Contributions of starburst galaxies and reflection-dominated active galactic nuclei to the cosmic x-ray background

Starburst galaxies and low-luminosity reflection-dominated active galactic nuclei (AGNs) have been proposed as models of the cosmic X-ray background (CXB). We calculate the CXB spectrum to be expected from an evolving population of both massive and low-mass X-ray binaries in starburst galaxies, and find that starbursts cannot produce much of the background at low energies (≤20 keV) without significant evolution. We find their contribution at higher energies to be negligible, even with strong evolution. We also investigate the consistency of joint starburst and AGN models with the spectrum of the CXB between 3 and 500 keV, by means of detailed χ² fits. If X-ray binaries in starburst galaxies do produce much of the low-energy CXB, some other type of source must be responsible for the high-energy background. If reflecting AGN are this other type of source, up to 89% of their emission must be reprocessed. Even without a starburst contribution, the AGN spectrum must still be about 86% reprocessed. However, for the simple cold disk reflection AGN models currently considered, the χ² fit statistical significance is too low, indicating the need for either alternative models or new elements in the standard cold disk reflection model.