Global constraints on diffusive particle acceleration by strong non-relativistic shocks. Issue 4 (19th November 2018)

• Record Type: Journal Article
• Authors: Zhang, Yiran; Liu, Siming
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 482:Issue 4(2019)
• Page Start: 5268
• Page End: 5274
• Publication Date: 2018-11-19
• Subjects: acceleration of particles -- shock waves -- cosmic rays -- ISM: supernova remnants; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/sty3136
• Languages: English
• ISSNs: 0035-8711

Abstract

ABSTRACT: Estimating the cosmic-ray acceleration efficiency ϵ in supernova remnants (SNRs) through observations is a challenging task in general. Based on the Rankine–Hugoniot shock conditions, we find an anticorrelation between ϵ and the power-law spectral index α of relativistic particle distribution produced via diffusive particle acceleration by non-relativistic shocks, implying more efficient acceleration in older SNRs with harder radio spectra. Then ϵ may be estimated from some hard radio spectral index measurements. Assuming the particle distribution in downstream of strong shocks to be a non-relativistic Maxwellian plus a relativistic power law with a high-energy cut-off, we also find that the injection rate for relativistic particles η needs to ≳10 −6 for a prominent decrease of the adiabatic index in SNRs, which implies higher compression ratio and lower values of α. This threshold of η increases with the shock speed u 1, which may explain the relatively harder radio spectra of older SNRs with lower u 1 . We show that η and/or the relativistic cut-off momentum p m need to be low for old SNRs, and expect a gradual increase of ϵ as SNR evolves with gradually decreasing η and p m .