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 star EV Lac, performed with the ASCA observatory in July 1998, during which an exceptionally intense flaring event (lasting approximately 12 ks) was observed: at the flare’s peak, the X-ray count rate in the ASCA GIS detectors was ' 300 times the quiescent value. The physical parameters of the flaring region have been derived by analyzing the decay, using both 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 ' 1.3 × 1010 cm (' 0.5 R∗), large but not exceptional even by solar standards. The flare decays is fast (with a measured e-folding time for the light curve of ≤ 2 ks), but nevertheless the hydrodynamic-based analysis shows strong evidence for sustained heating, with the shape of the light curve dominated by the time evolution of the heating rather than by the natural cooling of the flaring plasma. As a consequence, the quasi-static method yields a much larger estimate of the loop’s length (L ' 2 R∗). The event shows (similarly to some other well-studied large stellar flares) a light curve characterized 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 flare. The energetics of the event are exceptional, with the peak X-ray luminosity of the event reaching up to ' 25% of the 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.