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

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}_{{\rm{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}_{{\rm{E}}}\lesssim 6\,\mathrm{days} ) but also with very small angular Einstein radii ( ${\theta }_{{\rm{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 Monte Carlo simulations of Galactic events conducted with the combined ${t}_{{\rm{E}}} and ${\theta }_{{\rm{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 ˜80% for all events. We point out that routine lens mass measurements of short-timescale lensing events require survey-mode space-based observations.