The luminosity of type I supernovae

The luminosity of type I supernovae is recalculated, and the energy deposition by both gamma rays and positrons is determined using a numerical Monte Carlo program. Three independent physical phenomena are considered: the diffusive release of thermal radiation that determines the height and width of the optical peak; the initial fast decay of the optical peak by a factor of roughly 100 as determined by the progressive transparency to gamma rays, and the apparent 56 day half-life that results from the progressive escape of the positron fraction of the 77 day Co-56 decay. To obtain agreement with observations, each phenomenon requires that the expanding nebula be described by a single relation yielding a value of 0.22 plus or minus 0.05 for the product of the ejected mass (in solar masses), and the expansion velocity (in units of 10 to the 9th cm/s) taken to the -2nd power. It is concluded that the type I supernova light curve with its exponential tail is well explained by the radioactive decay of Ni-56.