The discovery of a 685 second orbital period from the X-ray source 4U 1820 - 30 in the globular cluster NGC 6624

Details The discovery of a 685 second orbital period from the X-ray source 4U 1820 - 30 in the globular cluster NGC 6624 The discovery of a 685 second orbital period from the X-ray source 4U 1820 - 30 in the globular cluster NGC 6624

Jan 1987

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1987apj...312l..17s&link_type=abstract

Astrophysical Journal, Part 2 - Letters to the Editor (ISSN 0004-637X), vol. 312, Jan. 1, 1987, p. L17-L21.

Astronomy and Astrophysics

Astronomy

191

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

Scientific paper

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.

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