Astron. Astrophys. 347, 424-433 (1999)

Appendix A: comparison with a PLE model

We have computed a canonical synthesis model of the XRB by adopting the XLF and PLE of Jones et al. (1997); since only AGNs with broad optical lines are included, there is no need to correct for the contribution of type 2 AGNs. This sample is smaller than Mi99a, and presumably low luminosity sources at high redshifts are underrepresented. Our model B assumes the XLF and PLE indicated above, includes QSO 2s as numerous as the Sey 2s, and adopts the absorption distribution of Risaliti et al. (1999) at all redshifts and all luminosities. In the cosmology adopted here, model B makes only of the soft XRB with type 1 AGNs, and one needs to fit the overall background (Fig. A1). Due to the large contribution of type 2 AGNs, the model XRB spectrum is very hard. Furthermore, due to the high "effective" luminosity implied by QSO 2s, the ASCA counts are reproduced. The discrepancy with the data in the 5-10 keV band, on the contrary, is not eliminated, although it is reduced to a level. Because of the preliminary nature of the HELLAS data one might debate about its significance. At any rate, one should stress that this marginal result can be obtained only by assuming a strong (a factor 4) differential evolution of QSO 2s with respect to QSO 1s, so that at the former would outnumber the latter by a factor 8.

Fig. A1. From top to bottom the same as in Figs. 2, 4, 5, but for model  B.

© European Southern Observatory (ESO) 1999

Online publication: June 30, 1999
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