Hardness ratio and Galactic N_H distribution of X-ray sources with optical or infrared catalogue counterparts with identification probability >0.9 (large grey dots and grey-filled histograms), and of X-ray sources without catalogue counterpart or with identification probability below 0.9 (small black dots and empty black histograms). We only considered sources with good X-ray quality (sum_flag = 0) and with errors smaller than 0.3 in X-ray colours. Blue and red lines show the expected position of AGNs (Γ = 1.9) and stars (70 Myr old population, see Sect. 2.2 for details) respectively. Most of the objects with no infrared or optical counterpart follow a distribution similar to that of AGNs.