Evidence of Systematic Errors in $Spitzer$ Microlens Parallax Measurements. (arXiv:1905.05794v1 [astro-ph.EP])
<a href="http://arxiv.org/find/astro-ph/1/au:+Koshimoto_N/0/1/0/all/0/1">Naoki Koshimoto</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bennett_D/0/1/0/all/0/1">David Bennett</a>
The microlensing parallax campaign with the $Spitzer$ space telescope aims to
measure masses and distances of microlensing planetary events seen towards the
Galactic bulge. The hope is to measure how the distribution of planets depends
on position within the Galaxy. In this paper, we compare 50 microlens parallax
measurements from 2015 $Spitzer$ campaign to three different Galactic models
commonly used in microlensing analyses, and we find that $geq 80,$% of these
events have microlensing parallax values higher than the medians predicted by
Galactic models. We use the Anderson-Darling (AD) and Kolmogorov-Smirnov (KS)
tests on the distributions of the prior probability that the each of the
$Spitzer$ parallax measurements is at least as large as the observed
microlensing parallax values. These tests indicate probabilities of $p_{rm AD}
< 3.0 times 10^{-8}$ and $p_{rm KS} < 4.1 times 10^{-6}$ that the data are
consistent with these Galactic models from the AD and KS tests respectively.
Given that many $Spitzer$ light curves show evidence of large correlated
errors, we conclude that this discrepancy is probably due to systematic errors
in the $Spitzer$ photometry. We consider a simple scheme to correct for this
problem by multiplying the reported error bars on the $Spitzer$ microlensing
parallax measurements by a constant factor, and we find that an error bar
renormalization factor of 3.4 provides reasonable agreement with all three
Galactic models. We expect, however, that corrections to the uncertainties in
the $Spitzer$ photometry itself are likely to be a more effective way to
address the systematic errors. We also argue that is important to include the
${bm pi_{rm E}}$ prior distributions when analyzing events with large
uncertainties or degeneracies in ${bm pi_{rm E}}$ measurements.
The microlensing parallax campaign with the $Spitzer$ space telescope aims to
measure masses and distances of microlensing planetary events seen towards the
Galactic bulge. The hope is to measure how the distribution of planets depends
on position within the Galaxy. In this paper, we compare 50 microlens parallax
measurements from 2015 $Spitzer$ campaign to three different Galactic models
commonly used in microlensing analyses, and we find that $geq 80,$% of these
events have microlensing parallax values higher than the medians predicted by
Galactic models. We use the Anderson-Darling (AD) and Kolmogorov-Smirnov (KS)
tests on the distributions of the prior probability that the each of the
$Spitzer$ parallax measurements is at least as large as the observed
microlensing parallax values. These tests indicate probabilities of $p_{rm AD}
< 3.0 times 10^{-8}$ and $p_{rm KS} < 4.1 times 10^{-6}$ that the data are
consistent with these Galactic models from the AD and KS tests respectively.
Given that many $Spitzer$ light curves show evidence of large correlated
errors, we conclude that this discrepancy is probably due to systematic errors
in the $Spitzer$ photometry. We consider a simple scheme to correct for this
problem by multiplying the reported error bars on the $Spitzer$ microlensing
parallax measurements by a constant factor, and we find that an error bar
renormalization factor of 3.4 provides reasonable agreement with all three
Galactic models. We expect, however, that corrections to the uncertainties in
the $Spitzer$ photometry itself are likely to be a more effective way to
address the systematic errors. We also argue that is important to include the
${bm pi_{rm E}}$ prior distributions when analyzing events with large
uncertainties or degeneracies in ${bm pi_{rm E}}$ measurements.
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