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

Y. Chou, J. E. Grindlay

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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.

Original languageEnglish
Pages (from-to)934-940
Number of pages7
JournalAstrophysical Journal
Issue number2 PART 1
StatePublished - 20 Dec 2001


  • Accretion, accretion disks
  • Stars: individual (4U 1820-30)
  • X-rays: stars


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