The X-ray spectrum of Cygnus-A

The X-ray source 2U 1957 + 40 is shown from observations with the Copernicus X-ray instrument to be associated with the powerful radiogalaxy Cygnus A. The angular size of the source is found to be less than 10' arc, the resolution of the telescopes for these observations. The spectrum can be fitted by a power law of slope 1.3 ± 0.6 or by thermal bremsstrahlung at a temperature ≽ 5 keV. The low energy cut-off corresponds to a neutral hydrogen column density of (5.5 ± 1.6) × 1021 cm−2 which may arise from absorption within the Galaxy. The total emission taking a Hubble constant of 50 km s−1 Mpc−1 is found to be (2.3 ± 0.8) × 1045 erg s−1, which is similar to the emission in the radio or optical bands. Various emission mechanisms are considered, and it is shown that synchrotron or inverse Compton radiation from the radio components or a hypothetical halo can be excluded. Thermal bremsstrahlung from a cloud of intergalactic gas is a possible process, which would make 2U 1957 + 40 similar to the other X-ray cluster sources. The kinetic pressure of this gas is sufficient to contain the radio components. Processes occurring in the nucleus of the galaxy are an alternative possibility, however, and it appears that a final determination of the emission process requires high resolution X-ray studies of the structure of the source.