Analyzing X‐Ray Pulsar Profiles: Geometry and Beam Pattern of Hercules X‐1

Abstract

We report on our analysis of a large sample of energy-dependent pulse proÐles of the X-ray binary pulsar Hercules X-1. We Ðnd that all data are compatible with the assumption of a slightly distorted magnetic dipole Ðeld as sole cause of the asymmetry of the observed pulse proÐles. Further, the analysis provides evidence that the emission from both poles is equal. We determine an angle between # m \ 20¡ the rotation axis and the local magnetic axis. One pole has an o †set d \ 5¡ from the antipodal position of the other pole. The beam pattern shows structures that can be interpreted as pencil-and fan-beam conÐgurations. Since no assumptions on the polar emission are made, the results can be compared with various emission models. A comparison of results obtained from pulse proÐles of di †erent phases of the 35 day cycle indicates di †erent attenuation of the radiation from the poles being responsible for the change of the pulse shape during the main-on state. These results also suggest the resolution of an ambiguity within a previous analysis of pulse proÐles of Cen X-3, leading to a unique result for the beam pattern of this pulsar as well. The analysis of pulse proÐles of the short-on state indicates that a large fraction of the radiation cannot be attributed to the direct emission from the poles. We give a consistent explanation of both the evolution of the pulse proÐle and the spectral changes with the 35 day cycle in terms of a warped precessing accretion disk.