The Great Flare of 2021 November 19 on AD Leonis: Simultaneous XMM-Newton and TESS observations

We present a detailed analysis of a superflare on the active M dwarf star AD Leonis. The event presents a rare case of a stellar flare that was simultaneously observed in X-rays (with XMM-Newton) and in the optical (with the Transiting Exoplanet Survey Satellite, TESS). The radiated energy in the 0.2 - 12 keV X-ray band (1.26 +/- 0.01 x 10(33) erg) and the bolometric value (E-F,E-bol=5.57 +/- 0.03 x 10(33) erg) place this event at the lower end of the superflare class. The exceptional photon statistics deriving from the proximity of AD Leo has enabled measurements in the 1 - 8 angstrom GOES band for the peak flux (X1445 class) and integrated energy (E-F,E-GOES=4.30 +/- 0.05 x 10(32) erg), which enables a direct comparison with data on flares from our Sun. From extrapolations of empirical relations for solar flares, we estimate that a proton flux of at least 10(5)cm(-2)s(-1)sr(-1) accompanied the radiative output. With a time lag of 300 s between the peak of the TESS white-light flare and the GOES band flare peak as well as a clear Neupert effect, this event follows the standard (solar) flare scenario very closely. Time-resolved spectroscopy during the X-ray flare reveals, in addition to the time evolution of plasma temperature and emission measure, a temporary increase in electron density and elemental abundances, and a loop that extends into the corona by 13% of the stellar radius (4 x 10(9) cm). Independent estimates of the footprint area of the flare from TESS and XMM-Newton data suggest a high temperature of the optical flare (25000 K), but we consider it more likely that the optical and X-ray flare areas represent physically distinct regions in the atmosphere of AD Leo.