Left panel: hardness-ratio diagram: HR₂ versus HR₁ for all the observed objects which have both values defined. The error bars represent one standard deviation. The four sources with HR₁ = 1 are from top to bottom #34, #33, #13 and #25. The four other hard sources are, in order of decreasing HR₁, #8, #5, #1 and #16. Middle panel: computed positions in the same hardness-ratio diagram of synthetic on-axis sources (optically thin thermal plasma emission mekal models): solid lines represent the position of mekal models as a function of the absorbing column density (N_H = 0., 0.25, 0.5, 1.0 and 2.0  ×  10²² cm^-2, from left to right), for several values of the temperature (kT = 0.2, 0.4, 0.6, 0.8, 1.2, 1.5, 2.0, 3.0 and 5.0 keV, from bottom to top). Right panel: idem but for powerlaw models (N_H = 0.1, 0.3, 1.0, 2.0, 3.0 and 7.0  ×  10²² cm^-2, from left to right), for several values of the photon index (Γ = 1.4, 1.7, 2.0, 3.0 and 4.0, from top to bottom).