V. G. Graffagnino
ROSAT observations of superflares on RS CVn systems
2000 (supervisor: G. Branduardi-Raymont)

The following thesis involves the analysis of X-ray observations of two RS CVn systems, made using the ROSAT satellite. These observations have revealed a number of long-duration flares lasting several days (much longer than previously observed in the X-ray band) and emitting energies which total a few percent of the available magnetic energy of the stellar system and thus far greater than previously encountered. Calculations based on the spectrally fitted parameters show that simple flare mechanisms and standard two-ribbon flare models cannot explain the observations satisfactorily and continued heating was observed during the outbursts. This is the first time that such departures from two-ribbon flare models have been identified unambiguously for such large flares. The general flare decay analysis method of Van den Oord et al (1988) was extended to include the effects of heating, and this was applied to the data in order to derive flare loop heights for these observations. Although these loop heights are dependent on the amount of heating assumed, the calculations were performed for a wide range of reasonable heating values. For these cases the flare heights obtained for these outbursts were of the order of the inter-binary separation and inter-binary flares are suggested as the cause of the outbursts. In the case of one of these systems (CF Tucanae), evidence for rotational modulation of the extreme ultra-violet lightcurve during quiescence was also found.