YOUNG SUPERNOVAE AS EXPERIMENTAL SITES FOR STUDYING THE ELECTRON ACCELERATION MECHANISM

Abstract

Radio emissions from young supernovae (~ 1 year after the explosion) show a peculiar feature in the relativistic electron population at a shock wave, where their energy distribution is steeper than typically found in supernova remnants (SNRs) and than the prediction from the standard diffusive shock acceleration (DSA) mechanism. This is especially established for a class of stripped envelope supernovae (SNe IIb/Ib/Ic) where a combination of high shock velocity and low circumstellar material (CSM) density makes it easier to derive the intrinsic energy distribution than in other classes of SNe. We suggest that this apparent discrepancy reflects the situation that the low energy electrons before accelerated by the DSA-like mechanism are responsible for the radio synchrotron emission from young SNe, and that studying young SNe sheds light on the still-unresolved electron injection problem in the acceleration theory of cosmic rays. We suggest that electron's energy distribution could be flattened toward the high energy, most likely around 100 MeV, which marks a transition from inefficient to efficient acceleration. Identifying this feature will be a major advance in understanding the electron acceleration mechanism. We suggest two further probes: (1) mm/sub-mm observations in the first year after the explosion, and (2) X-ray observations at about 1 year and thereafter. We show that these are reachable by ALMA and Chandra for nearby SNe.