2019A&A...628A..79S

We present short-cadence data of the ultra-active star AB Dor measured by the Transiting Exoplanet Survey Satellite (TESS). In the TESS light curves of AB Dor, we found numerous flare events in addition to time-variable rotational modulation with an amplitude of up to 7%. We identified eight superflares (releasing more than 10³⁴erg) and studied their morphologies and energetics. We compared these flares to both the most energetic solar flare seen in total solar irradiance measurements as well as to a very energetic flare on AB Dor observed by XMM-Newton, the superflare nature of which we also demonstrate. The total energy of both the solar flare and the event on AB Dor emitted in the optical exceed their respective X-ray outputs possibly by an order of magnitude, suggesting that the dominant energy loss of such flares actually occurs at optical wavelengths. Superflares are found to take place on AB Dor at a rate of about one per week, and due to the star's proximity and brightness can be studied in excruciating detail. Thus the TESS data offer a superb possibility to study the frequency and energetics of superflare events for stars in the solar neighborhood and at large.