Time-dependent radiative transfer for multi-level atoms using accelerated Lambda iteration

Abstract

We present a general formalism for computing self-consistent, numerical solutions to the time-dependent radiative transfer equation in low velocity, multi-level ions undergoing radiative interactions. Recent studies of time-dependent radiative transfer have focused on radiation hydrodynamic and magnetohydrodynamic effects without lines, or have solved time-independent equations for the radiation field simultaneously with time-dependent equations for the state of the medium. In this paper, we provide a fully time-dependent numerical solution to the radiative transfer and atomic rate equations for a medium irradiated by an external source of photons. We use Accelerated Lambda Iteration to achieve convergence of the radiation field and atomic states. We perform calculations for a three-level atomic model that illustrates important time-dependent effects. We demonstrate that our method provides an efficient, accurate solution to the time-dependent radiative transfer problem. Finally, we characterize astrophysical scenarios in which we expect our solutions to be important.