Binary and Long-Term (Triple?) Modulations of 4U 1820$-$30 in NGC 6624

Abstract

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 October 1996 and September 1997 show that the maximum of the 685s binary period modulation folded by the linear ephemeris from previous observations has phase shift of $-$0.20 $\pm$ 0.06. Combined with historical results (1976-1997), the binary period derivative is measured to be $\dot P / P =(-3.47 \pm 1.48) \times 10^{-8}$ yr$^{-1}$. The previous known ($\sim$176 d) long-term modulation is significant in the x-ray light curve obtained by analysis of the RXTE All-Sky Monitor (ASM) during the years 1996-2000. The RXTE/ASM ephemeris is extended by analysis of all historical data (Vela 5B and Ginga) to yield a period 171.033$\pm$0.326 days with no evidence for period change ($|{\dot{P}/ {P}}| < 2.20 \times 10^{-4}$ yr$^{-1}$). All reported x-ray burst activity is confined to within $\pm$23 d of the predicted minima. This stable long-term modulation is consistent with 4U 1820$-$30 being a hierarchical triple system with a $\sim$1.1 d period companion.