Search for continuous gravitational waves from neutron stars in globular cluster NGC 6544

We describe a directed search for continuous gravitational waves in data from the sixth initial LIGO science run. The target was the nearby globular cluster NGC 6544 at a distance of $\ensuremath{\approx}2.7\text{ }\text{ }\mathrm{kpc}$. The search covered a broad band of frequencies along with first and second frequency derivatives for a fixed sky position. The search coherently integrated data from the two LIGO interferometers over a time span of 9.2 days using the matched-filtering $\mathcal{F}$-statistic. We found no gravitational-wave signals and set 95% confidence upper limits as stringent as $6.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}25}$ on intrinsic strain and $8.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ on fiducial ellipticity. These values beat the indirect limits from energy conservation for stars with characteristic spin-down ages older than 300 years and are within the range of theoretical predictions for possible neutron-star ellipticities. An important feature of this search was use of a barycentric resampling algorithm which substantially reduced computational cost; this method is used extensively in searches of Advanced LIGO and Virgo detector data.