Electron Cooling in a Young Radio Supernova: SN 2012aw

Abstract

We present the radio observations and modeling of an optically bright Type II-P supernova (SN), SN 2012aw which exploded in the nearby galaxy Messier 95 (M95) at a distance of 10 Mpc. The spectral index values calculated using C, X, and K bands are smaller than the expected values for the optically thin regime. During this time, the optical bolometric light curve stays in the plateau phase. We interpret the low spectral-index values to be a result of electron cooling. On the basis of comparison between the Compton cooling timescale and the synchrotron cooling timescale, we find that the inverse Compton cooling process dominates over the synchrotron cooling process. We therefore model the radio emission as synchrotron emission from a relativistic electron population with a high energy cutoff. The cutoff is determined by comparing the electron cooling timescale, t cool, and the acceleration timescale, $\tilde{t}_{{\rm acc}}$. We constrain the mass-loss rate in the wind ($\dot{M}\sim 1.9\times 10^{-6}\ M_{\odot }\,{\rm yr}^{-1}$) and the equipartition factor between relativistic electrons and the magnetic field ($\tilde{\alpha }=\epsilon _e/\epsilon _B\sim 1.12\times 10^2$) through our modeling of radio emission. Although the time of explosion is fairly well constrained by optical observations within about two days, we explore the effect of varying the time of explosion to best fit the radio light curves. The best fit is obtained for the explosion date as 2012 March 15.3 UT.