The superorbital variability and triple nature of the X-ray source 4U 1820–303

Abstract

In Zdziarski et al. (2007a), we have performed a comprehensive analysis of the superorbital modulation in the ultracompact X‐ray source 4U 1820–303, consisting of a white dwarf accreting onto a neutron star. Based on RXTE data, we measured the fractional amplitude of the source superorbital variability (with a ∼170‐d quasi‐period) in the folded and averaged light curves, and found it to be by a factor of ∼2. As proposed before, that variability can be explained by oscillations of the binary eccentricity. In that work, we presented detailed calculations of the eccentricity‐dependent flow through the inner Lagrangian point, and found a maximum of the eccentricity of ≃0.004 is sufficient to explain the observed fractional amplitude. We then studied hierarchical triple models yielding the required quasi‐periodic eccentricity oscillations through the Kozai process. We found the resulting theoretical light curves to match well the observed ones. We constrained the ratio of the semimajor axes of the outer and in...