AN X-RAY SPECTROSCOPIC STUDY OF THE PRE-ECLIPSE DIPS OF HERCULES-X-1

Abstract

The X-ray binary pulsar Her X-1 was observed with Ginga on 1988 August 28 during the orbital phase of 0.76 to 0.85 in the main-on state of the 35 day cycle. During the observations the X-ray intensity varied by a factor of 5 or more on a time scale as short as 30 s, due mostly to the soft X-ray absorption in the pre-eclipse dip phase. From studies of pulse profiles and energy spectra, we find that there exists in the dip phase an unpulsed component which is approximately 3 % of the intensity of the nonabsorbed high level. We suggest that scattering of the source continuum by the optically thin hot corona is responsible for the unpulsed component. In the spectral analysis, we find that the high-state nonabsorbed spectra can be fitted by a power law without absorption, and the spectra observed in the different absorption states can be fitted by two power-law components with the same photon index. An iron-K emission line is required in both cases. The estimated equivalent width of the iron line varies from 0.18 to 0.55 keV in correlation with the change in the absorption column density along the line of sight. We suggest that the fluorescent iron line arises in a cool and relatively small region, like the Alfven surface, and may be partially intercepted by the optically thick gas cloud passing across the line of sight.