Discovery of near-ultraviolet counterparts to millisecond pulsars in the globular cluster 47 Tucanae

Abstract

We report the discovery of the likely white dwarf companions to radio millisecond pulsars 47 Tuc Q and 47 Tuc S in the globular cluster 47 Tucanae. These blue stars were found in near-ultraviolet images from the <i>Hubble Space Telescope</i> for which we derived accurate absolute astrometry, and are located at positions consistent with the radio coordinates to within 0.016 arcsec (0.2σ). We present near-ultraviolet and optical colours for the previously identified companion to millisecond pulsar 47 Tuc U, and we unambiguously confirm the tentative prior identifications of the optical counterparts to 47 Tuc T and 47 Tuc Y. For the latter, we present its radio-timing solution for the first time. We find that all five near-ultraviolet counterparts have U₃₀₀ − B₃₉₀ colours that are consistent with He white dwarf cooling models for masses ∼0.16–0.3 M⊙ and cooling ages within ∼0.1–6 Gyr. The Hα − R₆₂₅ colours of 47 Tuc U and 47 Tuc T indicate the presence of a strong Hα absorption line, as expected for white dwarfs with an H envelope.