Recent Microlensing Results from the MACHO Project

We describe a few recent microlensing results from the MA-CHO Collaboration. The aim of the MACHO Project was the identification and quantitative description of dark and luminous matter in the Milky Way using microlensing toward the Magellanic Clouds (LMC and SMC) and the Galactic bulge. We start with a discussion of the Hubble Space Telescope follow-up observations of the microlensing events toward the LMC detected in the first 5 years of the experiment. Using color-magnitude diagrams we attempt to distinguish between two possible locations of the microlensing sources toward the LMC: 1) in the LMC or 2) behind the LMC. We conclude that unless the extinction is extremely patchy, it is very unlikely that most of the LMC microlensing events have source stars behind the LMC. During examination of the HST images of the 13 LMC events we found a very red object next to the source star of event LMC-5. Based on astrometry, microlensing parallax fit and a spectrum , we argue that in this case we directly image the lens – a low-mass disk star. Then we focus on the majority of the events observed by the MACHO Project, which are detected toward the Galactic bulge. We determine the microlensing optical depth, which describes the amount of matter between us and the Galactic center. We argue that the microlensing optical depth toward the bulge is best measured using only a subclass of the events, namely the ones that have clump giant sources. They are numerous and belong to the brightest stars in the bulge, which makes them insensitive to blending bias. Our analysis of those events suggests that the optical depth toward the Galactic bulge is τ bulge = (1.4±0.3)×10 −6 , in good agreement with other observational constraints and with theoretical models. There are many long-duration events among the bulge candidates. We take advantage of this situation investigating the microlensing parallax effect. We show that the events with the strongest parallax signal are probably due to massive remnants. Events MACHO-96-BLG-5 and MACHO-98-BLG-6 might have been caused by the black holes with masses of order of 6 solar masses.