Abstract
We present the results of detailed spectral studies of the ultra-compact low mass X-ray binary (LMXB) 4U 1915-05 carried out with the Rossi X-ray Timing Explorer (RXTE) during 1996. 4U 1915-05 is an X-ray burster (XRB) known to exhibit a ~199-day modulation in its 2--12 keV flux. Observations 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-related spectral changes. We obtain good fits with a model consisting of a blackbody and an exponentially cut-off power law. The spectral parameters are strongly correlated with both the broad-band (2--50 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 (Bloser et al. 2000), we find that the two ultra-compact LMXBs occupy similar spectral states even though the transitions occur at very different total luminosities.
Highlights
Recent advances in broadband X-ray astronomy observations have led to rapid progress in the study of low-mass X-ray binaries (LMXBs) containing low magnetic Ðeld neutron stars (NSs)
In this paper we present the spectral results of RXT E observations of a interesting X-ray burster (XRB), the 50 minute binary 4U 1915[05
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Summary
Recent advances in broadband X-ray astronomy observations have led to rapid progress in the study of low-mass X-ray binaries (LMXBs) containing low magnetic Ðeld neutron stars (NSs). The broadband spectral sensitivities of the Rossi X-Ray T iming Explorer (RXT E, Bradt, Rothschild, & Swank 1993) and BeppoSAX (Boella et al 1997) have allowed detailed modeling of the complete X-ray to hard X-ray (1È200 keV) spectra of several XRBs with a variety of complex physical models including blackbody, disk blackbody, power-law, Comptonization, reÑection, and line components (e.g., Church et al 1998 ; Guainazzi et al 1998 ; in Ït Zand et al 1999 ; Olive et al 1999 ; Piraino et al 1999a, 1999b ; Barret et al 1999, 2000 ; Bloser et al 2000) These observations promise to reveal the relationship between the hard and soft spectral components, the geometry of the emitting regions, and the dependence of each on the mass accretion rate
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