A Model for the 35 Day Variations in the Pulse Profile of Hercules X-1

The substantial differences between the X-ray pulse profiles seen during the 'long-on' and the 'short-on' stages in the 35-day cycle of Herculas X-1 are explained by a model that contains a tilted, twisted, precessing accretion disk, and an obliquely magnetized, nonprecessing neutron star which rapidly rotates about a fixed axis, perpendicular to the binary plane. The novel aspect of this model is a raised portion of the inner edge of the accretion disk that rotates synchronously with the neutron star. This additional component modulates the relative intensities of the main pulse and interpulse by attenuation of the emergent flux. The neutron star is thus not required to undergo free precession or to be inclined to the binary plane. The structure within the main pulse and interpulse is assumed to depend on the details of the accretion column and is not addressed by this model.