2012ApJ...758...81M

In this paper, we develop a model for the radio and X-ray emissions from the 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 particle acceleration, the so-called diffusive shock acceleration (DSA) or Fermi mechanism, requires seed electrons with modest energy with γ ∼ 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., ε_e ≲ 10^–2 as compared to the conventionally assumed value of ε_e ∼ 0.1. Furthermore, independent of the low value of ε_e, we find that the X-ray luminosity cannot be attributed to any emission mechanisms suggested 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 γ ∼ 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.