Nonthermal radiation of young supernova remnants: the case of Cas A

TL;DR

This paper models the broad-band nonthermal radiation of young supernova remnant Cas A, showing that both forward and reverse shocks accelerate particles, with implications for gamma-ray origins and cosmic ray energy conversion.

Contribution

It introduces a detailed nonlinear diffusive shock acceleration model including electrons, protons, and oxygen ions, explaining multi-wavelength observations of Cas A.

Findings

Both shocks contribute to observed radiation.

High acceleration efficiency with 25% of explosion energy converted to cosmic rays.

Maximum proton energy limited to about 100 TeV.

Abstract

The processes responsible for the broad-band radiation of the young supernova remnant Cas A are explored using a new code which is designed for a detailed treatment of the diffusive shock acceleration of particles in nonlinear regime. The model is based on spherically symmetric hydrodynamic equations complemented with transport equations for relativistic particles. Electrons, protons and the oxygen ions accelerated by forward and reverse shocks are included in the numerical calculations. We show that the available multi-wavelength observations in the radio, X-ray and gamma-ray bands can be best explained by invoking particle acceleration by both forward and reversed shocks. Although the TeV gamma-ray observations can be interpreted by interactions of both accelerated electrons and protons/ions, the measurements by Fermi LAT at energies below 1 GeV give a tentative preference to the hadronic origin of gamma-rays. Then, the acceleration efficiency in this source, despite the previous claims, should be very high; 25% of the explosion energy (or approximately $3\cdot 10^{50}$ erg) should already be converted to cosmic rays, mainly by the forward shock. At the same time, the model calculations do not provide extension of the maximum energy of accelerated protons beyond 100 TeV. In this model, the acceleration of electrons is dominated by the reverse shock; the required $10^{48}$ erg can be achieved under the assumption that the injection of electrons (positrons) is supported by the radioactive decay of $^{44}$Ti.