Optical Detection of a Variable Millisecond Pulsar Companion in 47 Tucanae

Using results from radio and X-ray observations of millisecond pulsars in 47Tucanae, and extensive HST U, V, I imaging of the globular cluster core, wehave derived a common astrometric solution good to < 0.1". A close positionalcoincidence 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 ofextensive time series data for this optical candidate shows a 0.004 magnitudesemi-amplitude variation at the period and phase expected from the radioephemeris, and the optical variations are spatially coincident with thecandidate. This provides secure optical detection of the white dwarf companionto the millisecond pulsar, the first such detection in a globular cluster,allowing for comparisons to recent models for such companions with dependencieson mass and age.Comment: 8 pages, 3 figures, accepted in ApJ Letters. Revised content of discussion sectio