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 others2,3,4, 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 layers5,6 and also identifies the compact source as a very young (∼330-year-old) neutron star.
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W.C.G.H. thanks N. Badnell, P. Chang, and D. Lai for discussions. W.C.G.H. appreciates the use of the computer facilities at the Kavli Institute for Particle Astrophysics and Cosmology. W.C.G.H. acknowledges support from the Science and Technology Facilities Council (STFC) in the United Kingdom. C.O.H. acknowledges support from the Natural Sciences and Engineering Research Council (NSERC) of Canada.
Author Contributions W.C.G.H. calculated the new models and wrote the manuscript. C.O.H. reduced the data, fitted the models to the data, and contributed to the manuscript.
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Ho, W., Heinke, C. A neutron star with a carbon atmosphere in the Cassiopeia A supernova remnant. Nature 462, 71–73 (2009). https://doi.org/10.1038/nature08525
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