Spectral Evolution during Pre-Eclipse Dips in Hercules X-1

Hercules X-1 was observed by the Large Area Proportional Counter (LAC) instrument on the Ginga satellite during 1988 August when pre-eclipse dips of Her X-1 were taking place. A detailed analysis of the X-ray spectra during the dips is presented here. The dips are confirmed to be caused by absorption, and for the first time the evolution of absorption during a dip is determined. However, in addition to the absorbed component, there is a significant unabsorbed continuum component from an extended corona around the neutron star, which also changes with time. By analyzing the spectral changes, evidence is found for important properties that have not been previously detected in X-ray binaries: the power-law index spectrum increases with luminosity; the covering fraction of a scattering corona by the accretion disk is variable on several minute timescales; and the flourescent iron line energy is downshifted by Compton scattering.