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 2nd, during which dipping was very deep such 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 non-dip 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 X 1755-338 and X 1624-490, consisting of point-source blackbody emission from the neutron star plus extended Comptonised 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 non-dip data, together with the corresponding non-dip normalisations. Dipping is shown to be due to large increases of column density for the point-like blackbody, combined with the extended power law component being progressively covered by 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 modelling of XB 1916-053 and similar sources, in which the normalisation of the unabsorbed component is allowed to decrease markedly in dipping, behavior 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.

Dipping in CygnusX-2 in a multi-wavelength campaign due to absorption of extended ADC emission

“…In fact, fitting does not permit absorption of the blackbody emission and this is remarkable, given that in the dipping class of LMXB in which dips are caused by absorption in the outer disk bulge, the neutron star blackbody emission is always removed (e.g. Church et al 1997). The dominant Comptonized emission in the present observation is removed gradually as dipping proceeds, i.e.…”

“…Fitting showed that during dipping, k1 remained closely equal to unity: k1 = 1.01 ± 0.05 at 90% confidence but k2 decreased to ∼0.60, indicating that the Comptonized emission was removed while the blackbody was not affected. This contrasts markedly with spectral evolution in the dipping LMXB in which the blackbody is absorbed with a much higher column density that the Comptonized emission, indicating that the neutron star emission is subject to absorption along a path of denser material through central parts of the bulge in the outer disk (Church et al 1997). Absorption could be seen to take place at energies below 5 keV, demonstrating that photoelectric absorption is taking place.…”

“…In analysis of the dipping LMXB, spectral evolution in dipping could be well-described by a model in which the neutron star blackbody was absorbed and the ADC emission was progressively covered by absorber (e.g. Church et al 1997), i.e. a model abs * (abs * bb + pcf * cut) representing the effects of the absorbing bulge on the outer disk passing across the neutron star, and also progressively overlapping the extended ADC.…”

“…We can thus rule out the bulge as the absorber, however, the actual phase of ∼0.35 agrees with a position on the opposite side of the disk to the bulge where the accretion flow from the companion impacts. Interdipping is seen in dipping sources such as XB 1916-053 with smaller dips between the main dips repeating at the orbital cycle (Church et al 1997), caused possibly by ellipticity in the disk or by material overshooting the disk creating a secondary bulge. In the present case, we may be seeing interdipping, or possibly we may see dipping caused by clouds of material traversing the system above the accretion disk.…”

“…This is based on long-term studies of the dipping LMXBs in which absorption in the bulge in the outer disk causes dips in X-ray intensity at the orbital cycle (White & Swank 1982). Spectral evolution during dipping can be explained in terms of point-like blackbody emission from the neutron star and the dominant Comptonized emission of an extended ADC (Church & Bałucińska-Church 1997). The extended nature of the ADC is shown by measurement of dip ingress times giving sizes of 20 000−700 000 km depending on luminosity (Church & Bałucińska-Church 2004).…”