Progressive Covering of the ADC during Dipping in the LMXB XBT 0748-676

We report results of analysis of the ASCA observation of 1993, May 7th of the dipping 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 non-dip intensity evident and the depth of dipping reached 100%. At higher energies, eg 3 10 keV, the depth of dipping was less than 100%, and there were marked increases in hardness in dipping. We show that the non-dip 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 Comptonised emission from the accretion disk corona (ADC), with progressive covering of the ADC during dipping. Best fit values of kTbb = 1.99±0.16 keV and power law photon index Γ = 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 Comptonised emission, differs radically from the “absorbed plus unabsorbed” approach previously used extensively for XBT 0748-676 and similar sources, in which the normalisation 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 Comptonised emission, and in the band 1 - 10 keV, we do not need to invoke electron scattering to explain dipping. Subject headings: accretion, accretion disks — scattering — (stars:) binaries: close — stars: circumstellar matter — stars: individual (XBT 0748-676) — X-rays: stars