Compton Scattering of Fe K alpha Lines in Magnetic Cataclysmic Variables

Abstract

Compton scattering of X-rays in the bulk flow of the accretion column in magnetic cataclysmic variables (mCVs) can significantly shift photon energies. We present Monte Carlo simulations based on a nonlinear algorithm demonstrating the effects of Compton scattering on the H-like, He-like and neutral Fe K alpha lines produced in the post-shock region of the accretion column. The peak line emissivities of the photons in the post-shock flow are taken into consideration and frequency shifts due to Doppler effects are also included. We find that line profiles are most distorted by Compton scattering effects in strongly magnetized mCVs with a low white dwarf mass and high mass accretion rate and which are viewed at an oblique angle with respect to the accretion column. The resulting line profiles are most sensitive to the inclination angle. We have also explored the effects of modifying the accretion column width and using a realistic emissivity profile. We find that these do not have a significant overall effect on the resulting line profiles. A comparison of our simulated line spectra with high resolution Chandra/HETGS observations of the mCV GK Per indicates that a wing feature redward of the 6.4 keV line may result from Compton recoil near the base of the accretion column.Comment: Accepted for publication in MNRAS, 10 pages with 8 figure