The Diffuse Gamma-Ray Background from Supernovae

The diffuse extragalactic γ-ray background in the MeV region is believed to be due to photons from radioactivity produced in supernovae throughout the history of galaxies in the universe. In particular, γ-ray line emission from the decay chain 56Ni→56Co→56Fe provides the dominant photon source (Clayton & Silk). Although iron synthesis occurs in all types of supernovae, the contribution to the background is dominated by Type Ia events due to their higher photon escape probabilities. Estimates of the star formation history in the universe suggest a rapid increase by a factor ~10 from the present to a redshift zp ~1.5, beyond which it either remains constant or decreases slowly. Little is known about the cosmological star formation history for redshift exceeding z ~5. We integrate the observed star formation history to determine the cosmic γ-ray background (CGB) from the corresponding supernova rate history. In addition to γ-rays from short-lived radioactivity in Type Ia supernovae (SN Ia's) and Type II, Ib, and Ic supernovae (SN II's, SN Ib's, SN Ic's) we also calculate the minor contributions from long-lived radioactivities (26Al, 44Ti, 60Co, and electron-positron pair annihilation). The time-integrated γ-ray spectrum of model W10HMM (Pinto & Woosley) was used as a template for Type II supernovae, and for SN Ia's we employ model W7 (Nomoto et al.). Although progenitor evolution for Type Ia supernovae is not yet fully understood, various arguments suggest delays of order 1-2 Gyr between star formation and the production of SN Ia's. The effect of this delay on the CGB is discussed. We emphasize the value of γ-ray observations of the CGB in the MeV range as an independent tool for studies of the cosmic star formation history. If the delay between star formation and SN Ia activity exceeds 1 Gyr substantially and/or the peak of the cosmic star formation rate occurs at a redshift much larger than unity, the γ-ray production of SN Ia's would be insufficient to explain the observed CGB and a so far undiscovered source population would be implied. Alternatively, the cosmic star formation rate would have to be higher (by a factor 2-3) than commonly assumed, which is in accord with several upward revisions reported in the recent literature.