Candidate Brown-dwarf Microlensing Events with Very Short Timescales and Small Angular Einstein Radii

Abstract

Short-timescale microlensing events are likely to be produced by substellar brown dwarfs (BDs), but it is difficult to securely identify BD lenses based on only event timescales $t_{\mathrm{E}}$ because short-timescale events can also be produced by stellar lenses with high relative lens-source proper motions. In this paper, we report three strong candidate BD-lens events found from the search for lensing events not only with short timescales ($t_{\mathrm{E}}\lesssim 6\mathrm{days}$) but also with very small angular Einstein radii (${\theta }_{\mathrm{E}}\lesssim 0.05\mathrm{mas}$) among the events that have been found in the 2016--2019 observing seasons. These events include MOA-2017-BLG-147, MOA-2017-BLG-241, and MOA-2019-BLG-256, in which the first two events are produced by single lenses and the last event is produced by a binary lens. From the Bayesian analysis conducted with the combined $t_{\mathrm{E}}$ and ${\theta }_{\mathrm{E}}$ constraint, it is estimated that the lens masses of the individual events are $0.051_{-0.027}^{+0.100}{M}_{\odot }$, $0.044_{-0.023}^{+0.090}{M}_{\odot }$, and $0.046_{-0.023}^{+0.067}{M}_{\odot }/0.038_{-0.019}^{+0.056}{M}_{\odot }$ and the probability of the lens mass smaller than the lower limit of stars is $\sim 80\%$ for all events. We point out that routine lens mass measurements of short time-scale lensing events require survey-mode space-based observations.

arXiv comment9 pages, 6 figures

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