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 behavi or of the source was very unusual in that, in the band 1-3 keV, dipping ext ended around most of the orbital cycle with almost no nondip intensity evid ent, 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 increas es in hardness in dipping. We show that the nondip and dip spectra in sever al intensity bands are well fitted using the same physical model that we ha ve previously shown gives good explanations of several dipping sources, con sisting of point-source blackbody emission from the neutron star plus exten ded Comptonized emission from the accretion disk corona (ADC), with progres sive 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 sam e progressive covering factor as the extended continuum emission, providing strong evidence that the line originates in the ADC. Our approach of apply ing the two-component model and explicitly including progressive covering o f the Comptonized emission differs radically from the "absorbed + unabsorbe d" approach previously used extensively for XBT 0748-676 and similar source s, in which the normalization of the unabsorbed peak in dip spectra is allo wed to decrease by a large factor in dipping. This decrease has often been attributed to the effects of electron scattering. By using our two-componen t 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 invo ke electron scattering to explain dipping.