Mean SED derived from the WISSH sample. The shaded area gives the 68% confidence interval. The colored circles represent available data points, color-coded according to the filter in which they were observed. Photometry obtained by Gap Repair is represented as gray circles. The gray crosses indicate X-ray luminosities reconstructed from the α_OX relationship. The dotted black line indicates the Lyman limit λ_L = 912 Å: for shorter wavelengths the mean SED was truncated and extended as explained in Sect. 4.4.