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

5. Summary and conclusions

In this paper we have shown that the standard prescriptions for synthesizing the XRB from the integrated emission of AGNs are not consistent with a number of recent observational constraints, and some of them must be relaxed.

We have worked out models (A1 and A2) which take into account detailed input spectra of AGNs, the distribution observed in local Seyfert 2s, and the XLF and evolution newly determined from the largest ROSAT sample. The latter data do not define a unique parametrization, and the two models explore different variants. As prescribed by the standard model, the XLF and evolution of type 2 AGNs are taken from type 1s, and the spectra of both types are taken independent of redshift; the only fitting parameter is the number ratio R of type 2s to type 1s. We find that model A1 reproduces the XRB and the soft counts with a ratio R compatible with the local value, but underestimates the hard counts. Model A2 is less discrepant as far as the counts are concerned, but requires a ratio R definitely larger than observed locally. We have also computed a model adopting a canonical pure luminosity evolution (model B). In agreement with the results of Co95, model B can reproduce the XRB, the soft X-ray counts and the ASCA hard counts in the 2-10 keV band. It is also consistent within 2 (or discrepant at 2) with the preliminary BeppoSAX counts in the 5-10 keV band. Nevertheless, it requires a number of type 2 QSOs much higher than the local upper limit, and perhaps already ruled out by the deep X-ray surveys.

The discrepancies found in all models are to some extent model dependent, but all of them point in the same direction, and suggest that hard spectrum sources at intermediate or high redshifts are needed in addition to the predictions of the standard scenario. The X-ray spectrum of these additional sources could be flattened by absorption, or could be intrinsically hard. In the former hypothesis reasonable candidate counterparts could be rapidly evolving, "normal" Seyfert 2s. One should also note that a fraction of ULIRGs seem to be powered by AGNs, and their cosmological evolution seems faster than that of unabsorbed QSOs. The alternate hypothesis could instead require the presence of ADAFs. Optical identifications of the hard X-ray sources are still largely incomplete and do not allow yet to decide between the various possibilities.

© European Southern Observatory (ESO) 1999

Online publication: June 30, 1999
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