General relativistic radiative transfer in hot astrophysical plasmas: a characteristic approach

In this paper we present a characteristic method for solving the transfer equation in differentially moving media in a curved spacetime. The method is completely general, but its capabilities are exploited at best in presence of symmetries, when the existence of conserved quantities allows to derive analytical expressions for the photon trajectories in phase space. In spherically--symmetric, stationary configurations the solution of the transfer problem is reduced to the integration of a single ordinary differential equation along the bi--parametric family of characteristic rays. Accurate expressions for the radiative processes relevant to continuum transfer in a hot astrophysical plasma have been used in evaluating the source term, including relativistic e--p, e--e bremsstrahlung and Compton scattering. A numerical code for the solution of the transfer problem in moving media in a Schwarzschild spacetime has been developed and tested. Some applications, concerning ``hot'' and ``cold'' accretion onto non--rotating black holes as well as static atmospheres around neutron stars, are presented and discussed.Comment: 47 pages plus 9 postscript figures, uuencoded file, PlainTeX. Accepted for publication in the Astrophysical Journa

General Relativistic Radiative Transfer In Hot Astrophysical Plasmas: A Characteristic Approach

INTRODUCTION Radiative transfer in high energy, fast moving plasmas in a strong gravitational field is today at the basis of a large number of currently interesting astrophysical applications: accretion onto compact objects, jets, stellar collapse and supernova expanding envelopes are just some examples. Different approaches to the solution of the relativistic comoving frame transfer equation (CTE) in planar or spherical geometry have been suggested in the past (see e.g. Schmidt--Burgk 1978, Mihalas 1980, Thorne 1981, Schinder & Bludman 1989, Hauschildt & Wehrse 1991). They can be grouped, schematically, into three wide classes: direct solution of the CTE using discretization techniques, moment expansion and integration of the CTE along characteristic directions. Characteristics methods are based on the fact that the transfer equation is just the Boltzmann equation for the photon distribution function in phase--space. The hyperbolic character of the Boltzmann equation implie