Observation of the first gravitational microlensing event in a sparse stellar field : the Tago event

TL;DR

This paper reports the first observation of a gravitational microlensing event in a sparse stellar field, involving a bright, nearby star, discovered by an amateur astronomer, demonstrating the potential of wide-field surveys for detecting rare transient events.

Contribution

It presents the first detection of a microlensing event in a sparse stellar field involving a bright, nearby star, highlighting the role of amateur astronomers and wide-field surveys in such discoveries.

Findings

The event was caused by gravitational microlensing, not stellar variability.

The observed event rate aligns with expectations for stars down to V=12 mag.

High-magnification events are rarer than previously estimated, suggesting a higher true event rate.

Abstract

We report the observation of the first gravitational microlensing event in a sparse stellar field, involving the brightest (V=11.4 mag) andclosest (~ 1 kpc) source star to date. This event was discovered by an amateurastronomer, 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 ~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 exo-planets.