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 Hubble Space Telescope for which we derived accurate absolute astrometry, and are located at positions consistent with the radio coordinates to within 0. ′′016 (0.2 σ). We present near-ultraviolet and optical colors 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 colors that are consistent with He white dwarf cooling models for masses ∼ 0.16 − 0.3 M⊙and cooling ages between ∼ 0.1−6 Gyr. The Hα−R625 colors of 47 Tuc U and 47 Tuc T indicate the presence of a strong Hα absorption line, as expected for white dwarfs with a H envelope.