New modelling of a large long-duration X-ray flare on UX Ari

Abstract

We have carried out a new gas-dynamic simulation of the physical processes occurring in a fixed amount of plasma confined in coronal loops heated in the top. We chose the solution which explains both the time behaviour of the temperature and the energetics of the X-ray emission of a long-duration X-ray flare observed by BeppoSAX on UX Ari. This analysis makes it possible to obtain the length of the loop and the distribution of the temperature and 'density along the loop (with T max ≈ 100 MK and n e = (1 – 3) · 10 11 cm −3), as well as to estimate the X-ray flare area S ≈ 1022 cm 2. Franciosini et al. (2001) have developed a time-dependent model of magnetic reconnection for the decay phase of the same flare. Our gas-dynamic solution corresponds to reconnection of middle-scale magnetic fields which in the model by Franciosini et al. (2001) corresponds to a degree n of the Legendre polynomial close to 5. The magnetic field strength is estimated to be about 100 G near the loop top. We argue that the shrinkage-effect plays a more important role during large long-duration X-ray stellar flares than in solar two-ribbon flares.