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Context: The CCO candidate in the center of the supernova remnant shell HESSJ1731-347 / G353.6-0.7 shows no pulsations and exhibits a blackbody-like X-rayspectrum. If the absence of pulsations is interpreted as evidence for theemitting surface area being the entire neutron star surface, the assumption ofthe measured flux being due to a blackbody emission translates into a sourcedistance that is inconsistent with current estimates of the remnant's distance.Aims: With the best available observational data, we extended the pulse periodsearch down to a sub-millisecond time scale and used a carbon atmosphere modelto describe the X-ray spectrum of the CCO and to estimate geometricalparameters of the neutron star. Methods: To search for pulsations we used dataof an observation of the source with XMM-Newton performed in timing mode. Forthe spectral analysis, we used earlier XMM-Newton observations performed inimaging mode, which permits a more accurate treatment of the background. Thecarbon atmosphere models used to fit the CCO spectrum are computed assuminghydrostatic and radiative equilibria and take into account pressure ionizationand the presence of spectral lines. Results: Our timing analysis did not revealany pulsations with a pulsed fraction above ~8% down to 0.2 ms. This findingfurther supports the hypothesis that the emitting surface area is the entireneutron star surface. The carbon atmosphere model provides a good fit to theCCO spectrum and leads to a normalization consistent with the availabledistance estimates of the remnant. The derived constraints on the mass andradius of the source are consistent with reasonable values of the neutron starmass and radius. After the CCO in Cas A, the CCO in HESS J1731-347 / G353.6-0.7is the second object of this class for which a carbon atmosphere model providesa consistent description of X-ray emission.

A non-pulsating neutron star in the supernova remnant HESS J1731-347 / G353.6-0.7 with a carbon atmosphere