Numerical solution of the radiative transfer equation: Xray spectral formation from cylindrical accretion onto a magnetized neutron star
Abstract
Context. Predicting the emerging Xray spectra in several astrophysical objects is of great importance, in particular when the observational data are compared with theoretical models. This requires developing numerical routines for the solution of the radiative equation according to the expected physical conditions of the systems under study.
Aims: We have developed an algorithm solving the radiative transfer equation in the FokkerPlanck approximation when both thermal and bulk Comptonization take place. The algorithm is essentially a relaxation method, where stable solutions are obtained when the system has reached its steadystate equilibrium.
Methods: We obtained the solution of the radiative transfer equation in the twodimensional domain defined by the photon energy E and optical depth of the system τ using finitedifferences for the partial derivatives, and imposing specific boundary conditions for the solutions. We treated the case of cylindrical accretion onto a magnetized neutron star.
Results: We considered a blackbody seed spectrum of photons with exponential distribution across the accretion column and for an accretion where the velocity reaches its maximum at the stellar surface and at the top of the accretion column, respectively. In both cases higher values of the electron temperature and of the optical depth τ produce flatter and harder spectra. Other parameters contributing to the spectral formation are the steepness of the vertical velocity profile, the albedo at the star surface, and the radius of the accretion column. The latter parameter modifies the emerging spectra in a specular way for the two assumed accretion profiles.
Conclusions: The algorithm has been implemented in the xspec package for Xray spectral fitting and is specifically dedicated to the physical framework of accretion at the polar cap of a neutron star with a high magnetic field (≳ 10^{12} G). This latter case is expected to be typical of accreting systems such as Xray pulsars and supergiant fast Xray transients.
 Publication:

Astronomy and Astrophysics
 Pub Date:
 February 2012
 DOI:
 10.1051/00046361/201118008
 arXiv:
 arXiv:1111.6851
 Bibcode:
 2012A&A...538A..67F
 Keywords:

 methods: numerical;
 Xrays: binaries;
 radiative transfer;
 magnetic fields;
 Astrophysics  High Energy Astrophysical Phenomena
 EPrint:
 13 pages, 20 figures, accepted for publication in A&