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We report the serendipitous detection by GALEX of fast (<1 day) rising (>1mag) UV emission from two Type II plateau (II-P) supernovae (SNe) at z=0.185and 0.324 discovered by the Supernova Legacy Survey. Optical photometry and VLTspectroscopy 2 weeks after the GALEX detections link the onset of UV emissionto the time of shock breakout. Using radiation hydrodynamics and non-LTEradiative transfer simulations, and starting from a standard red supergiant(RSG; Type II-P SN progenitor) star evolved self-consistently from the mainsequence to iron core collapse, we model the shock breakout phase and the 55 hrthat follow. The small scale height of our RSG atmosphere model suggests thatthe breakout signature is a thermal soft X-ray burst (lambda_peak ~ 90\AA) witha duration of <~ 2000 s. Longer durations are possible but require either anextended and tenuous non-standard envelope, or an unusually dense RSG wind with\dot{M} ~ 10^(-3) Msun yr^(-1). The GALEX observations miss the peak of theluminous (M_FUV ~ -20) UV burst but unambiguously capture the rise of theemission and a subsequent 2 day long plateau. The postbreakout, UV-brightplateau is a prediction of our model in which the shift of the peak of thespectral energy distribution (SED) from ~100 to ~1000\AA and the ejectaexpansion both counteract the decrease in bolometric luminosity from ~10^11 to~10^9 L_sun over that period. Based on the observed detection efficiency of ourstudy we make predictions for the breakout detection rate of the GALEX TimeDomain Survey.

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