Detection of Rotation in a Binary Microlens: PLANET Photometry of MACHO 97-BLG-41

We analyze PLANET collaboration data for MACHO 97-BLG-41, the only microlensing event observed to date in which the source transits two disjoint caustics. The PLANET data, consisting of 46 V-band and 325 I-band observations from five southern observatories, span a period from the initial alert until the end of the event. Our data are incompatible with a static binary lens, but are well fit by a rotating binary lens of mass ratio q = 0.34 and angular separation d ≈ 0.5 (in units of the Einstein ring radius) in which the binary separation changes in size by δd = −0.070± 0.009 and in orientation by δθ = 5.61± 0.36 during the 35.17 days between the separate caustic transits. We use this measurement combined with other observational constraints to Univ. of Canterbury, Dept. of Physics & Astronomy, Private Bag 4800, Christchurch, New Zealand Kapteyn Astronomical Institute, Postbus 800, 9700 AV Groningen, The Netherlands Institut d’Astrophysique de Paris, INSU CNRS, 98 bis Boulevard Arago, F-75014, Paris, France South African Astronomical Observatory, P.O. Box 9, Observatory 7935, South Africa Ohio State University, Department of Astronomy, Columbus, OH 43210, U.S.A. Univ. of Tasmania, Physics Dept., G.P.O. 252C, Hobart, Tasmania 7001, Australia Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD. 21218 U.S.A. Anglo-Australian Observatory, P.O. Box 296, Epping NSW 1710, Australia Perth Observatory, Walnut Road, Bickley, Perth 6076, Australia European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany