Astronomy and Astrophysics – Astronomy
Scientific paper
Aug 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995apj...449..826y&link_type=abstract
Astrophysical Journal v.449, p.826
Astronomy and Astrophysics
Astronomy
10
Methods: Numerical, Radiative Transfer, X-Rays: Stars
Scientific paper
Models of the central engines in bright galactic X-ray sources and in active galactic nuclei require accurate solutions of the radiative transfer problem. Full solutions, that accommodate both diffusive and streaming photon transport through differentially moving gas, are usually too time-consuming to be used in radiation hydrodynamic simulations, and modelers therefore resort to approximate transfer schemes such as flux-limited diffusion. Exact solutions are needed to establish the accuracy and reliability of these schemes. In this paper we present exact solutions to the spherically symmetric, time-independent radiative transfer problem in differentially moving media. We compare these with solutions of truncated transfer equations, in which only terms of low order in the flow speed υ = βc have been retained. We find that for mildly relativistic inflows, solutions of the O(β2) radiative transfer equation closely approximate the exact solutions, but that the O(β) transfer equation fails to provide a good approximation, since it cannot adequately describe radiation trapping. Exact solutions are also compared with approximate solutions obtained through flux-limited diffusion and variable Eddington factor approaches to the transfer problem. We find that both methods provide good approximations to the exact solutions but only if they are derived from transfer equations in which terms of order β2 or higher have been retained. The O(β2) stationary-frame flux-limited diffusion method we present here is thus valid over a much wider range of conditions than existing O(β) stationary-frame methods.
Miller Guy S.
Yin Wei-Wei
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