The Spectrum of a Fast Shock Breakout from a Stellar Wind

TL;DR

This paper develops a semi-analytic model for the X-ray emission from fast, sub-relativistic shock breakouts in stellar winds, revealing non-thermal spectra and temperature evolution during the event.

Contribution

It introduces a new semi-analytic model that accounts for photon escape and predicts spectral and temperature evolution in shock breakout events from stellar winds.

Findings

Breakout spectra are non-thermal for shocks exceeding 0.1c.

The temperature decreases significantly during breakout.

A closure relation links duration, luminosity, and temperature of the breakout.

Abstract

The breakout of a fast ($>0.1 c$), yet sub-relativistic shock from a thick stellar wind is expected to produce a pulse of X-rays with a rise time of seconds to hours. Here, we construct a semi-analytic model for the breakout of a sub-relativistic, radiation-mediated shock from a thick stellar wind, and use it to compute the spectrum of the breakout emission. The model incorporates photon escape through the finite optical depth wind, assuming a diffusion approximation and a quasi-steady evolution of the shock structure during the breakout phase. We find that in sufficiently fast shocks, for which the breakout velocity exceeds about $0.1c$, the time-integrated spectrum of the breakout pulse is non-thermal, and the time-resolved temperature is expected to exhibit substantial decrease (roughly by one order of magnitude) during breakout, when the flux is still rising, because of the photon generation by the shock compression associated with the photon escape. We also derive a closure relation between the breakout duration, peak luminosity, and characteristic temperature that can be used to test whether an observed X-ray flare is consistent with being associated with a sub-relativistic shock breakout from a thick stellar wind or not. We also discuss implications of the spectral softening for a possible breakout event XRT 080109/SN 2008D.