BeppoSAX observation of a large long-duration X-ray flare from UX Arietis

We present two X-ray observations of the RS CVn binary system UX Arietis pe rformed with BeppoSAX in August 1997 and August 1998. A large are lasting f or about 1 day was detected during the first observation, while the star wa s quiescent during the second one. Hard X-ray emission up to similar to 50 keV was detected in the PDS instrument during the rise and at the peak of t he are. Time-resolved spectroscopy of the are with the LECS and MECS detect ors shows that temperatures as high as similar to 100 MK were reached at th e are peak; such high temperatures are responsible for the high-energy tail seen in the PDS. We confirm that the coronal abundance of UX Ari is strong ly subsolar (Z=Z. similar or equal to 0.2-0.3); no significant variation of the metallicity is observed during the are. We show that the new LECS resp onse matrices released in January 2000 seem to solve the problem of the hig h hydrogen column densities systematically found in previous BeppoSAX obser vations of stellar coronae. We find in fact for the quiescent spectrum N-H = 2.6 x 10(19) cm(-2), compatible with the expected value; during the are N -H is higher by a factor 5, probably due to the ejection of material. The a re evolution has been analysed using the two-ribbon are model by Poletto et al. (1988), finding a very good agreement between the model and the observ ations.