Candidate Brown-dwarf Microlensing Events with Very Short Timescales and Small Angular Einstein Radii
Han, Cheongho; Udalski, Andrzej; Lee, Chung-Uk; Zang, Weicheng; Albrow, Michael D.; Chung, Sun-Ju; Gould, Andrew; Hwang, Kyu-Ha; Jung, Youn Kil; Ryu, Yoon-Hyun; Shvartzvald, Yossi; Shin, In-Gu; Yee, Jennifer C.; Cha, Sang-Mok; Kim, Doeon; Kim, Dong-Jin; Kim, Seung-Lee; Lee, Dong-Joo; Lee, Yongseok; Park, Byeong-Gon; Pogge, Richard W.; Mróz, Przemek; Szymański, Michał K.; Skowron, Jan; Soszyński, Igor; Pietrukowicz, Paweł; Kozłowski, Szymon; Rybicki, Krzysztof A.; Iwanek, Patryk; Ulaczyk, Krzysztof; Wrona, Marcin; Fukui, Akihiko; KMTNet Collaboration; Poleski, Radek; Bozza, Valerio; Kim, Hyoun-Woo; Sumi, Takahiro; Bond, Ian A.; Abe, Fumio; Barry, Richard; Bennett, David P.; Bhattacharya, Aparna; Donachie, Martin; Fujii, Hirosane; Hirao, Yuki; Itow, Yoshitaka; Kondo, Iona; Koshimoto, Naoki; Li, Man Cheung Alex; Matsubara, Yutaka; Muraki, Yasushi; Miyazaki, Shota; Ranc, Clément; Rattenbury, Nicholas J.; Satoh, Yuki; Shoji, Hikaru; Suzuki, Daisuke; Tristram, Paul J.; Yamawaki, Tsubasa; Yonehara, Atsunori; Jee, M. James; OGLE Collaboration; Kamei, Yuhei; Nagakane, Masayuki; Suematsu, Haruno; Sullivan, Denis J.; Yamakawa, Takeharu; MOA Collaboration
South Korea, United States, Poland, Germany, New Zealand, Italy, Israel, China, United Kingdom, Japan, Spain
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}_{{\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.