A Search for Magnetically Active Binary Counterparts to Chandra X-Ray Sources in 47 Tucanae with HST Imaging

Abstract Using Hubble Space Telescope (HST) imaging, we searched for candidate magnetically active binary (AB) counterparts to low-luminosity Chandra X-ray sources in the globular cluster 47 Tucanae (NGC 104). We consider the catalog of 300 Chandra sources within the half-light radius of 47 Tuc compiled by C. O. Heinke et al. (2005). The 10-band HST imaging used to identify these sources spans the range from 275–658 nm. We generated a cross-matched catalog of objects detected by photometric reductions. We used this catalog to search for counterparts within Chandra source error circles using color–magnitude diagrams and a color–color plot. We found 88 likely ABs that lie primarily in the binary sequence region above the main sequence, 10 possible ABs within this region, 12 sub-subgiants, three subgiants, six blue stragglers, and two yellow stragglers. Of these 121 proposed counterparts, 67 are newly identified. One of the ABs appears to be a pre-cataclysmic variable. Our analysis of the spatial distribution of these proposed counterparts indicates that they are significantly more centrally concentrated than the main-sequence turnoff stars. The inferred characteristic mass for these objects is 1.3 ± 0.1 M ⊙ . A comparison of the numbers of X-ray detected ABs above L X = 10 30 erg s −1 in four globular clusters indicates that their relative numbers can be approximately predicted by the mass and binary fractions of each cluster, suggesting that ABs are a primordial population.