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To account for the non-thermal emission from the classical nova V2491 Cygni,we perform a series of numerical calculations of radiative transfer of\gamma-ray photons from the radioactive isotope $^{22}$Na in the matter ejectedfrom a white dwarf. Using a simple wind model for the dynamical evolution ofthe ejecta and a monte-carlo code, we calculate radiative transfer of the\gamma-ray photons in the ejecta. Repeated scattering of the \gamma-ray photonsby electrons in the ejecta, i.e., Compton degradation, results in an extremelyflat spectrum in the hard X-ray range, which successfully reproduces theobserved spectrum of the X-ray emission from V2491 Cygni. The amount of theisotope $^{22}$Na synthesized in the ejecta is required to be 3\times 10^{-5}M_\odot to account for the flux of the hard X-ray emission. Our model indicatesthat the ejecta become transparent to the \gamma-ray photons within severaltens days. Using the results, we briefly discuss the detectability of the\gamma-ray line emission by the {\it INTEGRAL} gamma-ray observatory and the{\it Fermi} Gamma-ray Space Telescope.

Compton degradation of gamma-ray line emission from radioactive isotopes in the classical nova V2491 Cygni