Analytical Solutions of Radiative Transfer Equations in Accretion Discs with Finite Optical Depth

TL;DR

This paper derives analytical solutions for radiative transfer in accretion discs with finite optical depth, examining effects of scattering, optical depth, and internal heating on radiative quantities and emergent spectra.

Contribution

It provides new analytical solutions for radiative transfer in accretion discs considering finite optical depth and scattering effects, enhancing understanding of their vertical structure.

Findings

Surface mean intensity varies with Eddington factor assumptions.

Scattering has negligible effect in radiative equilibrium case.

Optical depth influences the emergent spectrum significantly.

Abstract

The main purpose of this paper is to obtain analytical solutions for radiative transfer equations related to the vertical structure of accretion discs with finite optical depth. In the non-gray atmosphere, we employ the optical-depth dependent Eddington factor to define the relationship between the mean intensity and radiation stress tensor. Analytical solutions are achieved for two cases: (i) radiative equilibrium, and (ii) a disc with uniform internal heating and both cases are assumed to be in local thermodynamical equilibrium (LTE), too. These solutions enable us to study the probable role of scattering and disc optical depth on the emergent intensity and other radiative quantities. Our results show that for the first case, the surface value of mean intensity with constant Eddington factor is three times larger than that with a variable factor. Moreover, scattering has no role in the vertical radiative structure of discs with the assumptions of the first case. On the other hand, for the second case, we encounter reductions in all radiative quantities as the photon destruction probability decreases (which is equivalent to increasing scattering). Furthermore, for both cases with total optical depth less than unity, the outward intensity towards the polar direction becomes less than that from the edges of disc which is contrary to limb-darkening. In the end, we apply our results to find the spectrum from accretion systems, based on two dynamical models. Consequently, we can see how the total optical depth varies with frequency and causes remarkable changes in the emergent spectra.