Detection of an X-ray flare in the RS CVn binary Sigma Coronae Borealis

The detection of an X-ray flare in the RS CVn binary Sigma Coronae Borealis with the Monitor Proportional Counter on the Einstein Observatory is described. During the 513 min of observation, an X-ray flare of 208 min duration was detected at a significance level of 26σ in the 1.19–10.16 keV band. The rise time of the flare is between 25 and 70 min and the decay time is $$\geqslant34$$ min. The X-ray luminosity at the flare maximum is found to be $$6\times10^{30}\enspace \text {erg}\enspace \text s^{-1}$$ and the total energy radiated in X-rays during the flare is $$2\times10^{34}\enspace \text {erg}$$. The energy spectrum in the flaring state is found to be harder (temperature $$T\sim2.5\times10^7\enspace \text K$$) compared to the one observed in the quiescent state ($$T\sim6\times10^6\enspace \text K$$). Applying the coronal loop model, the loop parameters are calculated and compared for the X-ray flares observed in the various RS CVn binaries and the Sun. The significance of the differences in the observed and derived parameters of the X-ray flares is briefly discussed.