Discovery of X-ray emission lines from the RS CVn binary sigma CrB

X-ray observations of the RS CVn binary sigma CrB were carried out with a solid state spectrometer on board the Einstein Observatory in 1979. X-ray emission lines due to Mg XI, Si XIII, S XV and a blend of L-transition lines due to Fe XVII and Fe XVIII were detected in the sigma CrB spectrum. The observed X-ray spectrum is described by a two-temperature coronal X-ray emission model: the dominant low temperature component originates in a plasma at a temperature of 5.9 + or 0.5 x 10 to the 6th K; the weaker higher temperature arises from a plasma of not less than 35 x 10 to the 6th K. Intensity variations over a period of a few hours are also recorded and thought to be due to a variation in the number of loops concentrated around active regions on the star surface. Implications of the results are discussed in terms of the constant-pressure coronal loop model (Rosner, Tucker, Vaiana, 1978).