Triple Nature Of X-ray Source 4U 1820-303

Details Triple Nature Of X-ray Source 4U 1820-303 Triple Nature Of X-ray Source 4U 1820-303

Sep 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009dps....41.0510p&link_type=abstract

American Astronomical Society, DPS meeting #41, #5.10

Astronomy and Astrophysics

Astronomy

Scientific paper

The LMXB 4U 1820-303 is located near the center of globular cluster NGC 6624. The binary period is 685 s. This ultracompact X-ray binary consists of a white dwarf accreting via Roche-lobe overflow onto a neutron star. A very interesting feature of this source is luminosity variation by factor >2 at a superorbital period of 170 d. Since this variation is most likely intrinsic property of the source, hierarchical triple model was proposed to explain the superorbital variability. In this model there is a cyclic exchange of the angular momentum between the inner binary and the outer companion, known as Kozai cycles. Variations of the binary orbital eccentricity, which are result of Kozai cycles, are responsible for the superorbital variability.
Following Zdziarski, Wen & Gierlinski 2007 (hereafter Z07), in our model we included only general-relativistic (GR) periastron precession as a physical effect capable of suppressing the Kozai cycles. Then we included mass transfer, tides, and gravitational radiation. Results from Z07 were reproduced for slightly different inclination in both cases. This system is extremely sensitive to the initial inclination. We showed that this is a motion near the separatrix, so noted sensitivity to initial inclination is expected.
Puzzling synchronization between the observed period of superorbital variability (eccentricity oscillation period) and the period of the GR periastron precession of the binary was reported by Z07. We confirm this result and we discuss the origin of this apparent coincidence.

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