Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008dps....40.1902p&link_type=abstract
American Astronomical Society, DPS meeting #40, #19.02; Bulletin of the American Astronomical Society, Vol. 40, p.420
Astronomy and Astrophysics
Astronomy
Scientific paper
4U 1820-303 is an ultracompact X-ray binary consisting of a white dwarf accreting via Roche-lobe overflow onto a neutron star. The binary period is 685s. Besides the binary period, this source shows luminosity variations by a factor > 2 at a superorbital period of 170 d. To explain the superorbital variability Zdziarski, Wen & Gierlinski 2007 (hereafter Z07) proposed a hierarchical triple stellar model. In this hierarchical triple model the superorbital variability is due to variations of the binary orbital eccentricity. Following Z07 we calculate the variations of the binary eccentricity due to the gravity of a third star in the system. This results in a cyclic exchange of the angular momentum between the inner binary and the outer companion, known as Kozai cycles. As in Z07, we neglected a number of physical effects capable of suppressing the Kozai cycles, with the exception of general-relativistic (GR) periastron precession. We reproduced the results of Z07 using both analytical and numerical calculations, for slightly different initial inclination than that used by Z07. Our triple system is extremely sensitive to the initial inclination. We show that this is a motion near the separatrix, so sensitivity to the initial inclination is in fact expected.
Z07 discovered a remarkable and puzzling synchronization between the observed period of superorbital variability (eccentricity oscillation period in this model) and the period of the GR periastron precession of the binary, a result which we confirm. We will discuss the origin of this apparent coincidence.
Murray Nathan
Prodan Snezana
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