Resolving gravitational microlensing events with long-baseline optical interferometry - Prospects for the ESO Very Large Telescope Interferometer

Until now, the detailed interpretation of the observed microlensing events has suffered from the fact that the physical parameters of the phenomenon c annot be uniquely determined from the available astronomical measurements, i.e. the photometric lightcurves. The situation will change in the near-fut ure with the availability of long-baseline, sensitive optical interferomete rs, which should be able to resolve the images of the lensed objects into t heir components. For this, it will be necessary to achieve a milliarcsecond resolution on sources with typical magnitudes K greater than or similar to 12. Indeed, brighter events have never been observed up to now by micro-le nsing surveys. We discuss the possibilities opened by the use of long basel ine interferometry in general, and in particular for one such facility, the ESO VLT Interferometer, which will attain the required performance. We dis cuss the expected accuracy and limiting magnitude of such measurements. On the basis of the database of the events detected by the OGLE experiment, we estimate the number of microlenses that could be available for measurement s by the VLTI. We find that at least several tens of events could be observ ed each year. In conjunction with the photometric data, our ability to meas ure the angular separation between the microlensed images will enable a dir ect and unambiguous determination of both their masses and locations.