RXTE studies 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 Timin g Explorer (RXTE) during 1996. 4U 1915-05 is an X-ray burster (XRB) known t o exhibit a similar to 199 day modulation in its 2-12 keV flux. Observation s were performed with the PCA and HEXTE instruments on RXTE at roughly one- month intervals to sample this long-term period and study accretion rate-re lated spectral changes. We obtain good fits with a model consisting of a bl ackbody and an exponentially cut-off power law. The spectral parameters are strongly correlated with both the broadband (2-50 keV) luminosity and the position in the color-color diagram, with the source moving from a low, har d state to a high, soft state as the accretion rate increases. The blackbod y component appears to drive the spectral evolution. Our results are consis tent with a geometry in which the soft component arises from an optically t hick boundary layer and the hard component from an extended Comptonizing co rona. Comparing our results with those of a similar study of the brighter s ource 4U 1820-30, we find that the two ultracompact LMXBs occupy similar sp ectral states even though the transitions occur at very different total lum inosities.