A neutron star with a carbon atmosphere in the Cassiopeia A supernova remnant

Cas A: a neutron star with atmosphere The hot neutron stars left behind by supernova explosions are thought to posses a thin atmosphere, but despite extensive searches, observations have so far been unable to confirm the atmospheric composition of an isolated neutron star. Wynn Ho and Craig Heinke have now repaired that omission by analysing archival observations — made by the Chandra satellite observatory in 2004 — of the compact X-ray source in the centre of the Cassiopeia A supernova remnant. The spectrum of the Cas A neutron star corresponds to that predicted for a carbon atmosphere, and suggests that there is nuclear burning in the surface layers of what may be, at about 300 years old, the youngest known neutron star. The surface of hot neutron stars is known to be covered by a thin atmosphere but observations have been unable to confirm the atmospheric composition of isolated neutron stars. An analysis of archival observations of the compact X-ray source in the centre of the Cassiopeia A supernova remnant now reveals that an extremely young carbon-atmosphere neutron star (with low magnetic field) produces a good fit to the spectrum. 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 stars 1 . 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.