Orbital Evolution of Neutron-Star -- White-Dwarf Binaries by Roche-Lobe Overflow and Gravitational Wave Radiation

TL;DR

This paper models the orbital evolution of neutron-star-white-dwarf binaries considering mass transfer and gravitational wave emission, predicting their detectability by space-based GW detectors and explaining observed ultra-compact X-ray sources.

Contribution

It introduces a comprehensive model of NS-WD binary evolution including mass transfer effects and assesses their GW detectability, aligning with observations of sources like 4U 1820--30.

Findings

High-mass NS-WD binaries may coalesce after unstable mass transfer.

Most NS-WD binaries avoid coalescence due to orbital evolution reversal.

Detectability of NS-WD binaries depends on WD mass and distance, with high SNRs predicted for certain systems.

Abstract

We investigate the effects of mass transfer and gravitational wave (GW) radiation on the orbital evolution of contact neutron-star-white-dwarf (NS-WD) binaries, and the detectability of these binaries by space GW detectors (e.g., Laser Interferometer Space Antenna, LISA; Taiji; Tianqin). A NS-WD binary becomes contact when the WD component fills its Roche lobe, at which the GW frequency ranges from ~0.0023 to 0.72 Hz for WD with masses ~0.05-1.4 Msun. We find that some high-mass NS-WD binaries may undergo direct coalescence after unstable mass transfer. However, the majority of NS-WD binaries can avoid direct coalescence because mass transfer after contact can lead to a reversal of the orbital evolution. Our model can well interpret the orbital evolution of the ultra-compact X-ray source 4U 1820--30. For a 4-year observation of 4U 1820--30, the expected signal-to-noise-ratio (SNR) in GW characteristic strain is ~11.0/10.4/2.2 (LISA/Taiji/Tianqin). The evolution of GW frequencies of NS-WD binaries depends on the WD masses. NS-WD binaries with masses larger than 4U 1820--30 are expected to be detected with significantly larger SNRs. For a (1.4+0.5) Msun NS-WD binary close to contact, the expected SNR for a one week observation is ~27/40/28 (LISA/Taiji/Tianqin). For NS-WD binaries with masses of (1.4+>~1.1) Msun, the significant change of GW frequencies and amplitudes can be measured, and thus it is possible to determine the binary evolution stage. At distances up to the edge of the Galaxy (~100 kpc), high-mass NS-WD binaries will be still detectable with SNR>~1.