X‐Ray Dip Monitoring of XB 1916−053

We report on the long term monitoring of X-ray dips from the ultracompactlow-mass X-ray binary (LMXB) XB 1916-053. Roughly one-month intervalobservations were carried out with the Rossi X-ray Timing Explorer (RXTE)during 1996, during which the source varied between dim, hard states and moreluminous, soft states. The dip spectra and dip lightcurves were comparedagainst both the broadband luminosity and the derived mass accretion rate Mdot.The dips spectra could be fitted by an absorbed blackbody plus cut-off powerlaw non-dip spectral model, with additional absorption ranging from 0 to >100 x10^22 cm^-2. The amount of additional blackbody absorption was found to varywith the source luminosity. Our results are consistent with an obscuration ofthe inner disk region by a partially ionized outer disk. The size of thecorona, derived from the dip ingress times, was found to be ~10^9 cm. Thecorona size did not correlate with the coronal temperature, but seemed toincrease when \Mdot also increased. We discuss our findings in the context ofan evaporated accretion disk corona model and an ADAF-type model.Comment: Accepted for publication in ApJ, 15 page