A Generalized Analytical Model For Thermal And Bulk Comptonization In Accretion-Powered X-Ray Pulsars by Peter A. Becker et al. on Monday 28 November

We develop a new theoretical model describing the formation of the radiation

spectrum in accretion-powered X-ray pulsars as a result of bulk and thermal

Comptonization of photons in the accretion column. The new model extends the

previous model developed by the authors in four ways: (1) we utilize a conical

rather than cylindrical geometry; (2) the radiation components emitted from the

column wall and the column top are computed separately; (3) the model allows

for a non-zero impact velocity at the stellar surface; and (4) the velocity

profile of the gas merges with Newtonian free-fall far from the star. We show

that these extensions allow the new model to simulate sources over a wide range

of accretion rates. The model is based on a rigorous mathematical approach in

which we obtain an exact series solution for the Green's function describing

the reprocessing of monochromatic seed photons. Emergent spectra are then

computed by convolving the Green's function with bremsstrahlung, cyclotron, and

blackbody photon sources. The range of the new model is demonstrated via

applications to the high-luminosity source Her X-1, and the low-luminosity

source X Per. The new model suggests that the observed increase in spectral

hardness associated with increasing luminosity in Her X-1 may be due to a

decrease in the surface impact velocity, which increases the $P$d$V$ work done

on the radiation field by the gas.

arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2211.13894v1