Left panel: plot of k_bol vs. L_bol. Black square symbols represent type 2 AGNs, while colour-coded circles indicate the type 1 and type 1.9 sources for increasing outflow velocity. The black and red curves represent the best-fitting relations obtained by Lusso et al. (2012) for type 1 and type 2 AGNs in the COSMOS field, respectively. Right: distribution of the normalized (in log space) k_bol values with respect to those expected from the relation for X-ray selected AGNs (black curve in the left panel) for the BL X-ray/SDSS sample (grey histogram). Blue and black histograms represent the distributions for sources with and without outflow signatures. Error bars indicate Poissonian errors. For comparison, 3 × σ dispersion values derived by Lusso et al. (2012) for XMM-COSMOS sources are also shown with red dashed vertical lines.