Recent Microlensing Results from the MACHO Project
msra(2003)
摘要
We 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. We start with a discussion of
the HST 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: 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. 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, microlensing parallax fit, and a spectrum suggest that in this case
we directly image the lens - a low-mass disk star.
Then we focus on the majority of events observed by the MACHO Project, which
are detected toward the Galactic bulge. We argue that the microlensing optical
depth toward the bulge is best measured using events that have clump giant
sources, which are almost unaffected by blending. From this sample we derive a
low optical depth toward the Galactic bulge of (1.4 +/- 0.3) x 10^{-6}, in good
agreement with other observational constraints and with theoretical 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.
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关键词
black hole,milky way,spectrum
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