Progressive covering of the accretion disk corona during dipping in the low-mass X-ray binary XBT 0748-676

We report results of analysis of the ASCA observation of 1993 May 7 of the dipping low-mass X-ray binary (LMXB) source XBT 0748-676 and propose a new explanation of the spectral evolution in dipping in this source. The behavior of the source was very unusual in that, in the band 1-3 keV, dipping extended around most of the orbital cycle with almost no nondip intensity evident, and the depth of dipping reached 100%. At higher energies, e.g., 3-10 keV, the depth of dipping was less than 100%, and there were marked increases in hardness in dipping. We show that the nondip and dip spectra in several intensity bands are well fitted using the same physical model that we have previously shown gives good explanations of several dipping sources, consisting of point-source blackbody emission from the neutron star plus extended Comptonized emission from the accretion disk corona (ADC), with progressive covering of the ADC during dipping. Best-fit values of kT(bb) = 1.99 +/- 0.16 keV and power-law photon index Gamma = 1.70 +/- 0.16 are found. The strong excess below 1 keV was well fitted by a Gaussian line at 0.65 keV. In dipping, good fits were obtained by allowing it to be covered by the same progressive covering factor as the extended continuum emission, providing strong evidence that the line originates in the ADC. Our approach of applying the two-component model and explicitly including progressive covering of the Comptonized emission differs radically from the "absorbed + unabsorbed" approach previously used extensively for XBT 0748-676 and similar sources, in which the normalization of the unabsorbed peak in dip spectra is allowed to decrease by a large factor in dipping. This decrease has often been attributed to the effects of electron scattering. By using our two-component model, we show that the unabsorbed component is the uncovered fraction of the Comptonized emission, and in the band 1-10 keV, we do not need to invoke electron scattering to explain dipping.