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Astron. Astrophys. 353, 440-446 (2000)

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1. Introduction

The cosmic X-ray background (XRB) above [FORMULA] 1 keV is known to be produced by integrated emission of discrete sources. XRB synthesis models are usually based on the so-called unification scheme for Active Galactic Nuclei (AGN), which ascribes the different observational appearances to the orientation of a molecular torus surrounding the nucleus. The intersection of the line of sight with the torus determines a type 2 AGN (AGN2), while the direct observation of the nucleus identifies a type 1 AGN (AGN1) (Antonucci & Miller 1985). An appropriate combination of the two type of sources can reproduce the main characteristics of high-energy XRB, i.e. intensity and spectral shape (Setti & Woltjer 1989; Madau et al. 1994; Matt & Fabian 1994; Celotti et al. 1995; Comastri et al. 1995).

In this paper, the constraints on the evolution of AGN from the XRB and the source counts are examined, and a possible variation of the standard model (Comastri et al. 1995), in which the AGN2/AGN1 ratio changes with redshift, is explored. We made use of source counts data in the 0.5-2 keV band by ROSAT (Georgantopoulos et al. 1996), in the 0.3-3.5 keV band by Einstein (EMSS; Gioia et al. 1990), and the source counts provided by the BeppoSAX HELLAS survey in the 5-10 keV band (Fiore et al. 1999; Comastri et al. 1999). The XRB fitting procedure was based on the Gruber (1992) compilation of data, mostly from HEAO-1 in the 3-50 keV range (Marshall et al. 1980).

Throughout this paper, a Hubble constant [FORMULA] Km s-1 Mpc-1 and deceleration parameter [FORMULA] have been assumed.

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© European Southern Observatory (ESO) 2000

Online publication: December 17, 1999