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Register a new userOGLE-2015-BLG-1482L: the first isolated low-mass microlens in the Galactic bulge
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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 $0.10 pm 0.02 M_odot$ or a brown dwarf with the mass $55pm 9 M_{J}$, which are respectively located at $D_{rm LS} = 0.80 pm 0.19 textrm{kpc}$ and $ D_{rm LS} = 0.54 pm 0.08 textrm{kpc}$, 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 $rho$. Because the $rho$ degeneracy can be resolved only by relatively high cadence observations around the peak, while the textit{Spitzer} cadence is typically $sim 1,{rm day}^{-1}$, we expect that events for which the finite-source effect is seen only in the textit{Spitzer} data may frequently exhibit this $rho$ degeneracy. For OGLE-2015-BLG-1482, the relative proper motion of the lens and source for the low-mass star is $mu_{rm rel} = 9.0 pm 1.9 textrm{mas yr$^{-1}$}$, while for the brown dwarf it is $5.5 pm 0.5 textrm{mas yr$^{-1}$}$. Hence, the degeneracy can be resolved within $sim 10 rm yrs$ from direct lens imaging by using next-generation instruments with high spatial resolution.
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