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 performed with BeppoSAX in August 1997 and August 1998. A large flare lasting for about 1 day was detected during the first observation, while the star was quiescent during the second one. Hard X-ray emission up to ~50 keV was detected in the PDS instrument during the rise and at the peak of the flare. Time-resolved spectroscopy of the flare with the LECS and MECS detectors shows that temperatures as high as ~100 MK were reached at the flare 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 strongly subsolar ($Z/Z_{\odot} \simeq 0.2{-}0.3$); no significant variation of the metallicity is observed during the flare. We show that the new LECS response matrices released in January 2000 seem to solve the problem of the high hydrogen column densities systematically found in previous BeppoSAX observations of stellar coronae. We find in fact for the quiescent spectrum $N_{\rm H} = 2.6 \times 10^{19}$ cm-2, compatible with the expected value; during the flare NH is higher by a factor 5, probably due to the ejection of material. The flare evolution has been analysed using the two-ribbon flare model by Poletto et al. ([CITE]), finding a very good agreement between the model and the observations