An extreme X-ray flare observed on EV Lac by ASCA in July 1998

We present a long (150 ks elapsed time) X-ray observation of the dM3.5e sta r EV Lac, performed with the ASCA observatory in July 1998, during which an exceptionally intense flaring event (lasting approximately 12 ks) was obse rved: at the flare's peak, the X-ray count rate in the ASCA CIS detectors w as similar or equal to 300 times the quiescent value. The physical paramete rs of the flaring region have been derived by analyzing the decay, using bo th a "classic" quasi-static approach and an approach based on hydrodynamic simulations of decaying flaring loops. Notwithstanding the large peak X-ray luminosity, this second method shows that the flare's decay is compatible with its being produced in a relatively compact region of semi-length L sim ilar or equal to 1.3 x 10(10) cm (similar or equal to 0.5 R*), large but no t exceptional even by solar standards. The flare decays is fast (with a mea sured e-folding time for the light curve of less than or equal to 2 ks), bu t nevertheless the hydrodynamic-based analysis shows strong evidence for su stained heating, with the shape of the light curve dominated by the time ev olution of the heating rather than by the natural cooling of the flaring pl asma. As a consequence, the quasi-static method yields a much larger estima te of the loop's length (L similar or equal to 2 R*). The event shows (simi larly to some other well-studied large stellar flares) a light curve charac terized by two separate decay time constants (with the initial decay being faster) and a significant enhancement in the plasma metal abundance at the peak of the hare. The energetics of the event are exceptional, with the pea k X-ray luminosity of the event reaching up to similar or equal to 25% of t he bolometric luminosity of the star, making this one of the largest X-ray flare (in relative terms) observed to date on a main-sequence star.