Evidence of Systematic Errors in Spitzer Microlens Parallax Measurements

Abstract

Abstract The microlensing parallax campaign with the Spitzer space telescope aims to measure masses and distances of microlensing events seen toward the Galactic bulge, with a focus on planetary microlensing events. 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 the 2015 Spitzer campaign to three different Galactic models commonly used in microlensing analyses, and we find that ≥74% of these events have microlensing parallax values higher than the medians predicted by Galactic models. The Anderson–Darling tests indicate probabilities of p AD < 6.6 × 10−5 for these three Galactic models, while the binomial probability of such a large fraction of large microlensing parallax values is <4.6 × 10−4. 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 find formally acceptable probabilities of p AD > 0.05 for subsamples of events with bright source stars (I S ≤ 17.75) or Spitzer coverage of the light-curve peak. This indicates that the systematic errors have a more serious influence on faint events, especially when the light-curve peak is not covered by Spitzer. We find that multiplying an error bar renormalization factor of 2.2 by the reported error bars on the Spitzer microlensing parallax measurements provides reasonable agreement with all three Galactic models. However, corrections to the uncertainties in the Spitzer photometry itself are a more effective way to address the systematic errors.