English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Plus monthly

Global: Zendy Tools annual

Global: Zendy Tools monthly

Global: Zendy Free Plan

We develop a new theoretical model describing the formation of the radiationspectrum in accretion-powered X-ray pulsars as a result of bulk and thermalComptonization of photons in the accretion column. The new model extends theprevious model developed by the authors in four ways: (1) we utilize a conicalrather than cylindrical geometry; (2) the radiation components emitted from thecolumn wall and the column top are computed separately; (3) the model allowsfor a non-zero impact velocity at the stellar surface; and (4) the velocityprofile of the gas merges with Newtonian free-fall far from the star. We showthat these extensions allow the new model to simulate sources over a wide rangeof accretion rates. The model is based on a rigorous mathematical approach inwhich we obtain an exact series solution for the Green's function describingthe reprocessing of monochromatic seed photons. Emergent spectra are thencomputed by convolving the Green's function with bremsstrahlung, cyclotron, andblackbody photon sources. The range of the new model is demonstrated viaapplications to the high-luminosity source Her X-1, and the low-luminositysource X Per. The new model suggests that the observed increase in spectralhardness associated with increasing luminosity in Her X-1 may be due to adecrease in the surface impact velocity, which increases the $P$d$V$ work doneon the radiation field by the gas.

A Generalized Analytical Model For Thermal And Bulk Comptonization In Accretion-Powered X-Ray Pulsars