RXTE studies of long-term X-ray spectral variations in 4U 1820-30

We present the results of detailed spectral studies of the ultracompact low mass X-ray binary (LMXB) 4U 1820-30 carried out with the Rossi X-ray Timin g Explorer (RXTE) during 1996-1997. 4U 1820-30 is an "atoll" source X-ray b urster (XRB) located in the globular cluster NGC 6624. It is known to have an 11 minute binary period and a similar to 176 day modulation in its 2-12 keV flux. Observations were made with the PCA and HEXTE instruments on RXTE at roughly one-month intervals to sample this longterm period and study fl ux-related spectral changes. We find that the 176 day period corresponds to normal atoll source motion in the color-color diagram, indicating that the long period is due to mass accretion rate changes. There are clear correla tions between our fitted spectral parameters and both the broadband (2-50 k eV) flux and the position in the color-color diagram, as described by the p arameter S, introduced by Mendez et al. The hard X-ray tail becomes more pr ominent at low flux levels as the source moves into the island state. In ad dition, we find a strong correlation between the position in the color-colo r diagram and the frequencies of the kilohertz quasi-periodic oscillations (kHz QPOs) reported by Zhang et al. This lends further support to the notio n that evidence for the last stable orbit in the accretion disk of 4U 1820- 30 has been observed. For a model consisting of Comptonization of cool phot ons by hot electrons plus an additional blackbody component, we report an a brupt change in the spectral parameters at the same accretion rate at which the kHz QPOs disappear, suggesting a change in the accretion flow that aff ects both the spectrum and the timing properties. For a model consisting of a multicolor disk blackbody plus a cut-off power law, we find that the inn er disk radius reaches a minimum at the same accretion rate at which the kH z QPO frequency saturates, as expected if the disk reaches the last stable orbit. Both models face theoretical and observational problems when interpr eted physically for this system.