The Structure of Cooling Fronts in Accretion Disks

Recent work has shown that the speed of the cooling front in soft X-ray transients may be an important clue in understanding the nature of accretion disk viscosity. In a previous paper (Vishniac & Wheeler), we derived the scaling law for the cooling front speed. Here we derive a similarity solution for the hot inner part of disks undergoing cooling. This solution is exact in the limit of a thin disk, power-law opacities, and a minimum hot state column density, which is an infinitesimal fraction of the maximum cold state density. For a disk of finite thickness, the largest error is in the ratio of the mass flow across the cooling front to the mass flow at small radii. Comparison to the numerical simulations of Cannizzo et al. indicates that the errors in other parameters do not exceed (csF/rFΩF)q, that is, the ratio of the sound speed at the disk midplane to its orbital velocity, evaluated at the cooling front, to the qth power. Here q ≈ ½. Its precise value is determined by the relevant hot state opacity law and the functional form of the dimensionless viscosity.