Microlensing optical depth and event rate toward the Galactic bulge from eight years of OGLE-IV observations. (arXiv:1906.02210v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Mroz_P/0/1/0/all/0/1">P. Mroz</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Udalski_A/0/1/0/all/0/1">A. Udalski</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Skowron_J/0/1/0/all/0/1">J. Skowron</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Szymanski_M/0/1/0/all/0/1">M.K. Szymanski</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Soszynski_I/0/1/0/all/0/1">I. Soszynski</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Wyrzykowski_L/0/1/0/all/0/1">L. Wyrzykowski</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Pietrukowicz_P/0/1/0/all/0/1">P. Pietrukowicz</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kozlowski_S/0/1/0/all/0/1">S. Kozlowski</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Poleski_R/0/1/0/all/0/1">R. Poleski</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ulaczyk_K/0/1/0/all/0/1">K. Ulaczyk</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Rybicki_K/0/1/0/all/0/1">K. Rybicki</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Iwanek_P/0/1/0/all/0/1">P. Iwanek</a>
The number and properties of observed gravitational microlensing events
depend on the distribution and kinematics of stars and other compact objects
along the line of sight. In particular, precise measurements of the
microlensing optical depth and event rate toward the Galactic bulge enable
strict tests of competing models of the Milky Way. Previous estimates, based on
samples of up to a few hundred events, gave larger values than expected from
the Galactic models and were difficult to reconcile with other constraints on
the Galactic structure. Here, we used long-term photometric observations of the
Galactic bulge by the OGLE survey to select a homogeneous sample of 8,000
gravitational microlensing events. We created the largest and the most accurate
microlensing optical depth and event rate maps of the Galactic bulge. The new
maps ease the tension between the previous measurements and Galactic models.
They are consistent with some earlier calculations based on bright stars and
are systematically 30% smaller than the other estimates based on “all-source”
samples of microlensing events. The difference is caused by the careful
estimation of the source star population. The new maps agree well with
predictions based on the Besancon model of the Galaxy. Apart from testing the
Milky Way models, our maps may have numerous other applications, such as the
measurement of the initial mass function or constraining the dark matter
content in the Milky Way center. The new maps will also inform planning of the
future space-based microlensing experiments by revising the expected number of
events.
The number and properties of observed gravitational microlensing events
depend on the distribution and kinematics of stars and other compact objects
along the line of sight. In particular, precise measurements of the
microlensing optical depth and event rate toward the Galactic bulge enable
strict tests of competing models of the Milky Way. Previous estimates, based on
samples of up to a few hundred events, gave larger values than expected from
the Galactic models and were difficult to reconcile with other constraints on
the Galactic structure. Here, we used long-term photometric observations of the
Galactic bulge by the OGLE survey to select a homogeneous sample of 8,000
gravitational microlensing events. We created the largest and the most accurate
microlensing optical depth and event rate maps of the Galactic bulge. The new
maps ease the tension between the previous measurements and Galactic models.
They are consistent with some earlier calculations based on bright stars and
are systematically 30% smaller than the other estimates based on “all-source”
samples of microlensing events. The difference is caused by the careful
estimation of the source star population. The new maps agree well with
predictions based on the Besancon model of the Galaxy. Apart from testing the
Milky Way models, our maps may have numerous other applications, such as the
measurement of the initial mass function or constraining the dark matter
content in the Milky Way center. The new maps will also inform planning of the
future space-based microlensing experiments by revising the expected number of
events.
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