Ultra-compact X-Ray Binaries: A Review

Abstract Ultra-compact X-ray binaries (UCXBs) are a subclass of low-mass X-ray binaries (LMXBs) characterized by ultra-short orbital periods, typically less than 60−80 minutes. They consist of a compact mass-accretor and a hydrogen-poor mass-donor, in which the mass-accretor could be a neutron star (NS) or even a black hole (BH). UCXBs play an important role in multiple areas of astrophysics. In particular, they are considered strong, continuous gravitational wave (GW) sources in the low-frequency band, making them key targets for future space-based GW observatories such as LISA, TianQin and Taiji. As the most compact binaries, the formation and evolution of UCXBs remain highly uncertain. In this article, we summarize theoretical advances and observational status in UCXBs. Especially, we review four classic formation channels: the white dwarf donor channel, the He star donor channel, the evolved main-sequence donor channel, and the accretion-induced collapse channel. We also discuss recent progress in these channels, covering evolutionary scenarios, the initial parameter space of the progenitors, and associated objects. A comparison between observations and theoretical predictions is provided, along with a discussion on the observed BH-UCXB candidates and their formation path. The origin of UCXBs can be constrained by the chemical composition of mass-donors and their locations in diagrams of mass-transfer rate (or X-ray luminosity) versus orbital period. We also examine the implications of UCXBs for several astrophysical fields, including GW astronomy, multi-messenger astronomy, binary evolution, and NS physics under extreme conditions. Further progress will depend on multi-wavelength observations, the discovery of more samples, and more detailed theoretical simulations.