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arXiv · 1703.05887

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

S. -J. Chung·W. Zhu·A. Udalski·C. -U. Lee·Y. -H. Ryu·Y. K. Jung·I. -G. Shin·J. C. Yee·K. -H. Hwang·A. Gould·M. Albrow·S. -M. Cha·C. Han·D. -J. Kim·H. -W. Kim·S. -L. Kim·Y. -H. Kim·Y. Lee·B. -G. Park·R. W. Pogge·R. Poleski·P. Mróz·P. Pietrukowicz·J. Skowron·M. K. Szymański·I. Soszyński·S. Kozłowski·K. Ulaczyk·M. Pawlak·C. Beichman·G. Bryden·S. Calchi Novati·S. Carey·M. Fausnaugh·B. S. Gaudi·Calen B. Henderson·Y. Shvartzvald·B. Wibking
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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 $0.10 \pm 0.02 \ M_\odot$ or a brown dwarf with the mass $55\pm 9 \ M_{J}$, 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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S. -J. Chung, W. Zhu, A. Udalski, C. -U. Lee, Y. -H. Ryu, Y. K. Jung, I. -G. Shin, J. C. Yee, K. -H. Hwang, A. Gould, M. Albrow, S. -M. Cha, C. Han, D. -J. Kim, H. -W. Kim, S. -L. Kim, Y. -H. Kim, Y. Lee, B. -G. Park, R. W. Pogge, R. Poleski, P. Mróz, P. Pietrukowicz, J. Skowron, M. K. Szymański, I. Soszyński, S. Kozłowski, K. Ulaczyk, M. Pawlak, C. Beichman, G. Bryden, S. Calchi Novati, S. Carey, M. Fausnaugh, B. S. Gaudi, Calen B. Henderson, Y. Shvartzvald, B. Wibking. 2017-03-17. OGLE-2015-BLG-1482L: the first isolated low-mass microlens in the Galactic bulge. https://doi.org/10.3847/1538-4357%2Faa67fa

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