Comptonization and the Spectra of Accretion-Powered X-Ray Pulsars - AstrophysicsReport as inadecuate

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Abstract: Accretion-powered X-ray pulsars are among the most luminous X-ray sources inthe Galaxy. However, despite decades of theoretical and observational worksince their discovery, no satisfactory model for the formation of the observedX-ray spectra has emerged. In this paper, we report on a self-consistentcalculation of the spectrum emerging from a pulsar accretion column thatincludes an explicit treatment of the bulk and thermal Comptonization occurringin the radiation-dominated shocks that form in the accretion flows. Using arigorous eigenfunction expansion method, we obtain a closed-form expression forthe Greens function describing the upscattering of monochromatic radiationinjected into the column. The Greens function is convolved withbremsstrahlung, cyclotron, and blackbody source terms to calculate the emergentphoton spectrum. We show that energization of photons in the shock naturallyproduces an X-ray spectrum with a relatively flat continuum and a high-energyexponential cutoff. Finally, we demonstrate that our model yields goodagreement with the spectra of the bright pulsar Her X-1 and the low luminositypulsar X Per.

Author: Michael T. Wolff, Peter A. Becker, Kenneth D. Wolfram



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