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The surface of hot neutron stars is covered by a thin atmosphere. If there isaccretion after neutron star formation, the atmosphere could be composed oflight elements (H or He); if no accretion takes place or if thermonuclearreactions occur after accretion, heavy elements (for example, Fe) are expected.Despite detailed searches, observations have been unable to confirm theatmospheric composition of isolated neutron stars. Here we report an analysisof archival observations of the compact X-ray source in the centre of theCassiopeia A supernova remnant. We show that a carbon atmosphere neutron star(with low magnetic field) produces a good fit to the spectrum. Our emissionmodel, in contrast with others, implies an emission size consistent withtheoretical predictions for the radius of neutron stars. This result suggeststhat there is nuclear burning in the surface layers and also identifies thecompact source as a very young (~330-year-old) neutron star.

A Neutron Star with a Carbon Atmosphere in the Cassiopeia A Supernova Remnant