Gravitational waves from a spinning particle scattered by a relativistic star: Axial mode case

TL;DR

This study investigates how the spin of a test particle influences gravitational wave emission when scattered by a relativistic star, revealing spin-dependent energy spectra, waveforms, and excitation of quasi-normal modes.

Contribution

It provides the first detailed analysis of spin effects on gravitational waves from particles scattered by relativistic stars, focusing on axial modes and including ultracompact stars.

Findings

Energy spectrum has a broad peak at the orbital angular velocity.

Total gravitational wave energy increases with particle spin in the anti-parallel direction.

Quasi-normal modes are excited in ultracompact stars, depending on particle spin.

Abstract

We study gravitational waves from a spinning test particle scattered by a relativistic star using a perturbation method. The present analysis is restricted to axial modes. By calculating the energy spectrum, the waveforms and the total energy and angular momentum of gravitational waves, we analyze the dependence of the emitted gravitational waves on a particle spin. For a normal neutron star, the energy spectrum has one broad peak whose characteristic frequency corresponds to the angular velocity at the turning point (a periastron). Since the turning point is determined by the orbital parameter, there exists the dependence of the gravitational wave on a particle spin. We find that the total energy of $l = 2$ gravitational waves gets larger as the spin increases in the anti-parallel direction to the orbital angular momentum. For an ultracompact star, in addition to such an orbital contribution, we find the quasi-normal modes exited by a scattered particle, whose excitation rate to gravitational waves depends on the particle spin. We also discuss the ratio of the total angular momentum to the total energy of gravitational waves and explain its spin dependence.