ASCA observations of the iron K complex of Circinus X-1 near zero phase: spectral evidence for partial covering

We report on ASCA energy spectra of Cir X-1 taken near its zero phase on 1994 August 4-5. The ASCA SIS detectors allow a more detailed study of the iron K complex than has been possible before. We find that prior to a sudden upward flux transition the dominant iron K feature appears to consist 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}, while little absorption over that expected from the Galaxy is seen at lower X-ray energies. The differential absorption at high and low X-ray energies combined with the iron edge energy are strong evidence that partial covering is a crucial determinant of the behaviour observed from Cir X-1. The continuum spectral variability observed by ASCA can also be understood naturally in terms of partial covering column changes. There is evidence for a relatively weak emission line from neutral iron with an equivalent width of only about 65 eV. After the flux transition, the strength of the edge feature is greatly reduced, suggesting a large reduction in the amount of partial covering. For a large region of statistically acceptable chi-squared parameter space, the luminosity of Cir X-1, after correction for partial covering, need not change during the transition. We discuss models for the partial covering and suggest that X-ray scattering by electrons may be important. Aspects of the Cir X-1 spectrum are very similar to those of Seyfert 2 galaxies with Compton-thin tori.