Cosmic diffuse gamma-ray background in the MeV regime

The origin of the diffuse Gamma-ray background in the MeV regime has been an intriguing question in the scientific community with the primary candidate initially believed to be a thermonuclear supernovae (SNIa). However, the contribution from SNIa is not adequate to explain the observed MeV gamma-ray background. We revisit this issue and address the origin of the cosmic gamma-ray background (CGB) in the MeV regime by adding contributions from SNIa along with other sources such as radio quiet Active Galactic Nuclei (AGN), Flat spectrum radio quasars (FSRQs), core collapse supernovae (ccSNe), and Neutron star - neutron star (NS-NS) mergers. We show that the gamma-ray background is explained above 1.5 MeV. However, the modeled diffuse gamma-ray background is a few factors (~3) lower than the observed background in the range 0.1 MeV to 1.0 MeV highlighting the need for future missions to establish the CGB spectrum more reliably and to identify additional sources or even source classes in the under-explored MeV band.