2019MNRAS.486.4144P

We report the identification of a very low mass new binary system 2MASS J10274572+0629104, based on Kepler K2 photometry and Gaia DR2 astrometry. It is located at a distance of 90.0 ± 2.9 pc. The K2 light curve is consistent with a beat pattern of two periodic signals, and using Lomb-Scargle periodogram, we find two rotation periods of 0.2114 ± 0.0002 and 0.2199 ± 0.0003 d. We conclude that these rotation periods arise from two stars with similar spectral types of M6, and have nearly equal luminosity. It is the first ultracool binary system to be identified based on beat patterns in the light curve. Near-infrared spectroscopy yields RV = -9.8 ± 0.6 km s^–1, v sin i = 21.5 ± 1.1 km s^–1, T_eff = 3110 ± 40 K, and log g = 5.2 ± 0.2. The motions are consistent with a young age, as are the rotation periods, but the source does not appear to be part of any known moving group. Furthermore, we detected three strong white light flares in the K2 light curve, with estimated total (UV/optical/IR) energies of 2.6 x 10³³, 5.0 x 10³³, and 3.5 x 10³³ erg, respectively.