ION-SUPPORTED TORI - A THERMAL BREMSSTRAHLUNG MODEL FOR THE X-RAY-BACKGROUND

We discuss the possibility that a significant contribution to the hard X-ray background is the result of the integrated emission from a popu lation of galaxies undergoing advection-dominated accretion in their n uclei. Owing to poor coupling between ions and electrons and to effici ent radiative cooling of the electrons, the accreting plasma is two-te mperature, with the ions being generally much hotter than the electron s and forming an ion-supported torus. We show that the electron temper ature then saturates at approximate to 100 keV, independent of model p arameters. At this temperature the hard X-ray emission can be dominate d by bremsstrahlung radiation. A bremsstrahlung model with that temper ature gives an excellent fit to the spectrum of the XRB in the 3-60 ke V range, provided that there is some evolution associated with the spe ctral emissivity, which must peak at a redshift similar to 2. The aver age luminosity of objects implied by such a contribution is high for t he model sources, requiring high black hole masses and accretion rates . These constraints are relaxed if the gas is clumped, which is plausi ble when the accretion rate is close to the critical rate for the form ation of an advection-dominated flow. We speculate that the X-ray back ground is due to an advection-dominated accretion phase which follows the quasar phase for massive black holes.