Beating the spin-down limit on gravitational wave emission from the Vela pulsar

TL;DR

This paper reports the first direct upper limits on continuous gravitational wave emission from the Vela pulsar, surpassing the spin-down limit, using data from the Virgo detector and three independent analysis methods.

Contribution

It introduces three independent methods to set upper limits on gravitational waves from Vela, achieving results below the spin-down limit and providing new constraints on neutron star ellipticity.

Findings

Upper limits of approximately 2.1-2.2 x 10^{-24} on gravitational wave strain.

Limits are below the spin-down limit of 3.3 x 10^{-24}.

Constraints on the star's ellipticity around 10^{-3}.

Abstract

We present direct upper limits on continuous gravitational wave emission from the Vela pulsar using data from the Virgo detector's second science run. These upper limits have been obtained using three independent methods that assume the gravitational wave emission follows the radio timing. Two of the methods produce frequentist upper limits for an assumed known orientation of the star's spin axis and value of the wave polarization angle of, respectively, $1.9\ee{-24}$ and $2.2\ee{-24}$, with 95% confidence. The third method, under the same hypothesis, produces a Bayesian upper limit of $2.1\ee{-24}$, with 95% degree of belief. These limits are below the indirect {\it spin-down limit} of $3.3\ee{-24}$ for the Vela pulsar, defined by the energy loss rate inferred from observed decrease in Vela's spin frequency, and correspond to a limit on the star ellipticity of $\sim 10^{-3}$. Slightly less stringent results, but still well below the spin-down limit, are obtained assuming the star's spin axis inclination and the wave polarization angles are unknown.