4U 1820-30
Abstract
E. H. Morgan and R. A. Remillard, Center for Space Research, Massachusetts Institute of Technology, write: "SAS-3 observations of the x-ray burster and QPO source 4U 1820-30 in the globular cluster NGC 6624 have confirmed the EXOSAT period of 685 s reported by Priedhorsky et al. (IAUC 4247). Pointed observations with the horizontal tube collimator (1.2-6 keV) were detrended and Fourier analyzed. The following are observation dates, percent duty cycle, period in s, and percent modulation: 1976 Jan. 23.8-29.3 UT, 23, 684.86 +/- 0.25, 1.6; Mar. 27.5-Apr. 8.7, 5, 685.01 +/- 0.08, 1.3; Aug. 27.8-28.5, 52, 685.0 +/- 1.9, 1.8; Sept. 28.0-Oct. 1.8, 17, 685.01 +/- 0.22, 1.9; 1977 Feb. 22.0-22.8, 45, 685.0 +/- 2.0, 3.6; Sept. 19.5- 19.7, 58, 683.1 +/- 9.8, 4.0. An upper limit (3 sigma) of P-dot/P < 2.3 x 10**-4 yr**-1 is obtained by comparing our observations with the reported EXOSAT period (weighted mean 685.17 +/- 1.41, 3 sigma). Our results strongly support the interpretation by Priedhorsky et al. that the 11-min period is the orbital period of the binary system."
- Publication:
-
International Astronomical Union Circular
- Pub Date:
- September 1986
- Bibcode:
- 1986IAUC.4254....2M