Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 2012
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2012aas...21923701b&link_type=abstract
American Astronomical Society, AAS Meeting #219, #237.01
Astronomy and Astrophysics
Astronomy
Scientific paper
A new analytical model describing spectral formation in accretion-powered X-ray pulsars is presented. The new model expands on previous work published by the authors by utilizing an improved column geometry combined with a more realistic variation for the accretion velocity. The velocity in the new model approaches the local Newtonian free-fall velocity far above the star, and the accretion column has a conical geometry, which is a reasonable approximation to the magnetic dipole in an X-ray pulsar. The transport equation includes bulk and thermal Comptonization, and the spectral solution for the Green's function is obtained using a mathematically rigorous eigenfunction expansion method. The method includes the application of realistic boundary conditions at the stellar surface and also at a large altitude above the star. The spectrum of the radiation escaping through the column walls is computed by convolving the Green's function with bremsstrahlung, cyclotron, and blackbody source terms. The emergent spectrum displays a relatively flat continuum shape with a high-energy quasi-exponential cutoff, in agreement with the observations of the luminous pulsars Her X-1, LMC X-4, and Cen X-3. We show that the observed spectrum is dominated by Comptonized bremsstrahlung emission in these sources.
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