rays from SNRs and cosmic rays

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  1. Gamma-rays from SNRs and cosmic rays V.N.Zirakashvili Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radiowave Propagation, Russian Academy of Sciences (IZMIRAN), 142190 Troitsk, Moscow Region, Russia

  2. Outline • Acceleration of particles at forward and reverse shocks in SNRs • Amplification of magnetic fields • Radioactivity and electron acceleration • Modeling of DSA in SNRs • Modeling of broad-band emission

  3. Krymsky 1977; Bell 1978; Axford et al.1977; Blandford & Ostriker 1978 Diffusive Shock Acceleration Very attractive feature: power-law spectrum of particles accelerated, =(+2)/(-1), where  is the shock compression ratio, for strong shocks =4 and =2 Maximum energy for SN: D0.1ushRsh 3·1027cm2/s<Dgal Diffusion coefficient should be small in the vicinity of SN shock In the Bohm limit D=DB=crg/3 and for interstellar magnetic field

  4. Shock modification by the pressure of accelerated CRs. Axford 1977, 1981 Eichler 1984 Higher compression ratio of the shock, concave spectrum of particles.

  5. X-ray image of Tycho SNR (from Warren et al. 2005) 1. CD is close to the forward shock – evidence of the shock modification by CR pressure. 2. Thin non-thermal X-ray filaments at the periphery of the remnant – evidence of electron acceleration and of magnetic amplification.

  6. Magnetic field amplification by non-resonant streaming instability Bell (2004) used Achterberg’s results (1983) and found the regime of instability that was overlooked k rg >>1, γmax=jdB0/2cρVa Since the CR trajectories are weakly influenced by the small- scale field, the use of the mean jd is well justfied saturated level of instability

  7. MHD modeling in the shock transition region and downstream of the shock Zirakashvili & Ptuskin 2008 B u1=3000 km/s Va=10 km/s ηesc=0.14 0.02L Magnetic field is not damped and is perpendicular to the shock front downstream of the shock! Ratio=1.4 σB=3 3D 2562×512

  8. Wood & Mufson 1992 Dickel et al. 1991 Radial magnetic fields were indeed observed in young SNRs

  9. Schematic picture of the fast shock with accelerated particles It is a great challenge - to perform the modeling of diffusive shock acceleration in such inhomogeneous and turbulent medium. Spectra of accelerated particles may differ from the spectra in the uniform medium. Both further development of the DSA theory and the comparison with X-ray and gamma-ray observations are necessary

  10. CR acceleration at the reverse shock (e.g.Ellison et al. 2005)? Probably presents in Cas A (Helder & Vink 2008) Magnetic field of ejecta? B~R-2, 104G at R=1013 cm - 10-8G at R=1019cm=3pc +additional amplification by the non-resonant streaming instability (Bell 2004) Field may be amplified and become radial – enhanced ion injection at the reverse shock

  11. Radio-image of Cas A Atoyan et al. 2000 X-ray image of Cas A (Chandra) Inner bright radio- and X-ray-ring is related with the reverse shock of Cas A while the diffuse radio-plateau and thin outer X-ray filaments are produced by electrons accelerated at the forward shock.

  12. Radio-image of RX J1713.7-3946 (Lazendic et al. 2004) X-rays: XMM-Newton, Acero et al. 2009 Inner ring of X-ray and radio-emission is probably related with electrons accelerated at the reverse shock.

  13. Radioactivity and electron acceleration in SNRs (Zirakashvili & Aharonian (2011)) Forward and reverse shocks propagate in the medium with energeticelectrons and positrons. Cosmic ray positrons can be accelerated at reverse shocks of SNRs (Ellison et al. 1990). 44Ti t1/2=63 yr 1.6·10-4 Msol in Cas A, (Iyudin et al. 1994,Renaud et al. 2006) 1-5 ·10-5 Msol in G1.9+0.3 (Borkowski et al. 2010)

  14. “Radioactive” scenario in the youngest galactic SNR G1.9+0.3 X-ray image radio-image Thermal X-rays and 4.1 keV Sc line (product of 44Ti) are observed from bright radio-regions (ejecta) Borkowski et al. 2010

  15. Numerical model of nonlinear diffusive shock acceleration(Zirakashvili & Ptuskin 2011)(natural development of existing models of Berezhko et al. (1994-2006), Kang & Jones 2006, see also half-analytical models of Blasi et al.(2005); Ellison et al. (2010) ) Spherically symmetric HD equations + CR transport equation Acceleration at forward and reverse shocks Minimal electron heating by Coulomb collisions with thermal ions

  16. nH= 0.1 cm-3 B0= 5 μG T= 104 K ESN=1051 erg Mej=1.4Msol η = 0.01 Protons and electrons are injected at the forward shock, ions and positrons are injected at the reverse shock. Magnetic amplification in young SNRs is taken into account Numerical results u PCR BS

  17. Radial profiles at T=1000 years u PCR ρ Pg

  18. Spectra of accelerated particles p p

  19. Integrated spectra The input of the forward shock was considered earlier (Berezhko & Völk 2007, Ptuskin et al. 2010, Kang 2011) Spectrum of ions is harder than the spectrum of protons because the ejecta density decreases in time.

  20. Integrated spectra Alfven drift downstream of the forward shock results in the steeper spectra of particles accelerated at the forward shock.

  21. nH= 0.1 cm-3 Evolution of nonthermal emission from SNR

  22. TeV gamma-rays from young SNRs Aharonian et al 2008 (HESS) Aharonian et al 2007 (HESS) Particles accelerated up to100 TeV in these SNRs. Gamma-rays can be produced in pp collisions (hadronic models) or via the Inverse Compton scattering of IR and MWBR photons on the electrons accelerated (leptonic models) Vela Jr RX J1713.7-3946

  23. hadronic Fermi LAT results on RX J1713.7-3946 (Abdo et al. 2011) RX J1713.7-3946 leptonic Although the leptonic model is preferable, the hadronic model is not excluded because of the possible energy dependent CR penetration in to the clouds. The main problem of the hadronic model is the absence of thermal X-rays.

  24. Vela Jr.:Fermi results (Tanaka et al. 2011) Hadronic model (Berezhko et al. 2009) is in agreement with the measured gamma-ray spectra. The forward shock is modified by CR pressure. Leptonic model is excluded if the thin X-ray filaments observed in this remnant can be considered as the evidence of strong magnetic fields. Chandra (Bamba et al. 2005)

  25. Modeling of broad-band spectra of Cas A for the “radioactive” scenario of lepton injection (Zirakashvili et al. 2012 in preparation) ESN=1.7·1051 erg, Mej=2.1 Msol, t=330 yr, nH=0.8 cm-3, Vf=5700 km/s, Vb=3400 km/s, Bf=1.1 mG, Bb=0.24 mG Emission is mainly produced at the reverse shock of Cas A

  26. Broad-band modeling of Tycho SNR(Morlino & Caprioli 2011) Good candidate to be a proton accelerator

  27. Old SNRs (T>104 yr) IC443 (Abdo et al. 2010) W44 (Giuliani et al. 2011) Old SNRs show gamma-ray spectra with steeper parts or cut-offs. TeV protons accelerated earlier have already left the remnant. Emax~100 GeV in IC443 and Emax ~10 GeV in W44. Probably because of damping of MHD waves generated by accelerated particles. The spectral shape at E<1 GeV favors a hadronic origin of gamma-emission in SNR W44 (AGILE coll.).

  28. Summary • Non-resonant streaming instability produced by the electric current of run-away CR particles results in the significant magnetic amplification at fast SNR shocks. 2. The reverse shocks in SNRs can produce CR ions and positrons with spectrum that is harder in comparison with spectra of particles accelerated at the forward shock. 3. Both leptonic and hadronic origin of gamma-emission in SNRs RX J1713.7-3946, Vela Junior, Cas A are possible. 4. SNR Tycho and old SNRs are good candidates for a hadronic origin of gamma-emission.

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