Astronomy and Astrophysics, volume 311, 211-229 (1996/7-1)
Forty days in the life of CF Tucanae (=HD 5303). The longest stellar X-ray flare observed with ROSAT.
KUERSTER M. and SCHMITT J.H.M.M.
Abstract (from CDS):
A ROSAT PSPC observation of an exceptional long-duration flare on the partially eclipsing RS CVn-type binary CF Tuc (=HD 5303) is presented. With a total duration of 9days this flare is the longest coherent stellar X-ray flare ever observed. Also the rise time of ≃1.5days is exceptional. The event released 1.4x1037erg in the PCPC bandpass (0.1-2.4keV), thereby making this giant outburst one of the most energetic stellar flare events known. The measured e-folding decay time of 0.9day implies either a very extended flaring volume with low densities or the occurence of reheating. Neither modulation on the 2.8day rotation time scale nor eclipses are seen, with the immediate implication that either the size of the flaring volume was at least comparable to the size of the active star (most likely as large as the larger binary component) or that the flare occurred within the circumpolar region of its visible pole. Both scenarios point at an event whose morphology is quite different from that of solar flares. Result. from spectral analysis performed with a variety of thermal plasma models demonstrate that one-temperature models with an unusually low metal abundance of z=0.1 are at least as good as the canonical two-temperature models with solar abundances. We present flare modelling using two different approaches, i.e. the quasi-static cooling loop model by van den Oord & Mewe (1989) describing only the flare decay phase and the two-ribbon flare model by Kopp & Poletto (1984).