Cosmic gamma-ray background from Type Ia supernovae revisited: Evidence for missing gamma-rays at MeV

The observed cosmic gamma-ray background at ~ MeV has often been attributed to Type Ia supernovae (SNIa). Since SNIa is close to a standard candle, one can calculate the gamma-ray intensity of SNIa integrated over redshifts fairly accurately, once the evolution of the SNIa rate is known. The latest SNIa rate measured at z < ~1.6 (Dahlen et al. 2004) indicates that the previous calculations of the gamma-ray background consistently overestimated the SNIa rate. With the new rate, we find that the SNIa contribution is an order of magnitude smaller than observed, and thus new population(s) of sources should be invoked.