MOA-2010-BLG-328: Keck and HST Expose the Limits of Occams Razor in Microlensing

Abstract We present high-resolution follow-up data of the microlensing event MOA-2010-BLG-328, using Keck and the Hubble Space Telescope. Keck data, taken 8 yr after the event, reveal a strong lens detection enabling measurement of lens flux and source-lens relative proper motion. We find μ rel,Hel = 4.07 ± 0.34 mas yr −1 , with the lens ∼10 times fainter than the source. The lens was very faint in the Hubble passbands, and the small ∼35 mas lens-source separation made its detection difficult. However, we estimated lens magnitudes in Hubble bands by constraining its location to match the Keck K -band detection. K. Furusawa et al. reported a degenerate light curve, with viable models including microlensing parallax and lens orbital motion, or xallarap. We attempt to break this degeneracy by remodeling the event with constraints from high-resolution imaging, but find none of their models fit. Instead, fitting the follow-up data requires microlensing parallax, lens orbital motion, and a magnified binary companion to the host, with xallarap. The high-resolution data do break the “ecliptic degeneracy” common in Galactic bulge microlensing events with parallax signals. Models omitting any of these effects are excluded. However, a new degeneracy allows different combinations of parallax, orbital motion, xallarap, and source companion magnification to explain the light curve. These require either a nearby late M dwarf host or a more distant early M dwarf host, indistinguishable with NIR data alone.