XMM-Newton EPIC-pn colour-colour diagram. The data points represent the ratios of the light curves in hard colour depending on soft colour, from beginning (light green) to end (dark green) of the observation. We also show the theoretical expectation for different covering fractions CF (shades of red, varying from 0.97 to 0.99) and absorption column densities N_H,1 (shades of blue, varying from 3 × 10²² cm⁻² to 80 × 10²² cm⁻²) using our partial covering model from Eq. (4). We used a photon index Γ of 1.0. More details about the simulation and its interpretation are given in Sect. 5.1.1.