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Astron. Astrophys. 328, 565-570 (1997)

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1. Introduction

Since the first observations from space it has been known that stellar X-ray emission is ubiquitous throughout the entire H-R diagram. The EINSTEIN satellite determined X-ray luminosities for all spectral types in the range [FORMULA] erg s-1. Only early A-type dwarfs and late M supergiants do not show any detectable X-ray flux (Hünsch et al. 1997). Two approximate correlations for the X-ray luminosity of main sequence and evolved stars were found: for early type stars it is related to the bolometric luminosity and for late type stars to the rotational velocity (Pallavicini et al. 1981). RS CVn-systems were found to be among the brightest sources with X-ray luminosities of [FORMULA] 1031 erg s-1. The variability of RS CVn-type stellar coronae, especially the most prominent type of variability i.e. the occurence of energetic flares, was observed at several occasions in the past. Charles et al. (1979) first detected a flare on the active RS CVn-type binary DM UMa using a HEAO-1 observation. Further X-ray flares were found by EINSTEIN and EXOSAT for various spectral types, from dMe-stars, RS CVns and Algols to solarlike G-stars. The ROSAT contribution to the exploration of stellar X-ray flares already started during the ROSAT all-sky survey (RASS). On HD 197890, also known as "Speedy Mic", a young K0V Star of the AB Dor type, the strongest flare in terms of count rate was detected during the RASS (Kürster 1995). The peak energy-release rate of this event was [FORMULA] 2.2 [FORMULA] erg s-1 in the PSPC bandpass (0.1-2.4 keV) and lasted for 0.7 days, that is about two times longer than the rotational period of 0.3 days. The quiescent emission before and after the flare suggested modulation with a [FORMULA] 0.3 day period, which implies an inhomogeneous corona. The record for the highest measured count rate in a ROSAT PSPC observation in pointing mode, still holds a flare on Algol with 100 cts sec-1, which is equivalent to an output of 2 [FORMULA] erg s-1 (Ottmann & Schmitt 1996). A very energetic flare was observed on the T-Tauri star P1724, with a total energy released in the PSPC bandpass of 5 [FORMULA] erg, assuming that P1724 belongs to the Orion constellation, which is somewhat uncertain (Preibisch et al. 1995). The longest known X-ray flare was presented by Kürster & Schmitt (1996), who detected a flare on the RS CVn binary CF Tuc with a total duration of 9 days, three times longer than the rotational period. This event released 1.4 [FORMULA] 1037 erg in the PSPC bandpass. Graffagninio et al. (1995) even proposed the idea of an "interbinary" flare, since the size of the flare observed on HR 5110 had about the same dimension as the binary separation.

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© European Southern Observatory (ESO) 1997

Online publication: March 26, 1998