SEEING THE COLLISION OF A SUPERNOVA WITH ITS COMPANION STAR

The progenitors of Type Ia and some core collapse supernovae are thought to be stars in binary systems, but little direct observational evidence exists to confirm the hypothesis. We show that the collision of the supernova ejecta with its companion star should produce detectable emission in the hours and days following the explosion. The interaction occurs at distances similar to 10(11)-10(13) cm and shocks the impacting supernova debris, dissipating kinetic energy and reheating the gas. Initially, some radiation may escape promptly through the evacuated region of the shadowcone, producing a bright X-ray (0.1-2 keV) burst lasting minutes to hours with luminosity L similar to 10(44) ergs s(-1). Continuing radiative diffusion from deeper layers of shock-heated ejecta produces a longer lasting optical/UV emission, which exceeds the radioactively powered luminosity of the supernova for the first few days after the explosion. These signatures are prominent for viewing angles looking down upon the shocked region, or about 10% of the time. The properties of the emission provide a straightforward measure of the separation distance between the stars and hence (assuming Roche lobe overflow) the companion's radius. Current optical and UV data sets likely already constrain red giant companions. By systematically acquiring early time data for many supernovae, it may eventually be possible to empirically determine how the parameters of the progenitor system influence the outcome of the explosion.