Mass and radius posteriors for synthetic data created for R = 12 km and M = 1.5 M_⊙, which are shown here with cyan crosses. The left panel shows a spectral fit with fixed emitting area S_f = 1 and hydrogen mass fraction X = 0 (model A). The right panel shows a spectral fit with a free hydrogen fraction X (model B). In both panels, the dotted line encapsulates the 68%, the dashed line the 95%, and the solid line the 99.7% credible regions. The dashed blue lines show values of constant redshift: 1 + z = 1.2 and 1.3. The dotted orange lines are the contours of constant surface gravity: log g = 14.0, 14.3 and 14.6. The solid green line shows the critical radius R = 4GM/c². The dark orange region in the top left corner marks the region of parameter space forbidden by the requirement of causality (Haensel et al. 2007; Lattimer & Prakash 2007). The input (M,R) point is within the 95% credible regions in both cases, even when the hydrogen mass fraction is a free parameter.