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 $60-80\,$min. They consist of a compact mass-accretor and a hydrogen-poor mass-donor, in which the mass-accretor could be a neuron 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 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 for UCXB formation, and associated objects. A comparison between observed UCXBs and theoretical expectations is provided, along with a discussion on the observed BH-UCXB candidates. The origin of UCXBs can be constrained by the chemical composition of mass-donors and their locations in diagrams of mass-transfer rate and 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 UCXB samples, and more detailed theoretical simulations.

Figures (10)

• Figure 1: Formation of NS+He WD systems that can evolve into UCXBs.
• Figure 2: Parameter space for producing UCXBs through the WD donor channel, in which the initial NS mass is assumed to be $1.3\,M_{\odot}$. It has been suggested that models with $\beta=0$, $\xi= 2$ and $\alpha= 1$ can better reproduce the properties of the observed X-ray binaries (see Van2019MNRAS.483.5595V). Source: From ChenHaiLiang2021MNRAS.503.3540C.
• Figure 3: Formation of NS+He star systems that can evolve into UCXBs.
• Figure 4: Parameter space for producing UCXBs through the He star donor channel, in which the initial NS mass is assumed to be $1.4\,M_{\odot}$. The filled triangles are for those resulting in the formation of UCXBs. Open circles are those that the masses of He donor stars are too low to ignite He in their center. Crosses indicate systems that will experience high mass-transfer rates when the He star donors evolve into the subgiant stage, leading to the formation of intermediate-mass binary pulsars. Source: From Wang2021MNRAS.506.4654W.
• Figure 5: The evolutionary tracks of NS+He star binaries with different initial He star masses ($M_2$) and evaporation efficiencies ($f$), along which the systems may evolve through the UCXB phase. Different colors represent the NS+He star systems with different initial parameters, hereafter referred to as sets 1–4 (see also GuoYun-Lang2022MNRAS.515.2725G). Set 1 starts with $M_2^{\rm i}=0.32\,M_\odot$ and $\log (P_{\rm orb}^{\rm i}/\rm d)=-1.90$, while sets 2–4 correspond to progressively higher initial He star masses ($M_2^{\rm i}=0.40$, 0.50, and $0.60\,M_\odot$) and wider initial orbits ($\log (P_{\rm orb}^{\rm i}/\rm d)=-1.80$, $-1.65$, and $-1.55$). Colored dots mark the onset of the evaporation process for each set. Black symbols in region (B) indicate BWs with $M_2 < 0.01\,M_\odot$, while gray symbols in region (A) represent BWs with $M_2$ in the range $0.01$–$0.05\,M_\odot$. Observational data for BWs are taken from the ATNF Pulsar Catalogue: http://www.atnf.csiro.au/research/pulsar/psrcat (version 1.70, 2023 May; Manchester2005AJ....129.1993M). The error bars indicate the minimum and maximum companion masses, corresponding to orbital inclination angles of $90^{\circ}$ and $26^{\circ}$ (the 90% probability limit), respectively.