X-ray spectrum of sigma Geminorum: an RS Canum Venaticorum type binary

The binary system Sigma Gem has been observed on three occasions during 1984 November 17-21 with the medium-energy and the low-energy detectors on the Exosat Observatory. Spectral analysis of the data obtained over the energy range of 0.05-6.0 keV shows evidence for a bimodal temperature distribution of emission measure in an optically thin thermal equilibrium plasma with one component of about 5 million K and the other one of about 40 million K. It is found that the lower-temperature component is the dominant source of X-ray emission. The data are interpreted in terms of emission from two types of hot coronal loops with constant pressure on the surface of the visible red giant star in the binary. The two systems of coronal loops correspond to the two temperature plasma components measured. The observed variations in the luminosity of the system are probably due to the rotational effects on the smaller, cooler loops confined to certain regions on the surface of the late-type giant star.