PIXEL LENSING SEARCH FOR BRIGHT MICROLENSING EVENTS AND VARIABLES IN THE GALACTIC BULGE

We describe a new method to search for gravitational microlensing towa rd the Galactic bulge that employs a small camera rather than a conven tional telescope and probes new regions of parameter space. The small aperture (similar to 65 mm) permits detection of stellar flux variatio ns corresponding to magnitudes 7 less than or similar to I less than o r similar to 16, while current searches are restricted by saturation t o I greater than or similar to 15. The large pixel size (similar to 10 '') and similar to(6 deg)(2) held of view allows observation of the e ntire bulge with a few pointings. With this large pixel size (and with the even larger 30 '' point-spread function [PSF] that we advocate) m ost bulge stars are unresolved, so one is in the regime of pixel lensi ng: microlensing and other forms of stellar variation are detected fro m the difference of pixel counts in successive images. We identify thr ee principal uses of such a search. First, the observations are analog ous to normal pixel lensing observations of the bulge of M31 but are c arried out under conditions where the detected events can be followed up in detail. This permits crucial checks on the systematics of the M3 1 searches. Second, the search gives a complete inventory of bright bu lge variables. Third, ''extreme microlensing events'' (EMEs) can be fo und in real time. EMEs are events with maximum magnifications A(max) s imilar to 200 which, if they were observed intensively from two observ atories, could yield the mass, distance, and speed of the gravitationa l lens. The instrumentation required to carry out the observations is inexpensive. The observations could be made in parallel with existing microlensing searches and/or follow-up observations. The data reductio n is much simpler than in ordinary pixel lensing because the PSF can b e fixed by the optics and so does not vary with atmospheric conditions .