MICROLENSING SEARCH OF 10(6) QUASARS

By monitoring 10(6) quasars one could search for lensing by stars and massive compact halo objects (MACHOs) out to redshifts z similar to 4. If MACHOs have a present cosmological density Omega(L,0) = 1%, then the expected event rate is Gamma similar to 200 yr(-1). The expected event rate for known stars in galaxies is Gamma similar to 20 yr(-1) assuming that their present cosmological density is Omega(L,0) = 0.3%. Typical event times are t(e) similar to 3 yr for MACHOs and t(e) similar to 10 yr for stars. By comparing the optical depths to quasars at different redshifts, one could measure the star-formation and MACHO-formation history of the universe. By comparing the timescales of events found parallel and perpendicular to the Sun's motion relative to the microwave background, one could measure or constrain the characteristic scale of large-scale motions. The lensing events themselves would help probe the structure of quasars on scales of 50-1500 AU. The monitoring program could be carried out with a single dedicated 1 m telescope with a 4 deg(2) camera. Quasar lensing events can be unambiguously distinguished from quasar variability because in the former case the broad lines are unaffected while in the latter they respond to the variation in the continuum on timescales similar to 1 yr.