ar X iv : a st ro - p h / 96 09 13 9 v 2 4 O ct 1 99 6 An Ultraviolet - Excess Optical Candidate for the Luminous Globular Cluster X - ray Source in NGC

The intense, bursting X-ray source in the globular cluster NGC1851 was one of the first cluster sources discovered, but has remained optically unidentified for 25 years. We report here on results from Hubble Space Telescope WFPC2 multicolor images in NGC1851. Our high spatial resolution images resolve ∼200 objects in the 3 radius Einstein X-ray error circle, 40 times as many as in previous ground-based work. A color-magnitude diagram of the cluster clearly reveals a markedly UV-excess object with B∼21, (U−B) ∼ −0.9 only 2 from the X-ray position. The UV-excess candidate is 0 . 12 distant from a second, unremarkable star that is 0.5 mag brighter in B; thus ground-based studies of this field are probably impractical. Three other UV-excess objects are also present among the ∼16,000 objects in the surveyed region of the cluster, leaving a ∼5% probability that a UV-excess object has fallen in the X-ray error circle by chance. No variability of the candidate is seen in these data, although a more complete study is required. If this object is in fact the counterpart of the X-ray source, previous inferences that some globular cluster X-ray sources are optically subluminous with respect to low-mass X-ray binaries in the field are now strengthened. Subject headings: globular clusters: individual (NGC 1851) — stars: neutron — ultraviolet: stars — X-rays: bursts — X-rays: stars 1 Based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.