A blue straggler binary with three progenitors in the core of a globular cluster?

We show that the X-ray source W31 in the core of the globular cluster 47 Tucanae is physically associated with the bright blue straggler BSS 7. The two sources are astrometrically matched to 07061, with a chance coincidence probability of less than 1%. We then analyze optical time-series photometry obtained with the Hubble Space Telescope (HST) and find that BSS 7 displays a 1.56 day periodic signal in the I band. We also construct a broadband (far-ultraviolet through far-red) spectral energy distribution for BSS 7 and fit this with single and binary models. The binary model is a better fit to the data, and we derive the corresponding stellar parameters. All of our findings are consistent with BSS 7 being a detached binary consisting of a blue straggler primary with an X-ray-active, upper-main-sequence companion. The formation of such a system would necessarily involve at least three stars, which is consistent with recent JV-body models in which blue stragglers often form via multiple encounters that can involve both single and binary stars. However, we cannot yet entirely rule out the possibility that BSS 7 descended directly from a binary system via mass transfer. The system parameters needed to distinguish definitively between these scenarios may be obtainable from time-resolved spectroscopy.