SIMBAD references

Rossi X-Ray Timing Explorer (RXTE) All-Sky Monitor light curves of Circinus X-1 show that intensity dips below the ∼1 crab baseline occur near phase zero of the 16.55 day cycle of the source. RXTE Proportional Counter Array observations carried out between 1996 September 20 and 22 provided 60% observing efficiency for 48 hr around phase zero. These observations showed significant dipping activity during much of those 2 days. The dramatic spectral evolution associated with the dips is well fitted by variable and at times heavy absorption (N_H>10²⁴ cm^–2) of a bright component, plus an underlying faint component which is not attenuated by the variable column and whose flux is about 10% of that of the unabsorbed bright component. 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 dips, with equivalent width increasing from as low as 44 eV outside dips to more 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 bright component is radiation received directly from a compact source while the faint component may be attributed to scattered radiation. Our results are also generally consistent with those of Brandt et al., who found that a partial covering model could explain spectra obtained in ASCA observations of a low-to-high transition in Cir X-1. The relative brightness of the two components in our model requires a column density of ∼2x10²³ cm^–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 Kα fluorescence flux that is in rough agreement with the flux of the observed emission line. We also conclude that if the scattering medium is not highly ionized, our line of sight to the compact source does not pass through such a scattering cloud. Finally, we discuss simple pictures of the absorbers responsible for the dips themselves.