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

We analyze the single microlensing event OGLE-2015-BLG-1482 simultaneously observed from two ground-based surveys and from $\mathit{\text{Spitzer}}$. The $\mathit{\text{Spitzer}}$ 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 $\mathit{\text{Spitzer}}$. 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 $\mathit{\text{Spitzer}}$ 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 $0.10\pm 0.02M_{\odot }$ or a brown dwarf with the mass $55\pm 9 \ M_{J}$,whicharerespectivelylocatedat$D_{\rm LS} = 0.80 \pm 0.19
$\text{kpc}$$and$ D_{\rm LS} = 0.54 \pm 0.08\ $\text{kpc}$$, 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 $\mathit{\text{Spitzer}}$ 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 $\mathit{\text{Spitzer}}$ cadence is typically $\sim 1,{\rm day}^{-1}$, we expect that events for which the finite-source effect is seen only in the $\mathit{\text{Spitzer}}$ 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}$}$,whileforthebrowndwarfitis$5.5 \pm 0.5\ \textrm{mas yr${}^{-1}$}$\mathrm{.}Hence,thedegeneracycanberesolvedwithin$\sim 10\ \rm yrs$ from direct lens imaging by using next-generation instruments with high spatial resolution.