A millisecond pulsar optical counterpart with large-amplitude variability in the globular cluster 47 Tucanae

Using extensive Hubble Space Telescope imaging, combined with Chandra X-ray and Parkes radio data, we have detected the optical binary companion to a second millisecond pulsar (MSP) in the globular cluster 47 Tucanae. This faint (V = 22.3) blue (V-I= 0.7) star shows a large-amplitude (60%-70%) sinusoidal variation in both V and I. The period (3.19066 hr) and phase of the variation match those of the MSP 47 Tue W (which does not have an accurate radio timing position) to within 0.5 s and 1.2 minutes, respectively, well within the 1 σ errors. The phase dependence of the intensity and color implies that heating of a tidally locked companion causes the observed variations. The eclipsing nature of this MSP in the radio, combined with the relatively large companion mass (greater than 0.13 M_⊙) and the companion's position in the color-magnitude diagram, suggests that the companion is a main-sequence star, a rare circumstance for an MSP companion. This system is likely to have had a complex evolution and represents an interesting case study in MSP irradiation of a close companion. We present evidence for another optical variable with similar properties to the companion of 47 Tuc W. This variable also may be an MSP companion, although no radio counterpart has yet been detected.