Peter A. Becker | Department of Computational and Data Sciences, Department of Physics & Astronomy, George Mason University

Thursday March 29, 4:30 PM | Research 1 Room 301

A New Analytical Model for Bulk and Thermal Comptonization in Accretion-Powered X-Ray Pulsars

Accretion-powered X-ray pulsars are among the most luminous X-ray sources in the Galaxy. However, despite decades of theoretical and observational work since their discovery, no satisfactory model for the formation of the observed X-ray spectra has emerged. In this talk, I discuss the first self-consistent model for the formation of the observed X-ray pulsar spectra based on the radiative transfer and the dynamics occurring in the gas as it accretes onto one of the magnetic poles of the rotating neutron star. The model includes an explicit treatment of the bulk and thermal Comptonization occurring in the accretion shock. Using an eigenfunction expansion method, it is possible to obtain a closed-form expression for the Green's function describing the scattering of monochromatic seed radiation injected into the column. The Green's function is convolved with bremsstrahlung, cyclotron, and blackbody source terms to calculate the emergent photon spectrum. The energizati on of the photons in the shock, combined with cyclotron absorption, naturally produces an X-ray spectrum with a relatively flat continuum, leading up to a high-energy exponential cutoff. The results are in good agreement with the observed X-ray pulsar spectra over a very wide range in luminosity, including bright sources such as LMC X-4 and Her X-1 as well as low-luminosity sources such as X Per. It is shown that in the luminous sources, the emergent spectrum is dominated by Comptonized bremsstrahlung emission.

more about Peter A. Becker