Constraining the physics of the r-mode instability in neutron stars with X-ray and UV observations

TL;DR

This paper uses X-ray and UV observations of neutron stars in LMXBs to constrain the physics of r-mode instabilities, revealing discrepancies with standard models and exploring mechanisms that could reconcile theory with observations.

Contribution

It demonstrates how observational data can challenge existing models of neutron star instabilities and suggests modifications to the theoretical instability window.

Findings

Core temperatures suggest many systems are within the unstable region.

Standard models predict instability where observations do not support it.

Mechanisms are proposed to modify the instability window for consistency.

Abstract

Rapidly rotating Neutron Stars in Low Mass X-ray Binaries (LMXBs) may be an interesting source of Gravitational Waves (GWs). In particular, several modes of stellar oscillation may be driven unstable by GW emission, and this can lead to a detectable signal. Here we illustrate how current X-ray and ultra-violet (UV) observations can constrain the physics of the r-mode instability. We show that the core temperatures inferred from the data would place many systems well inside the unstable region predicted by standard physical models. However, this is at odds with theoretical expectations. We discuss different mechanisms that could be at work in the stellar interior, and we show how they can modify the instability window and make it consistent with the inferred temperatures.