RXTE Studies of Long‐Term X‐Ray Spectral Variations in 4U 1820−30

Abstract

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 Timing Explorer (RXTE) during 1996-1997. 4U 1820-30 is an "atoll" source X-ray burster (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 flux-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 correlations between our fitted spectral parameters and both the broadband (2-50 keV) flux and the position in the color-color diagram, as described by the parameter S, introduced by Mendez et al. The hard X-ray tail becomes more prominent at low flux levels as the source moves into the island state. In addition, we find a strong correlation between the position in the color-color diagram and the frequencies of the kilohertz quasi-periodic oscillations (kHz QPOs) reported by Zhang et al. This lends further support to the notion 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 photons by hot electrons plus an additional blackbody component, we report an abrupt change in the spectral parameters at the same accretion rate at which the kHz QPOs disappear, suggesting a change in the accretion flow that affects 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 inner disk radius reaches a minimum at the same accretion rate at which the kHz QPO frequency saturates, as expected if the disk reaches the last stable orbit. Both models face theoretical and observational problems when interpreted physically for this system.