Simultaneous X-ray and ultraviolet observations of Epsilon Orionis and Kappa Orionis.

Abstract

Simultaneous observations were made with the Einstein Observatory, the International Ultraviolet Explorer, and the Copernicus satellite of the supergiant stars η Ori (B0 Ia) and κ Ori (B0.5 Ia). The X-ray observations spanned periods of 7 hours and 10 hours, and the ultraviolet observations spanned 15 and 18 hours for ηOri and κ Ori, respectively. For either star, no variations were detected in the X-ray flux or spectrum or in the ultraviolet line profiles of Auger enhanced ionization states. For ηOri, the X-ray count rate was the same as it was during an observation 11 months earlier, while for κ Ori, the X-ray count rate was 46% larger than it was 11 months earlier. This change could be explained by an increase in the source temperature from 10^6.3 K to 10^6.4 K, or by an increase in the source emission measure of 46 %, or by a decrease in the column density of the intervening matter of several times 1020 cm^-2. The ultraviolet line profiles are found to be insensitive to changes of X-ray flux which are less than 50 %. Various constraints imposed on current models for the X-ray production in early-type stars are discussed.