Formation of millisecond pulsars with helium white dwarfs, ultra-compact X-ray binaries and gravitational wave sources

TL;DR

This paper investigates the formation and evolution of neutron star binaries, including millisecond pulsars and ultra-compact X-ray binaries, using a new magnetic braking model to address fine-tuning issues and assess gravitational wave detection prospects.

Contribution

It introduces a revised magnetic braking prescription that broadens initial conditions for UCXB formation, helping to resolve previous fine-tuning problems in binary evolution models.

Findings

Wider initial orbital period range for UCXB progenitors with new magnetic braking.

Strong magnetic braking models hinder formation of wide-orbit BMSPs.

UCXBs are promising gravitational wave sources detectable by future observatories.

Abstract

Close-orbit low-mass X-ray binaries (LMXBs), radio binary millisecond pulsars (BMSPs) with extremely low-mass helium WDs (ELM He~WDs) and ultra-compact X-ray binaries (UCXBs) are all part of the same evolutionary sequence. It is therefore of uttermost importance to understand how these populations evolve from one specie to another. Moreover, UCXBs are important gravitational wave (GW) sources and can be detected by future space-borne GW observatories. However, the formation and evolutionary link between these three different populations of neutron star (NS) binaries are not fully understood. In particular, a peculiar fine-tuning problem has previously been demonstrated for the formation of these systems. In this investigation, we test a newly suggested magnetic braking prescription and model the formation and evolution of LMXBs. We compute a grid of binary evolution models and present the initial parameter space of the progenitor binaries which successfully evolve all the way to produce UCXBs. We find that the initial orbital period range of LMXBs, which evolve into detached NS+ELM~He~WD binaries and later UCXBs, becomes significantly wider compared to evolution with a standard magnetic braking prescription, and thus helps to relieve the fine-tuning problem. However, we also find that formation of wide-orbit BMSPs is prohibited for strong versions of this new magnetic braking prescription, which therefore calls for a revision of the prescription. Finally, we present examples of the properties of UCXBs as Galactic GW sources and discuss their detection by the LISA, TianQin and Taiji observatories.