Progressive covering of the accretion disc corona during dipping in the LMXB XB 1916-053

Abstract

Results are reported for analysis of the extensive ROSAT observation of the dipping low-mass X-ray binary (LMXB) XB 1916--053. The dipping is 100 per cent deep showing that the emission regions are completely covered by the absorber. A good fit to the non-dip spectrum is obtained using a model consisting of a blackbody with kTBB = 1.95+0.740.34 keV and a power law with photon index 2.32±0.04. These components are identified with emission from the neutron star, and Comptonized emission from an extended accretion disc corona (ADC). Dip spectra are well-fitted by rapid absorption of the blackbody, and progressive covering of the extended component, as the absorber moves across the source, with a covering fraction that increases smoothly from 0 to ∼1.0. Progressive covering shows that the Comptonized emission region is extended, consistent with it originating in the accretion disc corona. The strong unabsorbed component in the dip spectra is well-modelled as the uncovered part of the Comptonized emission at all stages of dipping. There is no detectable change in the low-energy cut-off of the spectrum in dipping which supports the identification of the unabsorbed part of the spectrum with the uncovered part of the ADC emission. The absorbed part of the ADC emission is rapidly removed from the 0.1--2.0 keV band of the ROSAT Position Sensitive Proportional Counter (PSPC), which therefore selects only the uncovered part of the emission, and so the spectral evolution in dipping as viewed by the PSPC depends only on the covering fraction, determined by the geometric overlap between the source and absorber.