Spectral evolution of circinus X-1 along its orbit

We report on the spectral analysis of Circinus X-1 observed by the ASCA sat ellite 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 peria stron (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 show s three states along the orbital evolution. The first state is at the orbit al 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 energ y spectrum at these phases. In the second state, corresponding to the orbit al 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 absor ption 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 presen ce of a partial covering component, indicating that the emission from the c ompact object is partially absorbed by neutral matter, probably the atmosph ere of the companion star and/or the accreting matter from the companion.