Simple Photoelectric Absorption during Dipping in theASCAObservation of XB 1916−053

Abstract

We report results of analysis of the ASCA observation of the low-mass X-ray binary dipping source XB 1916-053 made on 1993 May 2, during which dipping was very deep, so that in the deepest parts of dips, the X-ray intensity in the band 0.5-12.0 keV fell to zero, demonstrating that all emission components were completely removed. The best-fit orbital period of the binary system, determined from the X-ray data, was found to be 3005 ± 10 s. The high-quality ASCA data allowed spectral evolution in dipping to be systematically investigated by spectral analysis in intensity bands covering the full range of dipping from intensities close to zero to nondip values. We have shown that the spectra can be well fitted by the same two-component model previously used to give good explanations of the very different dip sources X1755-338 and X1624-490, consisting of point-source blackbody emission from the neutron star and extended Comptonized emission, probably from the accretion disk corona. In the case of XB 1916-053, we show that all levels of dipping can be fitted using kT_bb = 2.14 ± 0.28 keV and power-law photon index = 2.42 ± 0.21, which are the best-fit values for nondip data, together with the corresponding nondip normalizations. Dipping is shown to result from large increases of column density for the pointlike blackbody, combined with the extended power-law component being progressively covered by the absorber, until, in the deepest parts of dips, the partial covering fraction approaches unity. This approach differs radically from the "absorbed-plus-unabsorbed" approach previously used in spectral modeling of XB 1916-053 and similar sources, in which the normalization of the unabsorbed component is allowed to decrease markedly in dipping, behavior that is generally attributed to the effects of electron scattering. Thus, we have shown that spectral evolution in XB 1916-053 can be explained simply in terms of photoelectric absorption, without the need for substantial electron scattering. This explanation is supported by calculation of the relative importance of photoelectric absorption and electron scattering in the absorbing region, which shows that little electron scattering is expected in the ASCA energy band.