Ultra-compact binary pulsars as continuous dual-line gravitational wave sources

TL;DR

This paper explores ultra-compact binary pulsars as potential dual-line gravitational wave sources, with high-frequency signals detectable by advanced GW detectors, offering insights into neutron star properties and the equation of state.

Contribution

It introduces the concept of ultra-compact binary pulsars as dual-line GW sources, linking low- and high-frequency signals to neutron star characteristics and GW detection prospects.

Findings

High-frequency GW signals from neutron stars are in the 10-100 Hz range.

Detection of dual-line GW signals can constrain neutron star properties.

Some ultra-compact binary pulsars could be observable as dual-line GW sources.

Abstract

Binary millisecond pulsars (MSPs) are detached binary systems consisting of a MSP and a He white dwarf. If the initial orbital periods of binary MSPs are less than 0.3 day, they would evolve toward ultra-compact binary pulsars due to the rapid orbital shrinkage by the gravitational wave (GW) radiation. During the orbital decay, the MSP with an ellipticity would spin down by the GW radiation and the magnetic dipole radiation. Our calculations indicate that the angular momentum loss is dominated by the GW radiation when the ellipticities of the neutron stars (NSs) are in the range of $(1-50)\times 10^{-7}$, and the frequencies of high-frequency GW signals from the rotating NSs are $10-100$ Hz when the binary pulsars can be visible as low-frequency GW sources. These high-frequency GW signals are possible to be detected by the aLIGO and the third-generation GW detectors such as Einstein Telescope, depending on the frequencies and the distances. Therefore, some ultra-compact binary pulsars have an opportunity to become intriguing dual-line GW sources. By detecting the low-frequency GW signals, the NS mass can be accurately derived. A dual-line detection of two band GW signals could provide a constraint on the moment of inertia and the ellipticity of the NS. Thus the dual-line GW sources can be potentially applied to constrain the equation of state of the NS.