1. School of Mathematics, University of Southampton, Southampton SO17 1BJ, UK
2. Department of Physics, University of Alberta, Room 238 CEB, 11322-89 Avenue, Edmonton, AB T6G 2G7, Canada

Correspondence to: Wynn C. G. Ho¹Craig O. Heinke² Correspondence and requests for materials should be addressed to W.C.G.H. (Email: wynnho@slac.stanford.edu) or C.O.H. (Email: cheinke@phys.ualberta.ca).

The surface of hot neutron stars is covered by a thin atmosphere. If there is accretion after neutron-star formation, the atmosphere could be composed of light elements (H or He); if no accretion takes place or if thermonuclear reactions occur after accretion, heavy elements (for example, Fe) are expected. Despite detailed searches, observations have been unable to confirm the atmospheric composition of isolated neutron stars¹. Here we report an analysis of archival observations of the compact X-ray source in the centre of the Cassiopeia A supernova remnant. We show that a carbon atmosphere neutron star (with low magnetic field) produces a good fit to the spectrum. Our emission model, in contrast with others^{2, 3, 4}, implies an emission size consistent with theoretical predictions for the radius of neutron stars. This result suggests that there is nuclear burning in the surface layers^{5, 6} and also identifies the compact source as a very young (330-year-old) neutron star.

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