Extracting equation of state parameters from black hole-neutron star mergers. I. Nonspinning black holes

TL;DR

This paper demonstrates that gravitational wave observations from black hole-neutron star mergers can accurately determine neutron star deformability and radius, providing constraints on the equation of state through waveform analysis.

Contribution

It introduces a method to extract the neutron star's deformability parameter Lambda from gravitational wave signals, linking waveform features to the neutron star's physical properties.

Findings

Lambda can be extracted with 10-40% accuracy from single events.

Neutron star radius R can be inferred from Lambda with high precision.

Future detectors like Einstein Telescope improve EOS parameter measurements by an order of magnitude.

Abstract

The late inspiral, merger, and ringdown of a black hole-neutron star (BHNS) system can provide information about the neutron-star equation of state (EOS). Candidate EOSs can be approximated by a parametrized piecewise-polytropic EOS above nuclear density, matched to a fixed low-density EOS; and we report results from a large set of BHNS inspiral simulations that systematically vary two parameters. To within the accuracy of the simulations, we find that, apart from the neutron-star mass, a single physical parameter Lambda, describing its deformability, can be extracted from the late inspiral, merger, and ringdown waveform. This parameter is related to the radius, mass, and l=2 Love number, k_2, of the neutron star by Lambda = 2k_2 R^5/3M_{NS}^5, and it is the same parameter that determines the departure from point-particle dynamics during the early inspiral. Observations of gravitational waves from BHNS inspiral thus restrict the EOS to a surface of constant Lambda in the parameter space, thickened by the measurement error. Using various configurations of a single Advanced LIGO detector, we find that neutron stars are distinguishable from black holes of the same mass and that Lambda^{1/5} or equivalently R can be extracted to 10-40% accuracy from single events for mass ratios of Q=2 and 3 at a distance of 100 Mpc, while with the proposed Einstein Telescope, EOS parameters can be extracted to accuracy an order of magnitude better.