Gravitational waves associated with the r-mode instability from neutron star-white dwarf mergers

TL;DR

This paper investigates gravitational wave emissions from neutron star-white dwarf mergers caused by r-mode instability, highlighting their potential detectability and association with electromagnetic phenomena like gamma-ray bursts and kilonovae.

Contribution

It introduces the study of GW emissions from r-mode instabilities in NS-WD mergers, emphasizing their strength and possible electromagnetic counterparts, which has been less explored compared to other binary types.

Findings

GW emissions from r-modes are stronger than in low-mass X-ray binaries.

These GW signals could be detected by advanced GW observatories.

Potential electromagnetic counterparts include long gamma-ray bursts and kilonova-like emissions.

Abstract

Neutron star-white dwarf (NS-WD) binaries evolve into either ultra-compact X-ray binaries undergoing stable mass transfer or direct mergers by unstable mass transfer. While much attention has been on gravitational wave (GW) emissions from NS-WD binaries with the former evolutionary pathway, this work explores GW emissions related to {\em r}-mode instability of the accreting NSs in NS-WD mergers particularly with WD's mass $\gtrsim 1M_{\odot}$. Due to considerably high accretion rates, the GW emissions associated with both {\em r}-modes and magnetic deformation intrinsically induced by {\em r}-modes presented in this work are much stronger than those in NS-WD binaries categorized as intermediate-mass or low-mass X-ray binaries, rendering them interesting sources for the advanced Laser Interferometer Gravitational Wave Observatory and upcoming Einstein Telescope. Moreover, these strong GW emissions might accompany some intriguing electromagnetic emissions such as peculiar long gamma-ray bursts (LGRBs), fast blue optical transients including kilonova-like emissions associated with peculiar LGRBs, and/or fast radio bursts.