Monte-Carlo simulations of fast Newtonian and mildly relativistic shock breakout from a stellar wind

TL;DR

This paper presents Monte-Carlo simulations of shock breakout emissions from stellar winds, revealing how shock velocity influences spectra and lightcurves, with implications for observations of supernovae like SN 2008D.

Contribution

First self-consistent Monte-Carlo simulations of spectra and lightcurves for fast Newtonian and mildly relativistic shock breakouts, incorporating radiative losses and shock dynamics.

Findings

Peak energy of emission scales with shock velocity from 1 keV to 100 keV.

Spectrum below peak is nearly flat, indicating bright optical/UV emission.

Lightcurves show a gradual rise over tens to hundreds of seconds.

Abstract

Strong explosion of a compact star surrounded by a thick stellar wind drives a fast ($>0.1c$) radiation mediated shock (RMS) that propagates in the wind, and ultimately breaks out gradually once photons start escaping from the shock transition layer. In exceptionally strong or aspherical explosions the shock velocity may even be relativistic. The properties of the breakout signal depend on the dynamics and structure of the shock during the breakout phase. Here we present, for the first time, spectra and lightcurves of the breakout emission of fast Newtonian and mildly relativistic shocks, that were calculated using self-consistent Monte-Carlo simulations of finite RMS with radiative losses. We find a strong dependence of the $\nu F_\nu$ peak on shock velocity, ranging from $\sim 1$ keV for $v_s/c=0.1$ to $\sim 100$ keV for $v_s/c=0.5$, with a shift to lower energies as losses increase. For all cases studied the spectrum below the peak exhibits a nearly flat component ($F_\nu \sim \nu^0$) that extends down to the break frequency below which absorption becomes important. This implies much bright optical/UV emission than hitherto expected. The computed lightcurves show a gradual rise over tens to hundreds of seconds for representative conditions. The application to SN 2008D/XRT 080109 and the detectability limits are also discussed. We predict a detection rate of about one per year with eROSITA.