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J.L. Quinn (Notre Dame), C. Alcock (LLNL), R.A. Allsman (Supercomputing Facility, ANU), D. Alves (STScI), T.S. Axelrod (MSSSO), D. Becker (Washington), D.P. Bennett (Notre Dame), K.H. Cook (LLNL), A.D. Drake, K.C. Freeman (MSSSO), M. Geha (LLNL), K. Griest (UCSD), M.J. Lehner (Sheffield), S.L. Marshall (LLNL), D. Minniti (P. Universidad Católica), C.A. Nelson (LLNL), B.A. Peterson (MSSSO), P. Popowski (LLNL), P.J. Quinn (ESO), S.H. Rhie (Notre Dame), C.W. Stubbs (Washington), W. Sutherland (Oxford), A.B. Tomaney (Washington), T. Vandehei (UCSD), D. Welch (McMaster Univ.)
Microlensing event MACHO-96-BLG-5 is the longest microlensing event observed to date, and it has a strong microlensing parallax signal which allows an estimate of the distance and mass of the lens. The mass estimate indicates that the that the lens is probably a black hole stellar remnants, and if so, it would be the first isolated black hole stellar remnant yet discovered. However, the lens mass estimate has a systematic uncertainty due to the fact that the source star of MACHO-96-BLG-5 is significantly blended with stars unresolved from ground based images. Here, we present the analysis of HST WFPC2 images of MACHO-96-BLG-5 which allows a probable identification of the lensed source star and removes this systematic uncertainty in the lens mass estimate. Since the blending of stellar images is common in microlensing data, the technique presented here will be useful for removing similar ambiguities from similar microlensing events. This work was funded by the NSF through the Center for Particle Astrophysics and by the DOE (through LLNL).