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Fig. 4

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Rest-frame 3−20 keV stacked spectra for the four categories, defined in Fig. 3. The vertical axis is in arbitrary unit of flux density. Only the XMM-Newton data were used. The spectral stacking is a straight sum of individual sources while a weighted mean of the available EPIC data, based on the signal to noise ratio, is taken for each source. Number of sources, typical redshift and luminosity of each category can be found in Table 2. For a reference, the spectral slope of the U category spectrum is α ≃ 0.8, where FE ∝ Eα, i.e., photon index Γ ≃ 1.8.

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