Probing Shock Breakout with Serendipitous GALEX Detections of Two SNLS Type II-P Supernovae
Résumé
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 modeling the radiative precursor, the small RSG atmospheric scale height suggests a < 2000 s duration. A duration of many hours would require an extended low-density envelope, incompatible with our RSG envelope structure. 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.