Gravitational waves from rapidly rotating neutron stars

TL;DR

This paper estimates gravitational wave emissions from rapidly rotating neutron stars in Low Mass X-ray Binaries, analyzing various mechanisms and their detectability, and concludes that some emissions could be observed with current interferometers.

Contribution

It provides the first comprehensive estimates of gravitational wave signals from different neutron star deformation mechanisms in LMXBs, considering observational constraints.

Findings

Minimal neutron star models are inconsistent with observations.

Magnetic mountains are detectable if the buried magnetic field is around 10^{12} G.

Thermal mountains may produce detectable gravitational waves with ground-based interferometers.

Abstract

Rapidly rotating neutron stars in Low Mass X-ray Binaries have been proposed as an interesting source of gravitational waves. In this chapter we present estimates of the gravitational wave emission for various scenarios, given the (electromagnetically) observed characteristics of these systems. First of all we focus on the r-mode instability and show that a 'minimal' neutron star model (which does not incorporate exotica in the core, dynamically important magnetic fields or superfluid degrees of freedom), is not consistent with observations. We then present estimates of both thermally induced and magnetically sustained mountains in the crust. In general magnetic mountains are likely to be detectable only if the buried magnetic field of the star is of the order of $B\approx 10^{12}$ G. In the thermal mountain case we find that gravitational wave emission from persistent systems may be detected by ground based interferometers. Finally we re-asses the idea that gravitational wave emission may be balancing the accretion torque in these systems, and show that in most cases the disc/magnetosphere interaction can account for the observed spin periods.