Gravitational signals emitted by a point mass orbiting a neutron star: effects of stellar structure

TL;DR

This study investigates how neutron star structure influences gravitational wave emission in binary systems, highlighting the significance of stellar modes and the limits of perturbative methods at high velocities.

Contribution

It provides a detailed analysis of stellar perturbations in general relativity, comparing neutron star models with black holes to understand gravitational wave differences.

Findings

Stellar structure affects gravitational wave power at velocities above 0.2c.

Differences from black holes are mainly due to excitation of stellar quasinormal modes.

Perturbative approach validity is limited at high orbital velocities.

Abstract

The effects that the structure of a neutron star would have on the gravitational emission of a binary system are studied in a perturbative regime, and in the frequency domain. Assuming that a neutron star is perturbed by a point mass moving on a close, circular orbit, we solve the equations of stellar perturbations in general relativity to evaluate the energy lost by the system in gravitational waves. We compare the energy output obtained for different stellar models with that found by assuming that the perturbed object is a black hole with the same mass, and we discuss the role played by the excitation of the stellar modes. Ouresults indicate that the stellar structure begins to affect the emitted power when the orbital velocity is v >0.2c (about 185 Hz for a binary system composed of two canonical neutron stars). We show that the differences between different stellar models and a black hole are due mainly to the excitation of the quasinormal modes of the star. Finally, we discuss to what extent and up to which distance the perturbative approach can be used to describe the interaction of a star and a pointlike massive body.