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

## TL;DR

This paper presents evidence that the neutron star in Cassiopeia A has a carbon atmosphere, indicating nuclear burning on its surface and consistent with theoretical neutron star radii, based on analysis of archival X-ray data.

## Contribution

It provides the first evidence supporting a carbon atmosphere on a young neutron star, challenging previous assumptions about atmospheric composition.

## Key findings

- Carbon atmosphere fits the X-ray spectrum well
- Emission size matches neutron star radius predictions
- Indicates nuclear burning on the neutron star surface

## Abstract

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. 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, 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 and also identifies the compact source as a very young (~330-year-old) neutron star.