Quasi-periodic accretion and gravitational waves from oscillating "toroidal neutron stars" around a Schwarzschild black hole

TL;DR

This paper uses relativistic hydrodynamics simulations to study oscillations of toroidal neutron stars around black holes, revealing potential gravitational wave signals detectable by current instruments.

Contribution

It introduces a detailed analysis of the dynamical response and gravitational wave emission from oscillating toroidal neutron stars, a novel class of astrophysical objects.

Findings

Oscillations induce quasi-periodic gravitational waves.

Gravitational wave strength depends on torus mass and perturbation.

Potential detectability of signals with current gravitational wave detectors.

Abstract

We present general relativistic hydrodynamics simulations of constant specific angular momentum tori orbiting a Schwarzschild black hole. These tori are expected to form as a result of stellar gravitational collapse, binary neutron star merger or disruption, can reach very high rest-mass densities and behave effectively as neutron stars but with a toroidal topology (i.e. ``toroidal neutron stars''). Our attention is here focussed on the dynamical response of these objects to axisymmetric perturbations. We show that, upon the introduction of perturbations, these systems either become unstable to the runaway instability or exhibit a regular oscillatory behaviour resulting in a quasi-periodic variation of the accretion rate as well as of the mass quadrupole. The latter, in particular, is responsible for the emission of intense gravitational radiation whose signal-to-noise ratio at the detector is comparable or larger than the typical one expected in stellar-core collapse, making these new sources of gravitational waves potentially detectable. We discuss a systematic investigation of the parameter space both in the linear and nonlinear regimes, providing estimates of how the gravitational radiation emitted depends on the mass of the torus and on the strength of the perturbation.