Precision Ephemerides For Gravitational Wave Searches: II. Cyg X-2

Abstract

Accreting neutron stars in low-mass X-ray binaries (LMXBs) are candidate high-frequency persistent gravitational wave sources. These may be detectable with next generation interferometers such as Advanced LIGO/VIRGO within this decade. However, the search sensitivity is expected to be limited principally by the uncertainty in the binary system parameters. We combine new optical spectroscopy of Cyg X-2 obtained with the Liverpool Telescope (LT) with available historical radial velocity data, which gives us improved orbital parameter uncertainties based on a 44-year baseline. We obtained an improvement of a factor of 2.6 in the orbital period precision and a factor of 2 in the epoch of inferior conjunction T_0. The updated orbital parameters imply a mass function of 0.65 +/- 0.01 M_sun, leading to a primary mass (M_1) of 1.67 +/- 0.22 M_sun (for i=62.5 +/- 4 deg). In addition, we estimate the likely orbital parameter precision through to the expected Advanced LIGO and VIRGO detector observing period and quantify the corresponding improvement in sensitivity via the required number of templates.