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

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 c ent deep showing that the emission regions are completely covered by the ab sorber. A good fit to the non-dip spectrum is obtained using a model consis ting of a blackbody with kT(BB) = 1.95L(-0.34)(+0.74) keV and a power law w ith photon index 2.32 +/- 0.04. These components are identified with emissi on from the neutron star, and Comptonized emission from an extended accreti on disc corona (ADC). Dip spectra are well-fitted by rapid absorption of th e blackbody, and progressive covering of the extended component, as the abs orber moves across the sourer, with a covering fraction that increases smoo thly from 0 to similar to 1.0. Progressive covering shows that the Comptoni zed emission region is extended, consistent with it originating in the accr etion disc corona. The strong unabsorbed component in the dip spectra is we ll-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 pa rt of the spectrum with the uncovered pan of the ADC emission. The absorbed part of the ADC emission is rapidly removed from the 0.1-2.0 keV band of t he ROSAT Position Sensitive Proportional Counter (PSPC), which therefore se lects only the uncovered part of the emission, and so the spectral evolutio n in dipping as viewed by the PSPC depends only on the covering fraction, d etermined by the geometric overlap between the source and absorber.