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 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 P/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 (|P/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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- December 2001
- DOI:
- 10.1086/324038
- arXiv:
- arXiv:astro-ph/0108405
- Bibcode:
- 2001ApJ...563..934C
- Keywords:
-
- Accretion;
- Accretion Disks;
- Stars: Individual: Alphanumeric: 4U 1820-30;
- X-Rays: Stars;
- Astrophysics
- E-Print:
- 24 pages, 9 figures, accepted for ApJ