The Shape of the Accretion Disk in Her X-1: Results from Modelling X-ray and EUV Emission

The bright X-ray binary pulsar Hercules X-1 shows a 35-day cycle in hard x-ray intensity: ``Main High state -- low state -- Short High state -- low state." This is caused by a tilted-twisted precessing accretion disk (see Scott, Leahy, Wilson, 2000, ApJ, 539, 392). What is less well known is that during low states the soft x-rays and extreme ultraviolet radiation are modulated at the orbital period (Leahy, Marshall 1999, ApJ, 521, 328). This radiation is mainly due to reflection off of the companion star HZ Her (Leahy, Marshall, Scott, 2000, ApJ, 542, 446). Here a model of the accretion disk and the companion star is constructed to agree with the observations of Her X-1 by the Extreme Ultraviolet Explorer (EUVE) and the RXTE/All-Sky-Monitor. The accretion disk shape is well constrained: by the shadowing effect on HZ Her in EUV and by the occultation of the neutron star in x-rays. This work supported by the Natural Sciences and Engineering Research Council of Canada.