1, 2, and 3 σ confidence contours from fitting the X-ray spectrum of PSR J1909–3744. This shows the degeneracy between the temperature and size of the emitting surface when using the black-body model and the nsatmos model. For the latter, the model with fixed neutron star radius (R = 10.6 km, model ^(z) in Table 2) was used. Lines of constant (un-absorbed) bolometric flux are shown in grey, ranging from 10⁻¹⁵ to 10⁻¹³ erg s⁻¹ cm⁻², in steps of 0.5 dex.