Relativistic shock breakout from a stellar wind

TL;DR

This paper develops an analytic model for relativistic shock breakout from stellar winds, predicting observable gamma-ray signals that could be detected by current satellites within 10-100 Mpc.

Contribution

It introduces a novel analytic framework for relativistic shock breakout, incorporating photon escape and pair creation effects, and predicts observational signatures.

Findings

Relativistic breakout occurs at optical depths ~ (me/mp)Γ_sh.

About 10^48 ergs are emitted as MeV gamma-rays during breakout.

Signals can be detected up to 10-100 Mpc with current gamma-ray satellites.

Abstract

We construct an analytic model for the breakout of a relativistic radiation mediated shock from a stellar wind, and exploit it to calculate the observational diagnostics of the breakout signal. The model accounts for photon escape through the finite optical depth wind, and treats the fraction of downstream photons escaping to infinity as an adiabatic parameter that evolves in a quasi-steady manner. It is shown that the shock is mediated by radiation even when a large fraction of the downstream photons escape, owing to self-generation and adjustment of opacity through accelerated pair creation. Relativistic breakout occurs at radii at which the total optical depth of the wind ahead of the shock is $\sim (m_e/m_p)\Gamma_{sh}$, provided that the local shock Lorentz factor $\Gamma_{sh}$ exceeds unity at this location. Otherwise the breakout occurs in the Newtonian regime. A relativistic breakout is expected in a highly energetic spherical explosion ($10^{52}-10^{53}$ erg) of a Wolf-Rayet star, or in cases where a smaller amount of energy ($\sim 10^{51}$ erg) is deposited by a jet in the outer layers of the star. The properties of the emission observed in such explosions during the relativistic breakout are derived. We find that for typical parameters about $10^{48}$ ergs are radiated in the form of MeV gamma-rays over a duration that can range from a fraction of a second to an hour. Such a signal may be detectable out to 10-100 Mpc by current gamma-ray satellites.