Determine angular-momentum-loss mechanisms in intermediate-period ultra-compact X-ray binaries

Determine the angular-momentum-loss mechanisms operating in ultra-compact X-ray binaries with orbital periods between 40 and 80 minutes, where the companion stars may be white dwarfs or helium stars and the mechanisms governing orbital evolution are uncertain.

Background

The paper distinguishes short-period ultra-compact X-ray binaries (UCXBs), with orbital periods below 40 minutes, from systems with periods between 40 and 80 minutes. In the short-period systems, gravitational radiation is assumed to dominate orbital angular-momentum loss, enabling the authors to calculate accretion rates. For the longer-period UCXBs, the companion may instead be a white dwarf or a helium star, and the relevant angular-momentum-loss mechanism is not established. Resolving this uncertainty would be necessary for reliably determining their mass-transfer and accretion rates.

References

For UCXBs with $ 40 \ \rm min < P_{\rm orb} < 80 \ min $, the companion stars are white dwarfs (WDs) or helium stars (He stars) and their angular momentum losing mechanisms are uncertain.

— Observational evidence of black hole event horizon from ultra-compact X-ray binaries  (2610.17618 - Ma et al., 13 Oct 2026) in Section 1, paragraph 3