Probing Shock Breakout with Serendipitous GALEX Detections of Two SNLS Type II-P Supernovae

Understand

We report the serendipitous detection by GALEX of fast (<1 day) rising (>1 mag) UV emission from two Type II plateau (II-P) supernovae (SNe) at z=0.185 and 0.324 discovered by the Supernova Legacy Survey.

• Optical photometry and VLT spectroscopy 2 weeks after the GALEX detections link the onset of UV emission to the time of shock breakout.
• Using radiation hydrodynamics and non-LTE radiative transfer simulations, and starting from a standard red supergiant (RSG; Type II-P SN progenitor) star evolved self-consistently from the main sequence to iron core collapse, we model the shock breakout phase and the 55 hr that follow.
• The small scale height of our RSG atmosphere model suggests that the breakout signature is a thermal soft X-ray burst (lambda_peak ~ 90\AA) with a duration of <~ 2000 s.