ON THE DETECTABILITY OF VERY MASSIVE COMPACT OBJECTS WITH GRAVITATIONAL MICROLENSING

If the dark halo of our Galaxy is made of compact objects as massive a s M similar to 10(6)M(.), their detection by means of ordinary microle nsing searches would take a very long time a's the characteristic time scab of such a lensing event is t(0) similar to 200 years. Fortunatel y, the very high magnification events of the numerous faint stars, whi ch are normally well below the detection threshold, have short duratio n peaks with a characteristic time scale t(1/2) similar to t(0)/A(max) where A(max) much greater than 1 is the peak magnification factor The two almost equally bright images are separated by similar to 2''(M/10 (6)M(.))(1/2), and they rotate very rapidly around the lens with the r elative proper motion enhanced by a factor similar to 2A(max). The sam e events will offer an opportunity to study spectroscopically stars wh ich are normally far too faint to be reached.