Supernova shock breakout through a wind

Shmuel Balberg,Abraham Loeb

doi:10.1111/j.1365-2966.2011.18505.x

Abstract

The breakout of a supernova shock wave through the progenitor star's outer envelope is expected to appear as an X-ray flash. However, if the supernova explodes inside an optically thick wind, the breakout flash is delayed. We present a simple model for estimating the conditions at shock breakout in a wind based on the general observable quantities in the X-ray flash light curve; the total energy EX, and the diffusion time after the peak, tdiff. We base the derivation on the self-similar solution for the forward–reverse shock structure expected for an ejecta plowing through a pre-existing wind at large distances from the progenitor's surface. We find simple quantitative relations for the shock radius and velocity at breakout. By relating the ejecta density profile to the pre-explosion structure of the progenitor, the model can also be extended to constrain the combination of explosion energy and ejecta mass. For the observed case of XRO08109/SN2008D, our model provides reasonable constraints on the breakout radius, explosion energy and ejecta mass, and predicts a high shock velocity which naturally accounts for the observed non-thermal spectrum.

Highlights

As a supernova shock wave propagates through the progenitor star, it eventually emerges through the outer envelope region which has a low optical depth
We have presented a simple model for the properties of shock breakout in a supernova explosion embedded within an optically thick wind
By assuming that the ejecta and wind form a forward–reverse shock structure (Chevalier 1982), we have shown that the breakout radius can be estimated based on the observationally determinable values of the total energy in the flash and the diffusion time-scale from the shock to the wind’s photosphere

Summary

INTRODUCTION

As a supernova shock wave propagates through the progenitor star, it eventually emerges through the outer envelope region which has a low optical depth. We apply the known self-similar solution of a supernova ejecta moving into a pre-existing wind material with a density profile of ρ ∝ r−2 (Chevalier 1982), and demonstrate how it can be used to relate the key observable features of the X-ray flash, the total energy EX and the diffusion time-scale tdiff to the parameters of the shock breakout. Where g and m are constants which depend on the initial conditions of the progenitor and the explosion If both the ejecta and the wind material are initially cold, a self-similar solution can be found to describe for the structures of both the forward shock (Parker 1963) and the reverse shock (Chevalier 1982).

THEFO RWA R D – REVERSESHOCK STRUCTURE

A SIMPLE PARAMETRIZATION OF THE BREAKOUT CONDITIONS

A SIMPLE PARAMETRIZATION OF THE EXPLOSION PROPERTIES

Findings

CONCLUSIONS AND DISCUSSION