THE CRUCIAL EFFECTS OF ADVECTION ON THE STRUCTURE OF HOT ACCRETION DISKS

Optically thick, radiation pressure-dominated accretion disks are know n to undergo a secular instability and to evolve into optically thin c onfigurations that can account for the gross X-ray and gamma-ray chara cteristics of black hole systems such as Cyg X-1 and 1E 1740.7-2942. R ecent analyses have shown that in the inner, bremsstrahlung self-Compt onized portions of these disks, an electron-positron pair runaway can occur above a critical accretion rate. However, these studies have mos tly ignored the impact of advection on the structure of the hot disk. Ongoing observations of these sources by, e.g., the Compton Gamma Ray Observatory, and the upcoming timing studies of X-ray novae (such as N ova Muscae) with XTE, are motivating efforts to better understand the physics of these systems. In this paper, we include the crucial effect s of proton thermal energy advection and show that the disk structure is modified substantially. Pair runaway seems to be completely suppres sed. Instead we confirm the existence of a physically different critic al accretion rate above which no self-consistent steady state solution exists. We suggest that the hot disk is probably dynamic above this r ate, which may provide observationally significant timing signatures.