COSMIC X-RAY-BACKGROUND FROM HOT GAS

In this paper we consider constraints on models of the cosmic X-ray background (XRB) in which the XRB is produced by optically thin thermal bremsstrahlung from hot gas. We show that models in which the gas is gravitationally confined in a spherical configuration and is heated only once are contradicted by the observed number of gravitationally lensed quasars together with the lower limit on the number of XRB sources required by limits on fluctuations in the XRB and the cosmic microwave background (CMB). In addition, we show that for models in which the gas is not gravitationally confined, the expansion time of the gas is much shorter than the radiative cooling time, so that such models cannot explain the XRB. We conclude that thermal bremsstrahlung models cannot account for the XRB if the emitting gas is heated only once.