Contributions of AGNs and SNe la to the cosmic X-ray and gamma-ray backgrounds

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Abstract
Consequences of recent measurements of the MeV cosmic gamma-ray background by COMPTEL and EGRET aboard GRO are studied. First, we show that the intensities measured by COMPTEL are close to the intensities predicted by models of Type Ia supernovae (SNe Ia), and that future refined spectral measurements may be able to distinguish the SNe Ia contribution from that of active galactic nuclei (AGNs). This situation contrasts with that for pre-COMPTEL measurements, which yielded an MeV background intensity about an order of magnitude higher, rendering the SNe Ia contribution unimportant. Secondly, we show that inclusion of the contribution from SNe Ia (as well as of blazars) resolves a previous disagreement between theory and observations of the background at a few hundred keV: namely, unbeamed radio-quiet AGNs with thermally cut-off spectra can explain most of the X-ray background up to ∼ 100 keV, but the prediction of that model falls to about one-third of the observed intensity at 400 keV. This discrepancy is now resolved. Overall, the sum of the contributions of radio-quiet AGNs, SNe Ia and blazars agrees with the current measurements of the cosmic background spectrum over seven orders of magnitude, from ∼ 1 keV to 10 GeV. This also implies that a previously claimed necessity to invoke an exotic spectral component in the MeV range (e.g. from matter-antimatter annihilation in a symmetric universe) has disappeared (as a consequence of the COMPTEL measurements showing no MeV hump in the background spectrum).

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