BEATING THE SPIN-DOWN LIMIT ON GRAVITATIONAL WAVE EMISSION FROM THE VELA PULSAR

We present direct upper limits on continuous gravitational wave emission fromthe Vela pulsar using data from the Virgo detector's second science run. Theseupper limits have been obtained using three independent methods that assume thegravitational wave emission follows the radio timing. Two of the methodsproduce frequentist upper limits for an assumed known orientation of the star'sspin 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, underthe 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 inferredfrom observed decrease in Vela's spin frequency, and correspond to a limit onthe star ellipticity of $\sim 10^-3$. Slightly less stringent results, butstill well below the spin-down limit, are obtained assuming the star's spinaxis inclination and the wave polarization angles are unknown.