An EXOSAT observation of an X-ray flare and quiescent emission from the RS CVn binary sigma2 CrB.

Abstract

The RS CVn binary sigma2 Coronae Borealis was observed with medium-energy and low-energy detectors and the transmission grating spectrometer aboard EXOSAT, and a strong X-ray flare rising within about five minutes and lasting more than two hours was detected. The temperature rises steeply within three minutes from 19 MK to a value well above 70 MK, while the emission measure attains a maximum of 6 x 10 to the 53rd/cu cm five minutes later. The e-folding time is about half an hour. The flare volume and density are estimated and, on the basis of the short rise time of the flare, can be classified as a compact flare. The flare energy requirements are discussed, and it is found that current-driven turbulence can produce such fast heating. An equivalent-circuit description is presented.