Spectral Evolution of Circinus X‐1 along Its Orbit

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 2.5 x 10(38) ergs s(-1) at the periastron (orbital phase 0.01) to 1.5 x 10(38) 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 similar to8.7 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 similar to8.3 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