2001ApJ...561..321I

We report on the spectral analysis of Circinus X-1 observed by the ASCA satellite in 1998 March along one orbital period. The luminosity of the source (in the 0.1-100 keV band) ranges from 2.5x10³⁸ ergs.s^–1 at the periastron (orbital phase 0.01) to 1.5x10³⁸ ergs.s^–1 at orbital phase 0.3. From the spectral analysis and the light curve, we argue that Cir X-1 shows three states along the orbital evolution. The first state is at the orbital phase interval 0.97-0.3: the luminosity becomes super-Eddington, and a strong flaring activity is present. In this state a shock could form in the inner region of the system because of the super-Eddington accretion rate, producing an outflow of ionized matter whose observational signature could be the prominent absorption edge at ∼8.7 keV observed in the energy spectrum at these phases. In the second state, corresponding to the orbital phase interval between 0.3 and 0.7, the accretion rate is sub-Eddington, and we observe a weaker outflow, with a smaller hydrogen column: the absorption edge is now at ∼8.3 keV with an optical depth a factor of 2.5-6 smaller. The third state corresponds to the orbital phase interval 0.78-0.97. In this state the best-fit model to the spectrum requires the presence of a partial covering component, indicating that the emission from the compact object is partially absorbed by neutral matter, probably the atmosphere of the companion star and/or the accreting matter from the companion.