Radiative Transfer in Accretion Disks Under Irradiation of the Central Star

Abstract

Radiative transfer in a geometrically thin accretion disk, which is irradiated by a central star, is examined under a non-gray frequency-dependent treatment. The emergent intensity as well as other radiative quantities were analytically obtained under the Eddington approximation. In the case where the irradiation heating dominates the viscous one, the emergent spectra generally have two components. One is a multi-color blackbody part, which is reprocessed light of the irradiation flux, with a lower peak-frequency than that of the central star. The other is a single blackbody part, which is scattered light, with the same peak-frequency as that of the central star. When the vertical optical depth of the disk is small, the emergent intensity decreases in the poleward direction, but increases in the edgeward direction (limb-brightning), due to a scattering effect of the irradiated light. The overall spectra were calculated for a passive disk around a white dwarf and a neutron star. They exhibit multi-color blackbody spectra modified by the scattering light in the high-frequency region.