Partial Covering during Long-Term Dips in Circinus X-1

Abstract

Using published observations with the Rossi X-Ray Timing Explorer satellite of the peculiar low-mass X-ray binary Circinus X-1, we investigate its spectral evolution during a long-term dip just before periastron. The spectra can be fitted well by a two-blackbody model, the Eastern model or the modified Western model, plus partial covering absorption. The derived partial covering fraction and equivalent hydrogen column of partial covering absorption are correlated with the position on the hardness-intensity diagram. The heavy equivalent hydrogen column of partial covering (NH>1024 cm-2) suggests that partial covering absorption is the crucial determinant for the long-term dip behavior observed in Cir X-1. A low-energy component with luminosity between 3% and 4% of the nondip spectrum is not affected by the partial covering matter. A prominent iron emission line around 6.4 keV with variable flux is detected in the spectrum. The covering matter is distributed not only in the inner system but also in the disk and extends as far as the iron-line-emitting region in the disk. Several possible candidates for covering matter are discussed.