HR diagrams. The lines display the hardness ratios calculated for different spectral models. Black, navy, and blue lines show a power-law model with a photon index of Γ =1, 2, and 3, respectively. Red, yellow, and green lines show the apec model (Smith et al. 2001) for a thermal plasma with a temperature of kT = 0.2, 1.0, and 2.0 keV, respectively. The upper plot shows sources in the foreground, which therefore have very low absorbing foreground column density N_H ; they appear to be softer than the others. The symbols and colours are the same as in Fig. 5. Additionally, those sources, which have no counterpart are shown with black squares.