Flare from the secondary star of an extreme period binary

Abstract Photometric observations were conducted on the short-period and highly active M dwarf BX Tri from 2016 to 2023. From ground-based observations and data from the Transiting Exoplanet Survey Satellite (TESS), a total of 196 sets of light curves were analyzed using Wilson–Devinney (W-D) method. The photometric analyses of BX Tri reveal a semidetached configuration, where the primary star accounts for a minimum of $80\%$ of the total luminosity in this binary system. The magnetic activity cycle of BX Tri was determined to be $9.4\:$yr, as indicated by the variability in spot longitudes. Three flares were detected by TESS. The long-lasting flare$_{1797}$ could originate from a polar region on the secondary star. Given the ongoing flare eruptions, BX Tri has the potential to merge into a single star through mergebursts.