Steep Cosmic-Ray Spectra with Revised Diffusive Shock Acceleration

Galactic cosmic rays (CRs) are accelerated at the forward shocks of supernova remnants (SNRs) via diffusive shock acceleration (DSA), an efficient acceleration mechanism that predicts power-law energy distributions of CRs. However, observations of nonthermal SNR emission imply CR energy distribution...

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Bibliographic Details
Published in:The Astrophysical journal 2021-11, Vol.922 (1), p.1
Main Authors: Diesing, Rebecca, Caprioli, Damiano
Format: Article
Language:English
Subjects:
Astrophysics
Cosmic ray showers
Cosmic rays
Galactic cosmic rays
High energy astrophysics
Phenomenology
Shocks
Spectra
Spectral index
Supernova
Supernova remnants
Supernovae
Thermal plasmas
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Summary:Galactic cosmic rays (CRs) are accelerated at the forward shocks of supernova remnants (SNRs) via diffusive shock acceleration (DSA), an efficient acceleration mechanism that predicts power-law energy distributions of CRs. However, observations of nonthermal SNR emission imply CR energy distributions that are generally steeper than E −2 , the standard DSA prediction. Recent results from kinetic hybrid simulations suggest that such steep spectra may arise from the drift of magnetic structures with respect to the thermal plasma downstream of the shock. Using a semi-analytic model of nonlinear DSA, we investigate the implications that these results have on the phenomenology of a wide range of SNRs. By accounting for the motion of magnetic structures in the downstream, we produce CR energy distributions that are substantially steeper than E −2 and consistent with observations. Our formalism reproduces both modestly steep spectra of Galactic SNRs (∝ E −2.2 ) and the very steep spectra of young radio supernovae (∝ E −3 ).
ISSN:0004-637X
1538-4357
DOI:10.3847/1538-4357/ac22fe