Hard X‐Rays and Gamma Rays from Type Ia Supernovae

Abstract

The γ-ray light curves and spectra are presented for a set of theoretical Type Ia supernova (SN Ia) models including deflagration, detonation, delayed detonation, and pulsating delayed detonations of Chandrasekhar-mass white dwarfs, as well as merger scenarios that may involve more than the Chandrasekhar mass and helium detonations of sub-Chandrasekhar-mass white dwarfs. The results have been obtained with a Monte Carlo radiation transport scheme that takes into account all relevant γ-transitions and interaction processes. The result is a set of accurate line profiles that are characteristic of the initial 56Ni mass distribution of the supernova models. The γ-rays probe the isotopic rather than merely the elemental distribution of the radioactive elements in the ejecta. Details of the line profiles (including the line width, shift with respect to the rest frame, and line ratios) are discussed. With sufficient energy and temporal resolution, different model scenarios can clearly be distinguished. Observational strategies are discussed for current and immediately upcoming generations of satellites (Compton Gamma Ray Observatory [CGRO] and International Gamma-Ray Astrophysical Laboratory [INTEGRAL]), as well as projected future missions including presently unavailable equipment such as Laue telescopes. With CGRO, it is currently possible with sufficiently early observations (near optical maximum) to distinguish helium detonations from explosions of Chandrasekhar-mass progenitors and of those involving mergers up to a distance of about 15 Mpc. This translates into one target of opportunity every 8 years. SNe Ia up to about 10 Mpc would allow detailed CGRO studies of line ratios of 56Co lines. INTEGRAL will be able to perform detailed studies of the 56Co line profiles with a range comparable to CGRO. The superior sensitivity of INTEGRAL for low energies makes detection and detailed study of the positron annihilation line and appropriate low-energy 56Ni lines possible up to about 10-15 Mpc for all models. This capability means that this lower energy range may be the most useful for INTEGRAL detection and study of SNe Ia. Such studies will allow the determination of the precise time of the explosion. Whereas the current generation of γ-ray detectors will allow the study of supernovae that are discovered by other means, a new generation of proposed γ-ray detectors with sensitivity of about 10-6 photons s-1 cm-2 would generate the opportunity to discover supernovae by their γ-ray emission up to a distance of ≈ 100 Mpc. This would allow a systematic study of the variety of SNe Ia in terms of their γ-ray properties, independent of their optical properties. In addition, since γ-rays are not obscured by the host galaxy, such experiments would, for the first time, provide absolute supernova rates. Relative rates as a function of the morphology of and position in the host galaxy could be studied directly.