Synchrotron X-ray diagnostics of cutoff shape of nonthermal electron spectrum at young supernova remnants

TL;DR

This paper develops analytical models for the cutoff shape of nonthermal electron spectra in young supernova remnants, aiming to identify the dominant acceleration limiting mechanism through future X-ray observations.

Contribution

It derives formulas for the cutoff shape based on different limiting mechanisms and discusses how upcoming X-ray data can constrain the acceleration process and magnetic field strength.

Findings

Analytical formulas relate cutoff shape to acceleration mechanisms.

Future X-ray observations can distinguish between different electron cutoff models.

Constraints on magnetic field strength around SNR shocks can be obtained.

Abstract

Synchrotron X-rays can be a useful tool to investigate electron acceleration at young supernova remnants (SNRs). At present, since the magnetic field configuration around the shocks of SNRs is uncertain, it is not clear whether electron acceleration is limited by SNR age, synchrotron cooling, or even escape from the acceleration region. We study whether the acceleration mechanism can be constrained by the cutoff shape of the electron spectrum around the maximum energy. We derive analytical formulae of the cutoff shape in each case where the maximum electron energy is determined by SNR age, synchrotron cooling and escape from the shock. They are related to the energy dependence of the electron diffusion coefficient. Next, we discuss whether information on the cutoff shape can be provided by observations in the near future which will simply give the photon indices and the flux ratios in the soft and hard X-ray bands. We find that if the power-law index of the electron spectrum is independently determined by other observations, then we can constrain the cutoff shape by comparing theoretical predictions of the photon indices and/or the flux ratios with observed data which will be measured by NuSTAR and/or ASTRO-H. Such study is helpful in understanding the acceleration mechanism. In particular, it will supply another independent constraint on the magnetic field strength around the shocks of SNRs.