* Astronomy

Title: Probing Shock Breakout with Serendipitous GALEX Detections of Two SNLS Type II-P Supernovae
Authors: Suvi Gezari, Luc Dessart, Stephane Basa, D. Chris Martin, James D. Neill, S.E. Woosley, D. John Hillier, Gurvan Bazin, Karl Forster, Peter G. Friedman, Jeremy Le Du, Alain Mazure, Patrick Morrissey, Susan G. Neff, David Schiminovich, Ted K. Wyder
(Version v2)

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 (SNLS). 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 hydrodymanics 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 hours that follow. Although our one-temperature treatment prevents us from modelling the radiative precursor, the small RSG atmospheric scale height suggests a < 2000 s duration. In our model, the breakout signature is a luminous (M_FUV ~ -20) thermal < 1 hr-long soft X-ray burst (lambda_peak ~ 90 A), not observed, but likely missed, by GALEX. However, GALEX unambiguously captured the subsequent emission. In particular, a 2-day long, post-breakout, UV-brightness plateau is observed in both SNe, and is a prediction of our model in which the shift of the peak of the spectral energy distribution (SED) from ~ 100 to ~ 1000 A and the ejecta expansion 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 our study we make predictions for the breakout detection rate of the GALEX Time Domain Survey.

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