Binary and long-term (triple?) modulations of 4U 1820-30 in NGC 6624

We present timing analysis results for Rossi X-Ray Timing Explorer (RXTE) observations of X-ray binary source 4U 1820-30 located in the globular cluster NGC 6624. The light curves of observations made between 1996 October and 1997 September show that the maximum of the 685 s binary period modulation folded by the linear ephemeris from previous observations has a phase shift of -0.20 ± 0.06. Combined with historical results (1976-1997), the binary period derivative is measured to be Ṗ/P = ( -3.47 ± 1.48) × 10^-8 yr^-1. The previous known (∼ 176 day) long-term modulation is significant in the X-ray light curve obtained by analysis of the RXTE All-Sky Monitor (ASM) during the years 1996-2000. The RXTE ASM ephemeris is extended by analysis of all historical data (Vela 5B and Ginga) to yield a period of 171.033 ± 0.326 days with no evidence for period change ( |Ṗ/P| < 2.20 × 10^-4 yr^-1). All reported X-ray burst activity is confined to within ± 23 days of the predicted minima. This stable long-term modulation is consistent with 4U 1820-30 being a hierarchical triple system with a ∼ 1.1 day period companion.