The origin of the cosmic gamma-ray background in the MeV range

Pilar Ruiz-Lapuente, Lih-Sin The, Dieter Hartmann, Marco Ajello, Ramon Canal, Friedrich K Röpke, Sebastian T. Ohlmann, Wolfgang Hillebrandt

Published 2015-02-21Version 1

There has been much debate about the origin of the diffuse gamma--ray background in the energy range from a few hundred keV to 10 MeV. At lower energies, AGNs and Seyfert galaxies can explain the background, but their contribution cuts off above $\simeq$ 0.3 MeV. In the MeV range, the spectrum drops sharply for increasing energies. It flattens beyond $\sim$ 10 MeV, and blazars appear to account for the fluxes observed there. That leaves an unexplained window for which different candidate sources have been proposed, including annihilations of weakly interacting massive particles (WIMPS). One candidate are Type Ia supernovae (SNe Ia). Previous estimates of SNe Ia to the cosmic gamma--ray background were based on a restricted number of SN Ia explosion models and, on very limited measurements of the SN Ia rates as a function of redshift $z$. In the present work, we use a wide variety of explosion models and the most recent measurements of the SN Ia rates, which now cover a wide redshift interval. If we adopt the central values of the current measurements of the SN Ia rates, the SN still fall short of the observed background, which, however, has the same spectral shape as the predicted one. Only for a fit running between the 1\,$\sigma$ and 2\,$\sigma$ upper limits of the highest observed SN Ia rates, with their large error bars, it is possible an agreement between model prediction and observations of the gamma--ray background. Uncertainties are discussed and the conclusion is that SNe Ia do, in any case, make a non--negligible contribution to the cosmic gamma--ray background in the MeV range.