On the supernova remnants with flat radio spectra

TL;DR

This paper reviews the phenomenon of flat radio spectra in Galactic supernova remnants, discussing various models and emphasizing the need for new high-frequency observations to clarify the underlying mechanisms.

Contribution

It provides a comprehensive overview of existing explanations for flat SNR spectra and highlights the importance of high-energy observations for understanding these remnants.

Findings

Multiple models explain flat spectra, including Fermi acceleration and thermal effects.

Current data are insufficient to conclusively determine the dominant mechanism.

High-frequency radio observations are crucial for resolving the debate.

Abstract

A considerable fraction of Galactic supernova remnants (SNRs) characterize flat spectral indices ($\alpha<0.5$). There are several explanations of the flat radio spectra of SNRs in the present literature. The most of models involve a significant contribution of the second-order Fermi mechanism but some of them also discuss high compressions (>4), contribution of secondary electrons left over from the decay of charged pions, as well as the possibility of thermal contamination. In the case of expansion in high density environment, intrinsic thermal bremsstrahlung could theoretically shape the radio spectrum of an SNR and also account for observable curved -- "concave up" radio spectra of some Galactic SNRs. This model could also shed a light on the question of flat spectral indices determined in some Galactic SNRs. On the other hand, present knowledge of the radio continuum spectra (integrated flux densities at different frequencies) of SNRs prevent definite conclusions about the significance of proposed models so the question on flat spectral indices still remains open. New observations, especially at high radio continuum frequencies, are expected to solve these questions in the near future. Finally, as there is a significant connection between the majority of Galactic SNRs with flat integrated radio spectrum and their detection in $\gamma$-rays as well as detection of radiative recombination continua in their X-ray spectra, the analysis of high energy properties of these SNRs is very important.