Abstract
Her X-1 exhibits a 35-d cycle in X-ray intensity: ‘main high state–low state–short high state–low state’, which is caused by a varying obscuration of the neutron star by a tilted–twisted precessing accretion disc. The 35-d X-ray light curve in Hercules X-1 has been well characterized by the Rossi X-ray Timing Explorer All-Sky Monitor (RXTE/ASM). Here, different models of the accretion disc and of the neutron star source region are constructed and applied to fit the RXTE/ASM observations. Such detailed modelling of the shape of the 35-d light curve has not been carried out previously. The results show that a detailed model can fit the details of the light curve, and yield numerical values for the parameters of the accretion disc, its atmosphere and the inner edge of the disc. The best model here has a two-layer outer disc, a thick inner disc edge and an extended central source.
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