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P. F. Bloser, J. E. Grindlay, P. Kaaret (CfA), W. Zhang, A. P. Smale, J. H. Swank (GSFC)
We present the results of detailed spectral studies of the ultra-compact low mass x-ray binary 4U 1820-30 carried out with RXTE during 1997. 4U 1820-30 is an x-ray burster located in the globular cluster NGC 6624. It is known to have an 11 minute binary period and a ~176 day variation in its 2-10 keV flux. Observations were made with the PCA and HEXTE instruments on RXTE at roughly one month intervals to sample this long-term period and study flux-related spectral changes. We find clear correlations between spectral parameters and the broad-band (0.1-200 keV) flux, with the hard X-ray tail becoming more prominent at low flux levels. In addition, we find strong correlations between the spectral shape and the frequencies of the QPOs reported by Zhang et al. (1998). These lend support to the notion that evidence for the last stable orbit in the accretion disk of 4U 1820-30 has been observed.
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