A Doppler Shift model to explain the Cyclotron Line Variability in X-Ray Pulsars

Abstract

A simple model to explain the phase-dependence of the cyclotron absorption line observed in many X-Ray pulsars is presented. It includes several relativistic effects, namely gravitational redshift, gravitational light deflection, and - most important - doppler shift of the photon energy in the infalling plasma. It is shown that previous estimates of neutron star magnetic fields neglecting the last effect give results which are too small by about a factor of two. Finally the developed equations are used to determine the neutron star radius and magnetic field of Her X-1, but the large uncertainties in the geometry prevent the results from seriously constraining the parameters.