Discovery of a Very Bright, Nearby Gravitational Microlensing Event

We report the serendipitous detection of a very bright, very nearbymicrolensing event. In late October 2006, an otherwise unremarkable A0 star ata distance ~1 kpc (GSC 3656-1328) brightened achromatically by a factor ofnearly 40 over the span of several days and then decayed in an apparentlysymmetrical way. We present a light curve of the event based on opticalphotometry from the Center for Backyard Astrophysics and the All Sky AutomatedSurvey, as well as near-infrared photometry from the Peters Automated InfraredImaging Telescope. This light curve is well-fit by a generic microlensingmodel. We also report optical spectra, and Swift X-ray and UV observations thatare consistent with the microlensing interpretation. We discuss and rejectalternative explanations for this variability. The lens star is probably alow-mass star or brown dwarf, with a relatively high proper motion of >20mas/yr, and may be visible using precise optical/infrared imaging taken severalyears from now. A modest, all-sky survey telescope could detect ~10 such eventsper year, which would enable searches for very low-mass planetary companions torelatively nearby stars.Comment: 11 pages, 5 figures. Accepted to ApJ, to appear in the May 1, 2008 issue (v678). Minor changes. Data available upon reques