Dip Spectroscopy of the Low‐Mass X‐Ray Binary XB 1254−690

Abstract

We observed the low-mass X-ray binary XB 1254-690 with the Rossi X-Ray Timing Explorer in 2001 May and December. During the first observation, strong dipping on the 3.9 hr orbital period and a high degree of variability were observed, along with "shoulders" ~15% deep during extended intervals on each side of the main dips. The first observation also included pronounced flaring activity. The nondip spectrum obtained using the PCA instrument was well described by a two-component model consisting of a blackbody with kT = 1.30 ± 0.10 keV plus a cutoff power-law representation of Comptonized emission with power-law photon index 1.10 ± 0.46 and a cutoff energy of 5.9 keV. The intensity decrease in the shoulders of dipping is energy-independent, consistent with electron scattering in the outer ionized regions of the absorber. In deep dipping, the depth of dipping reached 100% in the energy band below 5 keV, indicating that all emitting regions were covered by absorber. Intensity-selected dip spectra were well fitted by a model in which the pointlike blackbody is rapidly covered, while the extended Comptonized emission is progressively overlapped by the absorber, with the covering fraction rising to 95% in the deepest portion of the dip. The intensity of this component in the dip spectra could be modeled by a combination of electron scattering and photoelectric absorption. Dipping did not occur during the 2001 December observation, but remarkably, both bursting and flaring were observed contemporaneously.