The Crucial Effects of Advection on the Structure of Hot Accretion Disks

Optically thick, radiation pressure-dominated accretion disks are known to undergo a secular instability and to evolve into optically thin configurations that can account for the gross X-ray and γ-ray characteristics of black hole systems such as Cyg X-1 and 1E 1740.7-2942. Recent analyses have shown that in the inner, bremsstrahlung self-Comptonized portions of these disks, an electron-positron pair runaway can occur above a critical accretion rate. However, these studies have mostly 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 Nova Muscae) with XTE, are motivating efforts to better understand the physics of these systems. In this paper, we include the crucial effects of proton thermal energy advection and show that the disk structure is modified substantially. Pair runaway seems to be completely suppressed. Instead we confirm the existence of a physically different critical accretion rate above which no self-consistent steady state solution exists. We suggest that the hot disk is probably dynamic above this rate, which may provide observationally significant timing signatures.