Diving below the spin-down limit: Constraints on gravitational waves from the energetic young pulsar PSR J0537-6910

TL;DR

This study searches for continuous gravitational waves from pulsar PSR J0537-6910 using LIGO/Virgo data, setting new upper limits that surpass the spin-down limit and constrain the star's ellipticity.

Contribution

First to surpass the gravitational-wave spin-down limit for PSR J0537-6910, providing the most stringent constraints on its gravitational-wave emission and ellipticity.

Findings

No gravitational-wave signal detected.

Constraints exceed the spin-down limit by more than a factor of two.

Ellipticity limited to less than about 3e-5.

Abstract

We present a search for continuous gravitational-wave signals from the young, energetic X-ray pulsar PSR J0537-6910 using data from the second and third observing runs of LIGO and Virgo. The search is enabled by a contemporaneous timing ephemeris obtained using NICER data. The NICER ephemeris has also been extended through 2020 October and includes three new glitches. PSR J0537-6910 has the largest spin-down luminosity of any pulsar and is highly active with regards to glitches. Analyses of its long-term and inter-glitch braking indices provided intriguing evidence that its spin-down energy budget may include gravitational-wave emission from a time-varying mass quadrupole moment. Its 62 Hz rotation frequency also puts its possible gravitational-wave emission in the most sensitive band of LIGO/Virgo detectors. Motivated by these considerations, we search for gravitational-wave emission at both once and twice the rotation frequency. We find no signal, however, and report our upper limits. Assuming a rigidly rotating triaxial star, our constraints reach below the gravitational-wave spin-down limit for this star for the first time by more than a factor of two and limit gravitational waves from the $l=m=2$ mode to account for less than 14% of the spin-down energy budget. The fiducial equatorial ellipticity is limited to less than about 3e-5, which is the third best constraint for any young pulsar.