Brown dwarf companions in binaries detected in the 2021 season high-cadence microlensing surveys

<jats:p><jats:italic>Aims.</jats:italic> As a part of the project aiming to build a homogeneous sample of binary-lens (2L1S) events containing brown dwarf (BD) companions, we investigate the 2021 season microlensing data collected by the Korea Microlensing Telescope Network (KMTNet) survey.</jats:p> <jats:p><jats:italic>Methods.</jats:italic> For this purpose, we first identified 2L1S events by conducting systematic analyses of anomalous lensing events. We then selected candidate BD companion events by applying the criterion that the mass ratio of the lens components is lower than <jats:italic>q</jats:italic><jats:sub>th</jats:sub> ∼ 0.1.</jats:p> <jats:p><jats:italic>Results.</jats:italic> From this procedure, we find four events including KMT-2021-BLG-0588, KMT-2021-BLG-1110, KMT-2021-BLG-1643, and KMT-2021-BLG-1770, for which the estimated mass ratios are <jats:italic>q</jats:italic> ∼ 0.10, 0.07, 0.08, and 0.15, respectively. Event KMT-2021-BLG-1770 was selected as a candidate even though the mass ratio is slightly greater than <jats:italic>q</jats:italic><jats:sub>th</jats:sub> because the lens mass expected from the measured short timescale of the event, <jats:italic>t</jats:italic><jats:sub>E</jats:sub> ∼ 7.6 days, is low. From the Bayesian analyses, we estimate that the primary and companion masses are (<jats:italic>M</jats:italic><jats:sub>1</jats:sub>/<jats:italic>M</jats:italic><jats:sub>⊙</jats:sub>,<jats:italic>M</jats:italic><jats:sub>2</jats:sub>/<jats:italic>M</jats:italic><jats:sub>⊙</jats:sub>) = (0.54<jats:sub>−0.24</jats:sub><jats:sup>+0.31</jats:sup>, 0.053<jats:sub>−0.023</jats:sub><jats:sup>+0.031</jats:sup>) for KMT-2021-BLG-0588L, (0.74<jats:sub>−0.35</jats:sub><jats:sup>+0.27</jats:sup>, 0.055<jats:sub>−0.026</jats:sub><jats:sup>+0.020</jats:sup>) for KMT-2021-BLG-1110L, (0.73<jats:sub>−0.17</jats:sub><jats:sup>+0.24</jats:sup>,0.061<jats:sub>−0.014</jats:sub><jats:sup>+0.020</jats:sup>) for KMT-2021-BLG-1643L, and (0.13<jats:sub>−0.07</jats:sub><jats:sup>+0.18</jats:sup>, 0.020<jats:sub>−0.011</jats:sub><jats:sup>+0.028</jats:sup>) for KMT-2021-BLG-1770L. It is estimated that the probabilities that the lens companions are in the BD mass range are 82%, 85%, 91%, and 59% for the individual events. To confirm the BD nature of the lens companions found in this and previous works by directly imaging the lenses from future high-resolution adaptive-optics (AO) followup observations, we provide the lens-source separations expected in 2030, which is the approximate year of the first AO light on 30 m class telescopes.</jats:p>