Upper bounds on r-mode amplitudes from observations of low-mass X-ray binary neutron stars

TL;DR

This study sets upper limits on r-mode oscillation amplitudes in low-mass X-ray binary neutron stars, indicating gravitational wave signals are likely too weak for detection and providing insights into neutron star internal composition.

Contribution

It provides the first observational upper bounds on r-mode amplitudes in neutron stars, linking spin-down rates and luminosity to internal physics and potential exotic matter presence.

Findings

R-mode amplitudes range from 1e-8 to 1.5e-6 for masses below 2 M_sun.

Less than 1% of observed spin-down rates are due to r-modes.

Gravitational wave signals from these r-modes are likely undetectable with Advanced LIGO.

Abstract

We present upper limits on the amplitude of r-mode oscillations, and gravitational-radiation-induced spin-down rates, in low mass X-ray binary neutron stars, under the assumption that the quiescent neutron star luminosity is powered by dissipation from a steady-state r-mode. For masses $< 2 M_{\odot}$ we find dimensionless r-mode amplitudes in the range from about $1\times 10^{-8}$ to $1.5\times 10^{-6}$. For the accreting millisecond X-ray pulsar sources with known quiescent spin-down rates these limits suggest that $\lesssim 1%$ of the observed rate can be due to an unstable r-mode. Interestingly, the source with the highest amplitude limit, NGC 6440, could have an r-mode spin-down rate comparable to the observed, quiescent rate for SAX J1808-3658. Thus, quiescent spin-down measurements for this source would be particularly interesting. For all sources considered here our amplitude limits suggest that gravitational wave signals are likely too weak for detection with Advanced LIGO. Our highest mass model ($2.21\, M_{\odot}$) can support enhanced, direct Urca neutrino emission in the core and thus can have higher r-mode amplitudes. Indeed, the inferred r-mode spin-down rates at these higher amplitudes are inconsistent with the observed spin-down rates for some of the sources, such as IGR J00291+5934 and XTE J1751-305. In the absence of other significant sources of internal heat, these results could be used to place an upper limit on the masses of these sources if they were made of hadronic matter, or alternatively it could be used to probe the existence of exotic matter in them if their masses were known.