X-ray bursts from Type II supernovae

X-ray burst emission during the shock breakout phase of a Type II supernova is investigated for a 12-solar-mass red giant at the end of its nuclear evolution. A supernova model based on a one-dimensional spherically symmetric hydrodynamic code with fine zoning is employed, the detailed time-dependent evolution of the supernova is calculated, and the time-dependent development of the emitted spectrum is computed. By using the calculated structure of the model at any given time, a formal solution of the non-LTE equation of radiative transfer is performed frequency by frequency for the mean intensity. Spectra of the emergent radiation near luminosity maximum and at a somewhat later time are plotted and compared with blackbody spectra for temperatures corresponding to a Rosseland optical depth of 0.67. It is found that a precursor burst of soft X radiation (0.2 to 0.5 keV) with a duration of 1000 sec, a peak luminosity corresponding to a blackbody at 230,000 K, and a distinctly non-Planckian spectrum emerges from the photosphere within 75,000 sec of the explosion. The potential observability of such a burst with an X-ray satellite is estimated.