PRECISION EPHEMERIDES FOR GRAVITATIONAL-WAVE SEARCHES. I. Sco X-1

Rapidly rotating neutron stars are the only candidates for persistent high-frequency gravitational wave emission, for which a targeted search can be performed based on the spin period measured from electromagnetic (e.g., radio and X-ray) observations. The principal factor determining the sensitivity of such searches is the measurement precision of the physical parameters of the system. Neutron stars in X-ray binaries present additional computational demands for searches due to the uncertainty in the binary parameters. We present the results of a pilot study with the goal of improving the measurement precision of binary orbital parameters for candidate gravitational wave sources. We
\nobserved the optical counterpart of Sco X-1 in 2011 June with the William Herschel Telescope and also made use
\nof Very Large Telescope observations in 2011 to provide an additional epoch of radial-velocity measurements to
\nearlier measurements in 1999. From a circular orbit fit to the combined data set, we obtained an improvement of a
\nfactor of 2 in the orbital period precision and a factor of 2.5 in the epoch of inferior conjunction T0. While the new orbital period is consistent with the previous value of Gottlieb et al., the new T0 (and the amplitude of variation of the Bowen line velocities) exhibited a significant shift, which we attribute to variations in the emission geometry with epoch. We propagate the uncertainties on these parameters through to the expected Advanced LIGO-Virgo
\ndetector network observation epochs and quantify the improvement obtained with additional optical observations.