Spectroscopic analysis of a super - hot giant flare observed on Algol by BeppoSAX on 30 August 1997

We present an X-ray observation of the eclipsing binary Algol, obtained with the BeppoSAX observatory. During the observation a huge flare was observed, exceptional both in duration as well as in peak plasma temperature and total energy release. The wide spectral response of the different BeppoSAX instruments, together with the long decay time scale of the flare, allowed us to perform a detailed time-resolved Xray spectroscopic analysis of the flare. We derive the physical parameters of the emitting region together with the plasma density applying different methods to the observed flare decay. The X-ray emission from the flare is totally eclipsed during the secondary optical eclipse, so that the size of the emitting region is strongly constrained (as described in a companion paper) on purely geometrical arguments. The size of the flare thus derived is much smaller than the size derived from the analysis of the evolution of the spectral parameters using the quasi-static cooling formalism, showing that the time evolution of the flare is determined essentially from the temporal profile of the heating, with the intrinsic decay of the flaring loop having little relevance. The analysis of the decay with the technique recently developed for solar flares by Reale et al. (1997) on the other hand is in much better agreement with the eclipse-derived constraints. The very high signal-to-noise of the individual spectra strongly constrains some of the derived physical parameters. In particular, very significant evidence for a three-fold increase in coronal abundance and for a large increase in absorbing column density during the initial phases of the flare evolution is present.