VATT/Columbia Microlensing Survey of M31 and the Galaxy

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We describe the results and outline the methods used in a search for microlensing events affecting stars in the outer bulge and inner disk of M31, due both to masses in M31 and the Galaxy. These observations, from 1994 and 1995 on the Vatican Advanced Technology Telescope and KPNO 4m, rule out masses over much of the range from 10e-7 Mₛol to 0.08 Mₛol as the primary constituents of the mass of M31 and the Galaxy towards this field. Furthermore we find six candidate events consistent with microlensing due to masses of about 1 Mₛol, but we suspect that some of these may be cases where long-period red supergiant variables may be mistaken for microlensing events. Coverage from anticipated data should be helpful in determining if these sources maintain a constant baseline, and therefore are best described by microlensing events. We analyzed our data using the new technique of "difference image photometry" (also called "pixel lensing"). A brief overview of this technique is included in the Appendix I. This contribution summarizes two other recent papers (Tomaney & Crotts 1996, Crotts & Tomaney 1996).