Spectrum and light curve of a supernova shock breakout through a thick Wolf-Rayet wind

Abstract

Wolf-Rayet stars are known to eject winds. Thus, when a Wolf-Rayet star explodes as a supernova, a fast, $>30,000$ km/s, shock is expected to be driven through a wind. We study the signal expected from a fast supernova shock propagating through an optically thick wind, and find that the electrons behind the shock driven into the wind are cooled efficiently, by inverse Compton over soft photons that were deposited by the radiation mediated shock that crossed the star. Therefore, the bolometric luminosity is comparable to the kinetic energy flux through the shock, and the spectrum is found to be a power-law, which slope and frequency range depend on the number flux of soft photons available for cooling. Wolf-Rayet supernovae that explode through a thick wind have a high flux of soft photons, producing a flat spectrum, $\nu F_{\nu}=Const$, in the X-ray range $0.1\lesssim T\lesssim 50$ keV. As the shock expands into an optically thin wind, the soft photons are no longer able to cool the shock that plows through the wind, and the bulk of the emission takes the form of a standard core-collapse supernova (without a wind). However, a small fraction of the soft photons is upscattered by the shocked wind and produces a transient unique X-ray signature.