Identification of bright lenses from the astrometric observations of gravitational microlensing events

When a source star is gravitationally microlensed by a dark lens, the centr oid of the source star image is displaced relative to the position of the u nlensed source star, with an elliptical trajectory. Recently, routine astro metric follow-up measurements of these source star image centroid shifts by using high-precision interferometers have been proposed to measure the len s proper motion, which can resolve the lens parameter degeneracy in the pho tometrically determined Einstein time-scale. When an event is caused by a b right lens, on the other hand, the astrometric shift is affected by the lig ht from the lens, but one cannot identify the existence of the bright lens from the observed trajectory because the resulting trajectory of the bright lens event is also an ellipse. As results, lensing parameters determined f rom the trajectory differ from those of a dark lens event, causing an incor rect identification of the lens population. In this paper, we show that alt hough the shape and size of the astrometric centroid shift trajectory are c hanged because of the bright lens, the angular speed of centroid shifts aro und the apparent position of the unlensed source star is not affected by th e lens brightness. Therefore, one can identify the existence of a bright le ns and determine its brightness by comparing the lens parameters determined from the 'angular speed curve' with those determined from the trajectory o f observed centroid shifts, Once the lens brightness is determined, one can correct for the lens proper motion. As the proposed method provides inform ation about both the lens brightness (dark or bright) and the corrected val ues of the physical parameters of the lens, one can constrain the nature of massive compact halo objects (MACHOs) significantly better.