Cygnus-A was first detected as an X-ray source by the Uhuru satellite [3]. Data from ANS showed that the X-ray source was either extended or a point source not coincident with the radio source [4]. This was confirmed using HEAO-1 observations which demonstrated that the X-ray emission was centered on the radio source but was extended with a FWHM of about 2 arcmin and that the spectrum was consistent with a thermal plasma of temperature approximately 6.5 keV [5].
A breakthrough in the understanding of the X-ray emission from Cygnus-A was provided by the Einstein Observatory, whose imaging showed that the radio source lies at the center of about 10^14 Solar masses of hot gas extending over 1 Mpc (H0 = 50 km/s/Mpc) in one direction [6]. Thus the X-ray emission as seen in this image was due to the cluster of galaxies around the Cygnus-A galaxy and not to any active nucleus associated with the radio source. There was no evidence for excess emission from the nucleus or the radio lobes using data from the high resolution instrument on the Einstein Observatory [7].