Expansion and ongoing cosmic ray acceleration in HESS J1731-347

TL;DR

This paper presents evidence of magnetic field amplification and ongoing cosmic ray acceleration in the supernova remnant HESS J1731-347, supporting its role in accelerating particles to very high energies.

Contribution

It provides observational evidence of magnetic field amplification and direct measurements of shock velocity, indicating active cosmic ray acceleration in HESS J1731-347.

Findings

Detection of fast X-ray synchrotron variability indicating strong magnetic fields.

Measurement of high shock expansion velocity in a wind-blown bubble.

Evidence supporting ongoing hadronic cosmic ray acceleration.

Abstract

Diffusive shock acceleration in supernova remnants (SNRs) is considered one of the prime mechanisms for Galactic Cosmic Ray (GCR) acceleration. It is still unclear, however, whether SNRs can contribute to GCR spectrum up to the ``knee'' (1\,PeV) band as acceleration to such energies requires an efficient magnetic field amplification process around the shocks. The presence of such a process is challenging to test observationally. Here we report on the detection of fast variability of the X-ray synchrotron emission from the forward shock in the supernova remnant HESS J1731-347, which implies the presence of a strong ($\sim$0.2\,mG) field exceeding background values and thus of effective field amplification. We also report a direct measurement of the high forward shock expansion velocity of 4000-5500\,km/s confirming that the SNR is expanding in a tenuous wind bubble blown by the SNR progenitor, is significantly younger (2.4-9\,kyr) than previously assumed by some authors, and only recently started interaction with the dense material outside the bubble. We finally conclude that there is strong evidence for ongoing hadronic CR acceleration in this SNR.