X-ray Counterparts to Advanced LIGO/Virgo Transients

Advanced LIGO and Advanced Virgo will be all-sky monitors for gravitational waves from binary neutron star mergers within a few hundred Mpc. Because the gravitational wave detectors will localize mergers to tens or hundreds of square degrees, placing the sources in an astronomical context will require also finding electromagnetic counterparts. Here, we discuss work to characterize bright, variable, soft X-ray sources within the LIGO/Virgo horizon, using data from the XMM-Newton Slew Survey and the ROSAT All Sky Survey. The resulting list of apparently transient events establishes the rate of incidental coincidences expected within a large LIGO/Virgo error box, and may lead to the discovery of unusual X-ray sources.