Injection and Acceleration of Electrons at A Strong Shock: Radio and X-ray Study of Young Supernova 2011dh

TL;DR

This study models radio and X-ray emissions from Supernova 2011dh, revealing low electron acceleration efficiency and a need for a peaked pre-acceleration spectrum to explain observations.

Contribution

It introduces a new model for pre-accelerated electron spectra and links pre-acceleration to magnetic field amplification in supernova shocks.

Findings

Electron acceleration efficiency is less than 1%.

Pre-acceleration spectrum peaks at gamma ~ 20-30.

Radio and X-ray emissions are explained by synchrotron and inverse Compton processes.

Abstract

In this paper, we develop a model for the radio and X-ray emissions from Type IIb Supernova (SN IIb) 2011dh in the first 100 days after the explosion, and investigate a spectrum of relativistic electrons accelerated at a strong shock wave. The widely-accepted theory of the particle acceleration, so-called diffusive shock acceleration (DSA) or Fermi mechanism, requires seed electrons with modest energy with gamma ~ 1 - 100, and little is known about this pre-acceleration mechanism: We derive the energy distribution of relativistic electrons in this pre-accelerated energy regime. We find that the efficiency of the electron acceleration must be low, i.e., epsilon_e <~ 0.01 as compared to the conventionally assumed value of epsilon_e ~ 0.1. Furthermore, independently from the low value of epsilon_e, we find that the X-ray luminosity cannot be attributed to any emission mechanisms suggested so far as long as these electrons follow the conventionally-assumed single power-law distribution. A consistent view between the radio and X-ray can only be obtained if the pre-acceleration injection spectrum peaks at gamma ~ 20-30 and then only a fraction of these electrons eventually experience the DSA-like acceleration toward the higher energy -- then the radio and X-ray properties are explained through the synchrotron and inverse Compton mechanisms, respectively. Our findings support the idea that the pre-acceleration of the electrons is coupled with the generation/amplification of the magnetic field.