Optical Identification of the X-ray Burster in the Globular

We have obtained exposures of the field of X 0512-401 in the globular cluster NGC1851, in X-rays with the Chandra X-ray Observatory, and in the far-UV with the Hubble Space Telescope. We derive an accurate new X-ray position (within ∼1) for X 0512-401, which enables us to confirm that the only plausible candidate for the optical/UV counterpart is the Star A, which we previously identified from WFPC2 imaging. We find no evidence for X-ray or UV flux modulation on the ultra-short (. 1 hr) expected binary period, which implies a low system inclination. In addition, we have detected and spatially resolved an X-ray burst event, confirming the association of the burster, quiescent X-ray source, and optical object. The very large LX/Lopt of this object implies an extraordinarily compact system, similar to the sources in NGC6624 and NGC6712. Subject headings: globular clusters: individual (NGC1851) – stars: neutron – ultraviolet: stars – X-rays: stars – X-rays: bursts