Orbital and superorbital variability and their coupling in X-ray binaries

TL;DR

This paper reviews X-ray flux modulations in binaries on orbital and superorbital timescales, highlighting their physical origins and coupling, with specific case studies of Cyg X-1 and 4U 1820-303.

Contribution

It presents new insights into the coupling between orbital and superorbital variability in X-ray binaries, supported by detailed case studies.

Findings

Detection of accretion bulge in Cyg X-1 caused by stellar wind collision.

Interpretation of superorbital variability in 4U 1820-303 as accretion rate modulation.

Evidence of coupling between different variability timescales in X-ray binaries.

Abstract

We review X-ray flux modulation from X-ray binaries on time scales corresponding to the orbital period and those at longer time scales (so called superorbital). Those modulations provide a powerful tool to constrain geometry of the accretion flow. The most common cause of the superorbital variability appears to be precession. We then discuss two specific examples of discoveries of a coupling between the two types of variability and their physical interpretation. One is Cyg X-1, a black-hole binary with a high-mass companion, in which case we find the presence of an accretion bulge formed by collision of the stellar wind with the outer edge of the precessing accretion disc. The other is 4U 1820-303, a neutron star accreting from a low-mass white dwarf, in which case we interpret the superorbital variability as accretion rate modulation induced by interactions in a triple stellar system. Then, the varying accretion rate leads to changes of the size of the accretion bulge in that system, obscuring the centrally-emitted X-rays.