Supernova Remnants as Sources of Cosmic Rays and Nonthermal Emission
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ELEMENTARY PARTICLES AND FIELDS Experiment
Supernova Remnants as Sources of Cosmic Rays and Nonthermal Emission V. N. Zirakashvili1)* and V. S. Ptuskin1) Received July 12, 2019; revised July 12, 2019; accepted July 12, 2019
Abstract—Cosmic ray acceleration by astrophysical shocks in supernova remnants is briefly reviewed. Results of numerical modeling taking into account magnetic field amplification by streaming instability and shock modification are presented. Nonthermal emission produced by accelerated particles in young and old supernova remnants is compared with available data of radio, X-ray, and gamma-ray astronomy. We also discuss a possibility of particle acceleration to PeV energies at supernova shocks propagating in the interstellar bubbles created by stellar winds of supernova progenitors. DOI: 10.1134/S1063778819660530
1. INTRODUCTION The diffusive shock acceleration (DSA) process [1–4] is considered as the principal mechanism for production of galactic cosmic rays (CRs) in supernova remnants (SNRs). During the last decades the excellent results of X-ray and gamma-ray astronomy supplied the observational evidence of the presence of multi-TeV energetic particles in these objects (see e.g. [5] for a review). Most of existing DSA models were applied to young SNRs (see, however, [6]). This is probably because it is expected that CRs with highest energies are produced there. However, lower energy particles are produced in old SNRs either. So the investigation of CR acceleration in old SNRs is important for the calculation of overall CR spectra produced by SNRs. In this paper we show the aplication results using our non-linear DSA model [7, 8] designed for investigation of DSA in old and young SNRs. We apply it for bright old SNR W28 [9] and young SNR Cas A [10]. 2. STREAMING INSTABILITY, MAGNETIC AMPLIFICATION AND MAXIMUM ENERGIES Particles accelerated at astrophysical shocks generate magnetohydrodynamic (MHD) turbulence in the upstream region of the shock via the streaming instability [2]. In young SNRs this instability operates in the non-resonant regime [11]. The nonresonant instability is driven by the electric current jd 1)
Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation, Russian Academy of Sciences, Troitsk, Moscow, 108840 Russia. * E-mail: [email protected]
of highest-energy particles escaping to the upstream region of the shock. The magnetic field is amplified in the expanding magnetic spirals. The maximal rate of instability Γ and its wavenumber k are given by expressions [11] Γ=
jd B0 , 2ρ0 cVA0
k=
Γ , VA0
(1)
of the where ρ0 is the gas density, B0 is the strength √ regular magnetic field and VA0 = B0 / 4πρ0 is the ´ speed. The gyroradius of particles rg producAlfven ing the instability must be higher than the scale of the unstable spirals i.e. rg k 1. When the amplified magnetic fields become several times stronger than the regular magnetic field B0 , the collisions of neighboring expanding magnetic spirals occur. This results in the slower magnetic growth. This non
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