Hope College Physics Department Research Experiences for Undergraduates Summer 2010 Project Summary

Resonant Compton scattering with relativistic electrons in the strong magnetic fields is an efficient mechanism to explain the recently discovered rapid rise above 10 keV in the X-ray spectra of Anomalous X-ray Pulsars (AXPs) and Soft Gamma-ray Repeaters (SGRs). Currently, the scattering cross section being used within the community incorporates the relativistic effects of strong magnetic fields with the average relativistic width of the resonance. This average width does not provide an accurate depiction of the scattering as it ignores the spin effects present in the virtual intermediate state. The current cross section displays the magnetic suppression of the cross section below the Thomson limit. Our team has recently suggested the development of an exact QED cross section that incorporates the spin dependence of resonance width with the correct treatment using the required Sokolov & Ternov basis states, as shown in Baring, Gonthier and Harding (2007). In addition to numerical checks, this project explores these spin effects both near and far from resonance. This correct and exact treatment will be cast in compact and accurate analytical expressions for the astrophysics community to utilize. In addition, we will develop the specific analytics for the resonant scattering as well as asymptotic forms. The objective is to develop numerical methods to support collaborators at Rice University and eventually the development of a Monte Carlo simulation code to describe the full magnetospheric scattering that produce the spectra observed in AXPs and SGRs. The expressions developed support data collected by various observatories, including RXTE, INTEGRAL, XMM and BeppoSAX.

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