The Discovery of a Second Orbital Period from the X-Ray Source 4U 1820-30 in the Globular Cluster NGC 6624
Abstract
The discovery of a coherent 685 s periodicity in the X-ray source 4U 1820 -30, which is located in the globular cluster NGC 6624, is reported. The modulation has a peak-to-peak amplitude of up to 3 percent and was independently detected in three observations by the EXOSAT Observatory. This period is unlikely to represent the rotation of a neutron star, because accretion torques should cause it to change by at least 10 s/yr, but observations constrain any change to less than 0.4 s/yr. The period is thus identified as the orbital period of 4U 1820 - 30, the first to be found for a globular cluster X-ray source and the shortest yet identified for any binary system. In this compact system the mass-losing star is probably a low-mass (0.055 solar mass) helium white dwarf.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- January 1987
- DOI:
- 10.1086/184811
- Bibcode:
- 1987ApJ...312L..17S
- Keywords:
-
- Globular Clusters;
- Orbit Calculation;
- Stellar Orbits;
- Stellar Rotation;
- X Ray Binaries;
- Exosat Satellite;
- Periodic Variations;
- Power Spectra;
- Spaceborne Astronomy;
- Stellar Mass Accretion;
- White Dwarf Stars;
- Astrophysics;
- CLUSTERS: GLOBULAR;
- STARS: BINARIES;
- X-RAYS: SOURCES