X-ray emission from γ Cas

Abstract

THE Copernicus orbiting observatory contains two astronomical experiments, the Princeton ultraviolet spectrometer and the MSSL X-ray package. They are coaligned, and during ultraviolet observations the 3–8 keV collimated proportional counter routinely collects background data¹. When the observatory was pointed at the ultraviolet target γ Cas on December 21, 1975 and again on January 20, 1976, a clear excess of counts was seen in the 3–8 keV channel compared to preceding and following targets. This count rate corresponds to a flux at the Earth of ∼ 1.2×10⁻¹⁰ erg cm⁻²S⁻¹, and lies 5σ above the mean background level determined from several weeks of observations (Table 1).