χ² confidence regions in the mass–radius plane for the CCO in HESS J1731−347 obtained with the carbon atmosphere models for the fixed distances of 3.2 kpc (bottom left) and 4.5 kpc (top right). The indicated confidence levels (50, 68, and 90%) are for two parameters of interest. The crosses indicate the corresponding χ²-minima. The shaded area in the top left of the plot indicates the region excluded by the requirements of causality. The dashed contours indicate the confidence regions only with the data from the 2007 observations (presented in Klochkov et al. 2013). The thin curves indicate the mass–radius dependencies for some of the commonly used nuclear EOS.