Vertical Radiative Transfer in Accretion Disks: Roles of Scattering Effects

Abstract

Radiative transfer in a geometrically thin accretion disk with finite optical depth is reconsidered under a non-gray treatment, while focusing attention on the scattering effect. The emergent intensity as well as other radiative quantities are analytically obtained in simple and uniform-heating cases. In the simple case with a given Planck function, for a vertically isothermal atmosphere, the scattering effect is important, and the emergent intensity becomes a modified blackbody spectrum, as in many of the traditional cases. For a non-isothermal atmosphere, however, the scattering effect is not important, but the emergent intensity is mainly affected by the disk optical depth. In the case with uniform heating, the scattering effect is not important, as long as the disk optical depth is sufficiently large. For a finite optical depth, however, we find that the combination effect of the scattering and the finite optical depth drastically change the emergent intensity and the radiative quantities. Since the accretion disk is generally non-isothermal in the vertical direction, or has a finite optical depth, we should carefully calculate the disk spectra, while taking into account radiative transfer in the disk.