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Astron. Astrophys. 362, 799-808 (2000)


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The Beppo SAX 2-10 keV survey *

P. Giommi 1, M. Perri 1 and F. Fiore 1,2,3

1 BeppoSAX Science Data Center, ASI, Via Corcolle 19, 00131 Roma, Italy
2 Osservatorio Astronomico di Roma, Via Frascati 33, 00044 Monteporzio, Italy
3 Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA

Received 29 March 2000 / Accepted 15 June 2000

Abstract

We present the results of a 2-10 keV Beppo SAX survey based on 140 high galactic latitude MECS fields, 12 of which are deep exposures of "blank" parts of the sky. The limiting sensitivity is [FORMULA] [FORMULA] where about 25% of the Cosmic X-ray Background (CXB) is resolved into discrete sources. The logN-logS function, built with a statistically complete sample of 177 sources, is steep and in good agreement with the counts derived from ASCA surveys. A CXB fluctuation analysis allowed us to probe the logN-logS down to about [FORMULA] [FORMULA] where the contribution of discrete sources to the CXB grows to [FORMULA].

A hardness ratio analysis reveals the presence of a wide range of spectral shapes and that a fairly large fraction of sources appear to be heavily absorbed, some of which showing soft components.

A comparison of the flux distribution of different subsamples confirms the existence of a spectral hardening with decreasing flux. This effect is probably due to an increasing percentage of absorbed sources at faint fluxes, rather than to a gradual flattening of the spectral slope. Nearly all the sources for which adequate ROSAT exposures exist, have been detected in the soft X-rays. This confirms that soft spectral components are present even in strongly absorbed objects, and that a large population of sources undetectable below a few keV does not exist.

A [FORMULA] test provides evidence for the presence of cosmological evolution of a magnitude similar to that found in soft X-ray extragalactic sources. Evolution is present both in normal and absorbed sources, with the latter population possibly evolving faster, although this effect could also be the result of complex selection effects.

Key words: surveys – cosmology: observations – X-rays: galaxies – X-rays: general

* Table 1 is only available in electronic form at the CDS via anonymous ftp to ftp://cdsarc.u-strasbg.fr/pub/cat/J/A+A/362/799 (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/362/799

Send offprint requests to: P. Giommi (giommi@napa.sdc.asi.it)

© European Southern Observatory (ESO) 2000

Online publication: October 30, 2000

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