Twin X-ray flares and the active corona of AB Dor observed with BeppoSAX

We present two X-ray observations of the young, active star AB Dor, perform ed with BeppoSAX on Nov 9 and Nov 29 1997, and lasting 50 ksec and 140 ksec , respectively. Two, quite similar, large flares occurred during these obse rvations, characterized by a 100-fold increase of the X-ray flux, peak temp eratures of about 10(8) K, and peak emission measures of similar to 5 X 10( 54) cm(-3) . We have performed a detailed, time-resolved analysis of the LEGS and MECS X -ray spectra during the flares, and also in the subsequent quiescent phases . We have derived a metal abundance of the coronal plasma about half the ph otospheric one, with no significant increase during the flaring events. How ever, abundance variations of less than a factor similar to 3 cannot be exc luded, within our statistical uncertainties. The exponential decay of the X -ray light curves, and the time evolution of plasma temperature and emissio n measure are similar to those observed in compact solar flares. The analys is of the flare decay, following the method of Reale et al. (1997), indicat es the presence of sustained heating, and yields coronal loop maximum heigh ts smaller than (but comparable with) the stellar radius, in both cases. The flares occurred at different rotation phases, and in one case, the deca y was observed for one entire rotation period with no evidence of self-ecli pse by the star. We conclude that the flaring loops may be located in the c ircumpolar region of AB Dor, but they do not appear to be related in any wa y with the large magnetic structures which sustain the well-known prominenc es of AB Dor, at several stellar radii above the surface. During the quiescent phase, lasting two rotation periods after the second f lare, we have found evidence of significant low-level variability but no in dication of rotational modulation. This variability, together with the high plasma temperature (similar to 25 MK) and large emission measure(less than or similar to 10(53) cm(-3)), are interpreted in the framework of a satura ted corona.