AAS Meeting #193 - Austin, Texas, January 1999
Session 108. Gravitational Lensing
Display, Saturday, January 9, 1999, 9:20am-4:00pm, Exhibit Hall 1

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[108.02] Candidate Binary Microlensing Events from the MACHO Project

A.C. Becker (U.Washington), C. Alcock, R.A. Allsman, D. R. Alves, T.S. Axelrod, D.P. Bennett, K. H. Cook, A.J. Drake, K. C. Freeman, K. Griest, L.J. King, M.J. Lehner, S.L. Marshall, D. Minniti, B.A. Peterson, P. Popowski, M.R. Pratt, P.J. Quinn, A.W. Rodgers, C.W. Stubbs, W. Sutherland, A. Tomaney, T. Vandehei, D.L. Welch, D. Baines, A. Brakel, B. Crook, J. Howard, T. Leach, D. McDowell, S. McKeown, J. Mitchell, J. Moreland, E. Pozza, P. Purcell, S. Ring, A. Salmon, K. Ward, G. Wyper, A. Heller, S. Kaspi, O. Kovo, D. Maoz, A. Retter (Wise Obs.), S.H. Rhie (Notre Dame), P. Stetson (DAO/NRC), A. Walker (CTIO), MACHO Collaboration, RAPT Collaboration

We present the lightcurves of 22 gravitational microlensing events from the first six years of the MACHO Project gravitational microlensing survey which are likely examples of lensing by binary systems. These events were selected from a total sample of ~ 300 events which were either detected by the MACHO Alert System or discovered through retrospective analyses of the MACHO database. Many of these events appear to have undergone a caustic or cusp crossing, and 2 of the events are well fit with lensing by binary systems with large mass ratios, indicating secondary companions of approximately planetary mass. The event rate is roughly consistent with predictions based upon our knowledge of the properties of binary stars.

The utility of binary lensing in helping to solve the Galactic dark matter problem is demonstrated with analyses of 3 binary microlensing events seen towards the Magellanic Clouds. Source star resolution during caustic crossings in 2 of these events allows us to estimate the location of the lensing systems, assuming each source is a single star and not a short period binary.

* MACHO LMC-9 appears to be a binary lensing event with a caustic crossing partially resolved in 2 observations. The resulting lens proper motion appears too small for a single source and LMC disk lens. However, it is considerably less likely to be a single source star and Galactic halo lens. We estimate the a priori probability of a short period binary source with a detectable binary character to be ~10 %. If the source is also a binary, then we currently have no constraints on the lens location.

* The most recent of these events, MACHO 98-SMC-1, was detected in real-time. Follow-up observations by the MACHO/GMAN, PLANET, MPS, EROS and OGLE microlensing collaborations lead to the robust conclusion that the lens likely resides in the SMC.

The author(s) of this abstract have provided an email address for comments about the abstract: becker@astro.washington.edu

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