Observation of the First Gravitational Microlensing Event in a Sparse Stellar Field: The Tago Event

We report the observation of the first gravitational microlensing event in a sparse stellar field, involving the brightest (V = 11.4 mag) and closest (~1 kpc) source star to date. This event was discovered by an amateur astronomer, A. Tago, on 2006 October 31 as a transient brightening, by ~4.5 mag during a ~15 day period, of a normal A-type star (GSC 3656-1328) in the Cassiopeia constellation. Analysis of both spectroscopic observations and the light curve indicates that this event was caused by gravitational microlensing rather than an intrinsically variable star. Discovery of this single event over a 30 year period is roughly consistent with the expected microlensing rate for the whole sky down to V = 12 mag stars. However, the probability for finding events with such a high magnification (~50) is much smaller, by a factor of ~1/50, which implies that the true event rate may be higher than expected. This discovery indicates the potential of all sky variability surveys, employing frequent sampling by telescopes with small apertures and wide fields of view, for finding such rare transient events, and using the observations to explore Galactic disk structure and search for exoplanets.