Scattering and iron fluorescence revealed during absorption dips in Circinus X-1

Rossi X-Ray Timing Explorer (RXTE) All-Sky Monitor light curves of Circinus X-l show that intensity dips below the similar to 1 crab baseline occur ne ar phase zero of the 16.55 day cycle of the source. RXTE Proportional Count er Array observations carried out between 1996 September 20 and 22 provided 60% observing efficiency for 48 hr around phase zero. These observations s howed significant dipping activity during much of those 2 days. The dramati c spectral evolution associated with the dips is well fitted by variable an d at times heavy absorption (N-H > 10(24) cm(-2)) Of a. bright component, p lus an underlying faint component which is not attenuated by the variable c olumn and whose flux is about 10% of that of the unabsorbed bright componen t. A prominent iron emission line at 6.4-6.6 keV is evident during the dips . The absolute line flux outside the dips is similar to that during the dip s, with equivalent width increasing from as low as 44 eV outside dips to mo re than 400 eV during dips, indicating that the line is associated with the faint component. These results are consistent with a model in which the br ight component is radiation received directly from a compact source while t he faint component may be attributed to scattered radiation. Our results ar e also generally consistent with those of Brandt et al., who found that a p artial covering model could explain spectra obtained in ASCA observations o f a low-to-high transition in Cir X-l. The relative brightness of the two c omponents in our model requires a column density of similar to 2 x 10(23) c m(-2) if the faint component is due to Thomson scattering in material that mostly surrounds the source. We find that illumination of such a scattering cloud by the observed direct component would produce an iron K alpha fluor escence flux that is in rough agreement with the flux of the observed emiss ion line. We also conclude that if the scattering medium is not highly ioni zed, our line of sight to the compact source does not pass through such a s cattering cloud. Finally, we discuss simple pictures of the absorbers respo nsible for the dips themselves.