Optical detection of a variable millisecond pulsar companion in 47 Tucanae

Using results from radio and X-ray observations of millisecond pulsars in 47 Tucanae, and extensive Hubble Space Telescope U, V, and I imaging of the globular cluster core, we have derived a common astrometric solution good to less than 0."1. A close positional coincidence is found for 47 Tuc U, a 4.3 ms pulsar in a 0.429 day orbit, detected in radio and X-rays, with an m(v) = 20.9 blue star. Analysis of extensive time series data for this optical candidate shows a 0.004 mag semiamplitude variation at the period and phase expected from the radio ephemeris, and the optical variations are spatially coincident with the candidate. This provides secure optical detection of the white dwarf companion to the millisecond pulsar, the first such detection in a globular cluster, allowing for comparisons to recent models for such companions with dependencies on mass and age.