Hercules X-1: spectral variability of an X-ray pulsar in a stellar binary system

Hercules X-1 is an x-ray pulsar in a stellar binary system. It is believed to be a neutron star. Spectral--temporal correlations of the x-ray emission were obtained. The major results concern observations of iron band emission, spectral hardening (increase in effective x-ray temperature) within the x-ray pulse, and a transition from an x-ray low state to a high state. The spectrum obtained prior to the high state can be interpreted as reflected emission from a hot coronal gas surrounding an accretion disk, which itself shields the primary x-ray source from the line of sight during the low state. The spectral hardening within the x-ray pulse is probably indicative of the beaming mechanism at the neutron star surface. If the hardest spectrum by pulse phase is identified with the line of sight close to the Her X-1 magnetic dipole axis, then the X-ray ''pencil'' beam becomes harder with decreasing angle between the line of sight and the dipole axis. The intensity asymmetry in the x-ray pulse can be explained by the attenuation of the beam caused by an intervening region with Compton scattering optical depth less than or equal to 1. This region could be the accretion column. The iron band emission is the most puzzling result because the line energies change within the band (5.5 approx. 8.2 keV) without apparent periodicity. Iron line fluorescence provides the correct line intensity if the fluorescingregion is a shell of material at the Alfven radius which is optically thick to Compton scattering. The absence of correlations between the intensity of this feature and binary phase indicates that most of the emission is not produced in the atmosphere of the optical binary companion, HZ Herculis. Alternatively, line emission from the surface of Her X-1 is discussed