Relativistic scattering of a fast spinning neutron star by a massive black hole

TL;DR

This paper investigates the relativistic scattering of fast-spinning neutron stars by massive black holes, highlighting how spin effects influence orbital dynamics, gravitational wave emission, and potential observational signatures for testing general relativity.

Contribution

It introduces a quadratic-in-spin MPD formulation for neutron star scattering, analyzing spin effects on orbits, gravitational waves, and observational prospects in a relativistic context.

Findings

Spin-orbit and spin-spin coupling affect scattering angles.

Observable pulse-arrival-time variations are detectable within hours to months.

Gravitational waves from such systems are detectable by LISA up to 100 Mpc.

Abstract

The orbital dynamics of fast spinning neutron stars encountering a massive Black Hole (BH) with unbounded orbits are investigated using the quadratic-in-spin Mathisson-Papapetrou- Dixon (MPD) formulation. We consider the motion of the spinning neutron stars with astrophysically relevant speed in the gravity field of the BH. For such slow-speed scattering, the hyperbolic orbits followed by these neutron stars all have near the e = 1 eccentricity, and have distinct properties compared with those of e >> 1. We have found that compared with geodesic motion, the spin-orbit and spin-spin coupling will lead to a variation of scattering angles at spatial infinity, and this variation is more prominent for slow-speed scattering than fast-speed scattering. Such a variation leads to an observable difference in pulse-arrival-time within a few hours of observation, and up to a few days or months for larger BH masses or longer spinning periods. Such a relativistic pulsar-BH system also emits a burst of gravitational waves (GWs) in the sensitivity band of LISA, and for optimal settings, can be seen up to 100 Mpc away. A radio follow up of such a GW burst with SKA or FAST will allow for measuring the orbital parameters with high accuracy and testing the predictions of General Relativity (GR).