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Context: Central Compact Objects (CCOs) in supernova remnants are isolatedthermally emitting neutron stars (NSs). They are most probably characterized bya magnetic field strength that is roughly two orders of magnitude lower thanthat of most of the radio and accreting pulsars. The thermal emission of CCOscan be modeled to obtain constraints on the physical parameters of the starsuch as its mass, radius, effective temperature, and chemical composition.Aims: The CCO in HESS, J1731-347 is one of the brightest objects of this class.We present our analysis of two new XMM-Newton observations of the source whichincrease the total exposure time of the by a factor of five compared to theanalyses presented before. Methods: We use our numerical spectral models forcarbon and hydrogen atmospheres to fit the spectrum of the CCO. From our fits,we derive constraints on the physical parameters of the emitting star. We alsouse the new data to derive new upper limits on pulsations and to confirm theabsence of a long-term variability. Results: The analysis shows that atmospherespectral models are clearly preferred by the fit over the blackbody function.Under the assumption that the X-ray emission is uniformly produced by theentire star surface, hydrogen atmosphere models lead to uncomfortably largedistances of the CCO, above 7-8 kpc. On the other hand, the carbon atmospheremodel formally excludes distances above 5-6 kpc and is compatible with thesource located in the Scutum-Crux (3 kpc) or Norma-Cygnus (4.5 kpc) Galacticspiral arm. We provide and discuss the corresponding confidence contours in theNS mass-radius plane. The measured effective temperature indicates that the NSis exceptionally hot for the estimated age of 30 kyr. We discuss possiblecooling scenarios to explain this property, as well as possible additionalconstraints on the star mass and radius from cooling theory.

The neutron star in HESS J1731-347: CCOs as laboratories to study the equation of state of superdense matter