AN ULTRAVIOLET-EXCESS OPTICAL CANDIDATE FOR THE LUMINOUS GLOBULAR-CLUSTER X-RAY SOURCE IN NGC-1851

The intense, bursting X-ray source in the globular cluster NGC 1851 wa s one of the first cluster sources discovered, but has remained optica lly unidentified for 25 years. We report here on results from Hubble S pace Telescope WFPC2 multicolor images in NGC 1851. Our high spatial r esolution images resolve similar to 200 objects in the 3 '' radius Ein stein X-ray error circle, 40 times as many as in previous ground-based work. A color-magnitude diagram of the cluster clearly reveals a mark edly UV-excess object with B similar to 21, (U-B) similar to -0.9, onl y 2 '' from the X-ray position. The UV-excess candidate is 0 ''.12 dis tant from a second, unremarkable star that is 0.5 mag brighter in B; t hus ground-based studies of this field are probably impractical. Three other UV-excess objects are also present among the similar to 16,000 objects in the surveyed region of the cluster, leaving a similar to 5% probability that a UV-excess object has fallen in the X-ray error cir cle 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 glo bular cluster X-ray sources are optically subluminous with respect to low-mass X-ray binaries in the field are now strengthened.