Magnetic activity and rotational modulation of the secondary star in the V471 Tau eclipsing binary system observed by K2 and TESS

Abstract

V471 Tau is a rare post common envelope binary consisting of a white dwarf and a K2V star, on the verge of entering a cataclysmic phase. This study presents the results of photometric analyses of V471 Tau using data obtained from the Kepler (K2 mission) and TESS space telescopes. The system, observed with high precision and sensitivity by both satellites, exhibits sinusoidal rotational modulation due to cool starspots outside eclipses, as well as short term flares. These phenomena were detected, analyzed, and modeled separately. For the flares, we applied a One Phase Exponential Association (OPEA) model and derived the corresponding parameters. Unlike previous studies, we constructed the cumulative flare frequency distribution using “flare equivalent durations”. The spot activity of the K2V component was analyzed using photometric data from both satellites, revealing three distinct spots consistently detected in each dataset. The long term photometric behavior of the stellar spots was further examined using data spanning a decade. The results show that one spot undergoes significant longitudinal migration, whereas the other two remain nearly constant in position. In addition, ellipsoidal effects and variations in the depth of the primary minimum were investigated to evaluate their contribution to the overall light curve morphology.