Probing MACHOs toward the Galactic Bulge

Abstract

If the massive compact halo object (MACHO) fraction of the Galactic dark halo is f ~ 20%, as suggested by some microlensing experiments, then about 1.2% of lensing events toward the Galactic bulge are due to MACHOs. For the 40% of these that lie nearby (Dl < 4 kpc), measurement of their distance Dl would distinguish them from bulge lenses, while measurement of their transverse velocity vl would distinguish them from disk lenses. Hence, it would be possible to identify about 0.5%(f/20%) of all events as due to MACHOs. I show that a planned experiment using the Space Interferometry Mission (SIM PlanetQuest) could thereby detect 1 or 2 such events. This is at the margin of what is required because of a small but nonnegligible background from spheroid stars.