X-ray spectroscopy of dips of Cir X-1

Abstract

We present X-ray spectral analyses of the low-mass X-ray binary Cir X-1 during X-ray dips, using the Rossi X-ray Timing Explorer (RXTE) data. Each dip was divided into several segments, and the spectrum of each segment was fitted with a three-component blackbody model, in which the first two components are affected by partial covering and the third one is unaffected. A Gaussian emission line is also included in the spectral model to represent the Fe K alpha line at similar to 6.4keV. The fitted temperatures of the two partially covered components are about 2 keV and I keV, while the uncovered component has a temperature of similar to 0.5-0.6keV. The equivalent blackbody emission radius of the hottest component is the smallest and that of the coolest component is the largest. During the dips the fluxes of the two hot components are linearly correlated, while that of the third component does not show any significant variation. The Fe line flux remains constant, within the errors, during the short dips. However, during the long dips the line flux varies significantly and is positively correlated with the fluxes of the two hot components. These results suggest: (1) that the temperature of the X-ray emitting region decreases with radius, (2) that the Fe Ka line emitting region is close to the hot continuum emitting region, and (3) that the size of the Fe line emitting region is larger than that of the obscuring matter causing the short dips but smaller than the region of that causing the long dips.