Discovery of Astrometric Accelerations by Dark Companions in the Globular Cluster Centauri

We present results from the search for astrometric accelerations of stars in omega Centauri using 13 yr of regularly scheduled Hubble Space Telescope WFC3/UVIS calibration observations in the cluster core. The high-precision astrometry of similar to 160,000 sources was searched for significant deviations from linear proper motion. This led to the discovery of four cluster members and one foreground field star with compelling acceleration patterns. We interpreted them as the result of the gravitational pull by an invisible companion and determined preliminary Keplerian orbit parameters, including the companion's mass. For the cluster members, our analysis suggests periods ranging from 8.8 to 19+ yr and dark companions in the mass range of similar to 0.7 to similar to 1.4M(circle dot). At least one companion could exceed the upper mass boundary of white dwarfs and can be classified as a neutron star candidate.