Compton Degradation of Gamma-ray Line Emission from Radioactive Isotopes in the Classical Nova V2491 Cygni

Abstract

To account for the non-thermal emission from the classical nova V2491 Cygni, we perform a series of numerical calculations of radiative transfer of γ-ray photons from the radioactive isotope ²²Na in the matter ejected from a white dwarf. Using a simple wind model for the dynamical evolution of the ejecta and a Monte Carlo code, we calculate radiative transfer of the γ-ray photons in the ejecta. Repeated scattering of the γ-ray photons by electrons in the ejecta, i.e., Compton degradation, results in an extremely flat spectrum in the hard X-ray range, which successfully reproduces the observed spectrum of the X-ray emission from V2491 Cygni. The amount of the isotope ²²Na synthesized in the ejecta is required to be 3 × 10^-5 M _sun to account for the flux of the hard X-ray emission. Our model indicates that the ejecta become transparent to the γ-ray photons within several tens days. Using the results, we briefly discuss the detectability of the γ-ray line emission by the INTEGRAL gamma-ray observatory and the Fermi Gamma-ray Space Telescope.