CORRELATION BETWEEN ENERGY SPECTRAL STATES AND FAST TIME VARIABILITY AND FURTHER EVIDENCE FOR THE MARGINALLY STABLE ORBIT IN 4U-1820-30

We report the results from a long-term monitoring data set obtained wi th the Rossi X-Ray Timing Explorer (RXTE) from the bright globular clu ster binary source 4U 1820-30. During this monitoring, the source flux varied from 2.9 x 10(-9) to 8.5 x 10(-9) ergs cm(-2) s-(1) in the 2-1 0 keV band, which correspond to, respectively, count rates of 1600 and 4500 counts s(-1) in the nominal 2-60 keV RXTE proportional counter a rray band. Its energy spectral state was observed to encompass both th e banana branches and the island state. Kilohertz quasiperiodic oscill ations (kHz QPOs) were observed to exist in the upper banana and lower banana branches, as well as in the island state. In particular, we re port that the kHz QPOs were present only when the soft color (defined as the flux ratio of the 3.9-5.0 keV band to the 1.7-3.9 keV band) was below 0.78 without regard to the overall count rate and without regar d to whether the source is in any one of the three branches or states. Their centroid frequency was well correlated with the overall count r ate below 2500 counts s(-1). When the count rate was above 2500 counts s(-1), the QPO frequency was consistent with a constant independent o f the overall count rate. We take this as further evidence that the in ner edge of the accretion had reached the marginally stable orbit, and therefore the constant 1060 +/ 20 Hz is the marginally stable orbit f requency.