Three-dimensional Simulation of Gamma-Ray Emission from Asymmetric Supernovae and Hypernovae

Abstract

Hard X- and γ-ray spectra and light curves resulting from radioactive decays are computed for aspherical (jetlike) and energetic supernova models (representing the prototypical hypernova SN 1998bw), using a three-dimensional energy- and time-dependent Monte Carlo scheme. The emission is characterized by (1) early emergence of high-energy emission, (2) large line-to-continuum ratio, and (3) large cutoff energy by photoelectric absorption in hard X-ray energies. These three properties are not sensitively dependent on the direction to the observer. On the other hand, fluxes and line profiles depend sensitively on the direction to the observer, showing larger luminosity and larger degree of blueshift for an observer closer to the polar (z) direction. Strategies to derive the degree of asphericity and the direction to the observer from (future) observations are suggested on the basis of these features, and an estimate of the detectability of the high-energy emission by INTEGRAL and future observatories is presented. Also presented is an examination of the applicability of a gray effective γ-ray opacity for computing the energy deposition rate in the aspherical SN ejecta. Detailed three-dimensional computations show that an effective γ-ray opacity κ_γ~0.025-0.027 cm² g^-1 reproduces the detailed energy-dependent transport for both spherical and aspherical (jetlike) geometry.