An Algorithm for Radiation Magnetohydrodynamics Based on Solving the Timedependent Transfer Equation
Abstract
We describe a new algorithm for solving the coupled frequencyintegrated transfer equation and the equations of magnetohydrodynamics in the regime that lightcrossing time is only marginally shorter than dynamical timescales. The transfer equation is solved in the mixed frame, including velocitydependent source terms accurate to {O}(v/c). An operator split approach is used to compute the specific intensity along discrete rays, with upwind monotonic interpolation used along each ray to update the transport terms, and implicit methods used to compute the scattering and absorption source terms. Conservative differencing is used for the transport terms, which ensures the specific intensity (as well as energy and momentum) are conserved along each ray to roundoff error. The use of implicit methods for the source terms ensures the method is stable even if the source terms are very stiff. To couple the solution of the transfer equation to the MHD algorithms in the ATHENA code, we perform direct quadrature of the specific intensity over angles to compute the energy and momentum source terms. We present the results of a variety of tests of the method, such as calculating the structure of a nonLTE atmosphere, an advective diffusion test, linear wave convergence tests, and the wellknown shadow test. We use new semianalytic solutions for radiation modified shocks to demonstrate the ability of our algorithm to capture the effects of an anisotropic radiation field accurately. Since the method uses explicit differencing of the spatial operators, it shows excellent weak scaling on parallel computers.
 Publication:

The Astrophysical Journal Supplement Series
 Pub Date:
 July 2014
 DOI:
 10.1088/00670049/213/1/7
 arXiv:
 arXiv:1403.6126
 Bibcode:
 2014ApJS..213....7J
 Keywords:

 accretion;
 accretion disks;
 magnetohydrodynamics: MHD;
 methods: numerical;
 radiative transfer;
 Astrophysics  Instrumentation and Methods for Astrophysics
 EPrint:
 15 pages, 16 figures, submitted to ApJS