2023A&A...679A.152D

Diffusive shock acceleration in supernova remnants (SNRs) is considered one of the prime mechanisms of galactic cosmic ray (GCR) acceleration. It is still unclear, however, whether SNRs can contribute to the 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 in the X-ray synchrotron emission from the forward shock in the SNR HESS J1731-347, which implies the presence of a strong (∼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^–1, 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 interacting with the dense material outside of the bubble. We finally conclude that there is strong evidence for ongoing hadronic GCR acceleration in this SNR.