Recent microlensing results from the MACHO Project
Piotr Popowski (MPI ) - Tuesday January 13 at 12:00 noon
I describe a few recent microlensing results from the MACHO 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 and Galactic bulge.
I start with a discussion of the HST follow-up observations of microlensing
events toward the LMC which were detected in the first 5 years of the
experiment. We try to understand the nature of the lenses by locating
the lensed source stars along the line-of-sight. Using color-magnitude
diagrams we attempt to distinguish between two possible locations of the
microlensing sources: 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 events have sources behind the LMC, in disagreement
with one of the popular LMC self-lensing scenarios.
During an 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. Astrometry,
a microlensing parallax fit, and a spectrum suggest that in this case we
directly image the lens - a low-mass disk star.
Then I focus on the majority of events observed by the MACHO Project,
which are detected toward the Galactic bulge. I argue that the
microlensing optical depth toward the crowded bulge fields is best
measured using events that have clump giant sources, which are almost
unaffected by blending. From this sample, we derive a low microlensing
optical depth toward the Galactic bulge. This new optical depth is in
good agreement with other observational constraints, and removes the
long-standing tension between microlensing and Galactic models.
The presence of many long-duration events among the bulge candidates
allows us to investigate the microlensing parallax effect. 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
6-solar-mass black holes.
The seminar will be held in 501 Campbell Hall.
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