The enigmatic spin evolution of PSR J0537-6910: r-modes, gravitational waves and the case for continued timing

TL;DR

This paper investigates the unusual spin-down behavior of pulsar PSR J0537-6910, proposing that gravitational waves from r-modes could explain its braking index and suggesting future observations to test this hypothesis.

Contribution

It introduces the idea that gravitational wave emission from r-modes may account for the pulsar's high braking index, linking observational data with theoretical models.

Findings

Braking index close to 7 observed in pulsar timing data

Potential detectability of gravitational waves from r-modes with future observations

Advocates joint NICER and LIGO-Virgo campaigns for testing hypotheses

Abstract

We discuss the unique spin evolution of the young X-ray pulsar PSR J0537-6910, a system in which the regular spin down is interrupted by glitches every few months. Drawing on the complete timing data from the Rossi X-ray Timing Explorer (RXTE, from 1999-2011), we argue that a trend in the inter-glitch behaviour points to an effective braking index close to $n=7$, much larger than expected. This value is interesting because it would accord with the neutron star spinning down due to gravitational waves from an unstable r-mode. We discuss to what extent this, admittedly speculative, scenario may be consistent and if the associated gravitational-wave signal would be within reach of ground based detectors. Our estimates suggest that one may, indeed, be able to use future observations to test the idea. Further precision timing would help enhance the achievable sensitivity and we advocate a joint observing campaign between the Neutron Star Interior Composition ExploreR (NICER) and the LIGO-Virgo network.