Orbital evolution of neutron-star-white-dwarf binaries by Roche lobe overflow and gravitational wave radiation

We investigate the effects of mass transfer and gravitational wave (GW) radiation on the orbital evolution of contact neutronstar-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 similar to 0.0023 to 0.72 Hz for WD with masses similar to 0.05-1.4 M-circle dot. 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-yr observation of 4U 1820-30, the expected signal-to-noise-ratio (SNR) in GW characteristic strain is similar to 11.0/10.4/2.2 (LISA/Taiji/Tianqin). The evolution of GW frequencies of NS-WD binaries depends on the WI) 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)M-circle dot NS-WD binary close to contact, the expected SNR for a one week observation is similar to 27/40/28 (LISA/Taijircianqin). For NS-WD binaries with masses of (1.4+ greater than or similar to 1.1) M-circle dot, 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 (similar to 100 kpc), high-mass NS-WD binaries will be still detectable with SNR greater than or similar to 1.

Automated summary

The study explores the impact of mass transfer and gravitational wave radiation on the orbital evolution of contact neutron star-white dwarf binaries. Results show that while some high-mass binaries may directly coalesce after unstable mass transfer, the majority can avoid this fate through a reversal of orbital evolution. Monitoring the GW frequencies and amplitudes of NS-WD binaries near contact can provide insights into their evolutionary stage.