paper

OGLE-2015-BLG-1482L: the first isolated low-mass microlens in the Galactic bulge

arXiv:1703.05887 · doi:10.3847/1538-4357/aa67fa

Abstract

We analyze the single microlensing event OGLE-2015-BLG-1482 simultaneously observed from two ground-based surveys and from \textit{Spitzer}. The \textit{Spitzer} data exhibit finite-source effects due to the passage of the lens close to or directly over the surface of the source star as seen from \textit{Spitzer}. Such finite-source effects generally yield measurements of the angular Einstein radius, which when combined with the microlens parallax derived from a comparison between the ground-based and the \textit{Spitzer} light curves, yields the lens mass and lens-source relative parallax. From this analysis, we find that the lens of OGLE-2015-BLG-1482 is a very low-mass star with the mass or a brown dwarf with the mass , which are respectively located at and , and thus it is the first isolated low-mass microlens that has been decisively located in the Galactic bulge. The fundamental reason for the degeneracy is that the finite-source effect is seen only in a single data point from \textit{Spitzer} and this single data point gives rise to two solutions for . Because the degeneracy can be resolved only by relatively high cadence observations around the peak, while the \textit{Spitzer} cadence is typically , we expect that events for which the finite-source effect is seen only in the \textit{Spitzer} data may frequently exhibit this degeneracy. For OGLE-2015-BLG-1482, the relative proper motion of the lens and source for the low-mass star is $μ_{\rm rel} = 9.0 \pm 1.9\ \textrm{mas yr$^{-1}$}$, while for the brown dwarf it is $5.5 \pm 0.5\ \textrm{mas yr$^{-1}$}$. Hence, the degeneracy can be resolved within from direct lens imaging by using next-generation instruments with high spatial resolution.

arXiv comment18 pages, 4 figures, accepted for publication in ApJ

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