New Method Used to Detect Exomoon Around 4 Jupiter Mass Free Floating Gas Giant

NEW METHOD TO MEASURE PROPER MOTIONS OF MICROLENSED SOURCES: APPLICATION TO CANDIDATE FREE-FLOATING-PLANET EVENT MOA-2011-BLG-262

Authors:

Skowron et al

Abstract:

We develop a new method to measure source proper motions in microlensing events, which can partially overcome problems due to blending. It takes advantage of the fact that the source position is known precisely from the microlensing event itself. We apply this method to the event MOA-2011-BLG-262, which has a short timescale t E = 3.8 day, a companion mass ratio q = 4.7 × 10–3, and a very high or high lens-source relative proper motion μrel = 20 mas yr–1 or 12 mas yr–1 (for two possible models). These three characteristics imply that the lens could be a brown dwarf or a massive planet with a roughly Earth-mass "moon." The probability of such an interpretation would be greatly increased if it could be shown that the high lens-source relative proper motion was primarily due to the lens rather than the source. Based on the long-term monitoring data of the Galactic bulge from the Optical Gravitational Lensing Experiment, we measure the source proper motion that is small, ${\boldsymbol {\mu }}_s = (-2.3, -0.9)\pm (2.8,2.6)\,{\rm mas\:yr^{-1}}$ in a (north, east) Galactic coordinate frame. These values are then important input into a Bayesian analysis of the event presented in a companion paper by Bennett et al.