Neutron Star$-$Neutron Star and Neutron Star$-$Black Hole Mergers: Multiband Observations and Early Warnings

TL;DR

This paper explores how joint space and ground-based gravitational wave observations can improve measurements of neutron star properties and sky localization, enhancing our understanding of dense matter and enabling early alerts for astrophysical events.

Contribution

It demonstrates the benefits of multiband GW detection, combining space-borne and ground-based detectors, for constraining neutron star parameters and improving localization accuracy.

Findings

Multiband observations constrain quadrupole parameters.

Decihertz detectors reduce tidal deformability errors.

ET improves sky localization by about tenfold.

Abstract

The detections of gravitational waves (GWs) from binary neutron star (BNS) systems and neutron star--black hole (NSBH) systems provide new insights into dense matter properties in extreme conditions and associated high-energy astrophysical processes. However, currently information about NS equation of state (EoS) is extracted with very limited precision. Meanwhile, the fruitful results from the serendipitous discovery of the $\gamma$-ray burst alongside GW170817 show the necessity of early warning alerts. Accurate measurements of the matter effects and sky location could be achieved by joint GW detection from space and ground. In our work, based on two example cases, GW170817 and GW200105, we use the Fisher information matrix analysis to investigate the multiband synergy between the space-borne decihertz GW detectors and the ground-based Einstein Telescope (ET). We specially focus on the parameters pertaining to spin-induced quadrupole moment, tidal deformability, and sky localization. We demonstrate that, (i) only with the help of multiband observations can we constrain the quadrupole parameter; and (ii) with the inclusion of decihertz GW detectors, the errors of tidal deformability would be a few times smaller, indicating that many more EoSs could be excluded; (iii) with the inclusion of ET, the sky localization improves by about an order of magnitude. Furthermore, we have systematically compared the different limits from four planned decihertz detectors and adopting two widely used waveform models.