On the plasma temperature in supernova remnants with cosmic-ray modified shocks

TL;DR

This paper investigates how strong cosmic-ray acceleration at supernova remnant shocks reduces the thermal plasma temperature, potentially explaining the lack of thermal X-ray emission observed in some remnants.

Contribution

It provides analytical estimates showing that particle acceleration can significantly lower post-shock temperatures, contrasting with pure gas models.

Findings

Post-shock gas temperature can be reduced to six times the ambient temperature.

Thermal X-ray emission can be suppressed by strong particle acceleration.

The overall remnant dynamics are relatively unaffected by particle acceleration.

Abstract

Context: Multiwavelength observations of supernova remnants can be explained within the framework of the diffusive shock acceleration theory, which allows effective conversion of the explosion energy into cosmic rays. Although the models of nonlinear shocks describe reasonably well the nonthermal component of emission, certain issues, including the heating of the thermal plasma and the related X-ray emission, remain still open. Aims: To discuss how the evolution and structure of supernova remnants is affected by strong particle acceleration at the forward shock. Methods: Analytical estimates combined with detailed discussion of the physical processes. Results: The overall dynamics is shown to be relatively insensitive to the amount of particle acceleration, but the post-shock gas temperature can be reduced to a relatively small multiple, even as small as six times, the ambient temperature with a very weak dependence on the shock speed. This is in marked contrast to pure gas models where the temperature is insensitive to the ambient temperature and is determined by the square of the shock speed. It thus appears to be possible to suppress effectively thermal X-ray emission from remnants by strong particle acceleration. This might provide a clue for understanding the lack of thermal X-rays from the TeV bright supernova remnant RX J1713.7-3946.