Evidence of Halo Microlensing in M31

We report results from intensive monitoring of two fields on either side ofM31,emphasizing microlensing involving stars and masses in M31. These resultsstem from the three-year VATT/Columbia survey of variability on 3d to 2mtimescales. Observations were conducted intensively from 1997-1999, withbaselines 1995-now,at the Vatican Advanced Technology Telescope, MDM 1.3-meter,and Isaac Newton telescopes, totaling about 200 epochs. The two fields cover560 square arcmin total, along the minor axis on either side of M31. Candidatemicrolensing events are subject to tests to distinguish microlenses fromvariable stars. Fourprobable microlensing events total, compared to carefullycomputed event rate and efficiency models, indicate a marginally significantmicrolensing activity above expectations for the stellar lenses alone in M31(and the Galaxy) acting as lenses. Maximum likelihood analyses of the eventdistribution in timescale and across the face of M31 indicates a microlensingdark matter halo fraction consistent with that seen in our Galaxy towards theLMC (Alcock et al. 2000). For a nearly singular isothermal sphere model, wefind a microlensing halo mass fraction f=0.29 +0.30/-0.13 of total dark matter,and a poorly constrained lensing component mass 0.02 to 1.5 solar mass, 1 sigmalimits). This study is the prototype for a larger one approaching completion;between them there is significant evidence for asymmetry in the distribution ofmicrolensing events across the face of M31, and a large population of halomicrolensing objects. (Abridged)Comment: 25 pages, LaTex; 11 figures. Revised, refereed manuscript resubmitted to The Astrophysical Journal. Corrected Figure 8 and other minor point