Late-time evolution of ultracompact X-ray binaries

TL;DR

This paper investigates the late-stage evolution of ultracompact X-ray binaries (UCXBs), exploring why older systems are not observed, and suggests that effects like the magnetic propeller and evaporation could explain their scarcity.

Contribution

The study applies theories of dynamical instability, magnetic propeller effect, and donor evaporation to model UCXB evolution, offering new explanations for the lack of observed long-period systems.

Findings

Magnetic propeller effect can prevent long-term UCXB visibility.

Donor evaporation may lead to system disruption or faintness.

These effects could explain the absence of old, long-period UCXBs.

Abstract

Ultracompact X-ray binaries (UCXBs) have orbital periods shorter than about 80 minutes and typically consist of a neutron star that accretes hydrogen-poor matter from a white dwarf companion. Angular momentum loss via gravitational wave radiation drives mass transfer via Roche-lobe overflow. The late-time evolution of UCXBs is poorly understood -- all 13 known systems are relatively young and it is not clear why. One question is whether old UCXBs actually still exist, or have they become disrupted at some point? Alternatively they may be simply too faint to see. To investigate this, we apply the theories of dynamical instability, the magnetic propeller effect, and evaporation of the donor, to the UCXB evolution. We find that both the propeller effect and evaporation are promising explanations for the absence of observed long-period UCXBs.