GAMMA-RAY TRANSFER AND ENERGY DEPOSITION IN SUPERNOVAE

Solutions to the energy-independent (gray) radiative transfer equations are compared to results of Monte Carlo simulations of the Ni-56 and Co-56 radioactive decay gamma-ray energy deposition in supernovae. The comparison shows that an effective, purely absorptive, gray opacity, kappa(gamma), similar to (0.06 +/- 0.01)Y-e cm(2) g(-1), where Y-e is the total number of electrons per baryon, accurately describes the interaction of gamma-rays with the cool supernova gas and the local gamma-ray energy deposition within the gas. The nature of the gamma-ray interaction process (dominated by Compton scattering in the relativistic regime) creates a weak dependence of kappa(gamma) on the optical thickness of the (spherically symmetric) supernova atmosphere: The maximum value of kappa(gamma) applies during optically thick conditions when individual gamma-rays undergo multiple scattering encounters and the lower bound is reached at the phase characterized by a total Thomson optical depth to the center of the atmosphere tau(e) less than or similar to 1. However, the constant asymptotic value, kappa(gamma) = 0.050Y(e) cm(2) g(-1), reproduces the thermal light curve due to gamma-ray deposition for Type Ia supernova models to within 10% for the epoch from maximum light to t = 1200 days. Our results quantitatively confirm that the quick and efficient solution to the gray transfer problem provides an accurate representation of gamma-ray energy deposition for a broad range of supernova conditions.