Shock steepening and prompt thermal emission in supernovae

Abstract

A calculation of supernova shock propagation down the exponential density gradient at the outside of the presupernova giant envelope predicts a strong quasi-thermal burst of ultraviolet and soft X-ray luminosity accompanying the emergence of the radiative shock at the stellar photosphere. Burst duration is of the order of several thousand seconds, with characteristic luminosities of 10 to the 45th power ergs/s in the thermal continuum, peaked at about 500 A. Total UV energy radiated is of the order of several times 10 to the 48th power ergs. Expansion velocities in the outer 0.005 solar mass of material reach nearly 10,000 km/s, in excellent agreement with peak values inferred from Doppler-shifted lines in the spectra of these objects. No underlying source of energy (pulsar, etc.) is required; the simple shock-wave model for supernova light curves can account for all of the major features of observed Type II events. A hard X-ray burst (kT about 10-100 keV) results from the transition to an ion-viscous shock in the outermost surface layers.