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Session 36 - New Insights on X-ray Binaries - I.
Oral session, Wednesday, June 11
North Main Hall C/D,
We report on high resolution ASCA SIS spectroscopy of the iron K complex of Cir X-1 near one of its zero phase transitions. ASCA allows the best study of the iron K complex to date, and interpretation of the observed spectral features gives information about the nature of much of the violent flux variability from Cir X-1 near zero phase, (2) the nature of some of the observed continuum spectral variability, (3) the orientation of the accretion disk and (4) the role of the accretion disk corona. Prior to a sudden upward flux transition the dominant iron K feature consists of a large edge from neutral or nearly-neutral iron. The depth of the edge corresponds to an absorption column of about 1.5E24 cm^-2, but little absorption over the interstellar value is seen at lower X-ray energies. The differential absorption at high and low X-ray energies is strong evidence that partial covering is a crucial determinant of the behavior observed from Cir X-1. The continuum spectral variability observed by ASCA can also be understood naturally in terms of partial covering column changes. After the flux transition, the strength of the edge feature is greatly reduced, suggesting a large reduction in the amount of partial covering. We discuss models for the partial covering and suggest, by analogy with Compton-thin Seyfert 2 galaxies, that an edge-on disk orientation combined with X-ray scattering by electrons may lead to the apparent partial covering.
