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

We present two X-ray observations of the young, ac- tive star AB Dor, performed 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 observations, characterized by a 100-fold increase of the X-ray flux, peak temperatures of about 10 8 K, and peak emission measures of 5 10 54 cm 3 . We have performed a detailed, time-resolved analysis of the LECS and MECS X-ray spectra during the flares, and also in the subsequent quiescent phases. We have derived a metal abun- dance of the coronal plasma about half the photospheric one, with no significant increase during the flaring events. However, abundance variations of less than a factor 3 cannot be ex- cluded, within our statistical uncertainties. The exponential de- cay of the X-ray light curves, and the time evolution of plasma temperature and emission measure are similar to those observed in compact solar flares. The analysis of the flare decay, follow- ing the method of Reale et al. (1997), indicates the presence of sustained heating, and yields coronal loop maximum heights smaller than (but comparable with) the stellar radius, in both cases. The flares occurred at different rotation phases, and in one case, the decay was observed for one entire rotation period with no evidence of self-eclipse by the star. We conclude that the flar- ing loops may be located in the circumpolar region of AB Dor, but they do not appear to be related in any way with the large magnetic structures which sustain the well-known prominences of AB Dor, at several stellar radii above the surface. During the quiescent phase, lasting two rotation periods after the second flare, we have found evidence of significant low- level variability but no indication of rotational modulation. This variability, together with the high plasma temperature ( 25 MK) and large emission measure (< 10 53 cm 3 ), are interpreted in the framework of a saturated corona.