RXTEStudies of X‐Ray Spectral Variations with Accretion Rate in 4U 1915−05

We present the results of detailed spectral studies of the ultracompact low-mass X-ray binary (LMXB) 4U 1915[05 carried out with the Rossi X-Ray T iming Explorer (RXT E) during 1996.4U 1915[05 is an X-ray burster (XRB) known to exhibit a D199 day modulation in its 2È12 keV Ñux.Observations were performed with the PCA and HEXTE instruments on RXT E at roughly one-month intervals to sample this long-term period and study accretion rate-related spectral changes.We obtain good Ðts with a model consisting of a blackbody and an exponentially cut-o † power law.The spectral parameters are strongly correlated with both the broadband (2È50 keV) luminosity and the position in the color-color diagram, with the source moving from a low, hard state to a high, soft state as the accretion rate increases.The blackbody component appears to drive the spectral evolution.Our results are consistent with a geometry in which the soft component arises from an optically thick boundary layer and the hard component from an extended Comptonizing corona.Comparing our results with those of a similar study of the brighter source 4U 1820[30, we Ðnd that the two ultracompact LMXBs occupy similar spectral states even though the transitions occur at very di †erent total luminosities.