EDP Sciences
Free access
Issue
A&A
Volume 394, Number 3, November II 2002
Page(s) 835 - 849
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20021235


A&A 394, 835-849 (2002)
DOI: 10.1051/0004-6361:20021235

Exploring the spectral properties of faint hard X-ray sources with $\vec XMM$- $\vec Newton$

E. Piconcelli1, 2, M. Cappi1, L. Bassani1, F. Fiore3, G. Di Cocco1 and J. B. Stephen1

1  IASF/CNR, via Piero Gobetti 101, 40129 Bologna, Italy
2  Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
3  Osservatorio Astronomico di Roma, Via Frascati 33, 00044 Monteporzio, Italy

(Received 15 March 2002 / Accepted 26 August 2002 )

Abstract
We present a spectroscopic study of 41 hard X-ray sources detected serendipitously with high significance ( >5 $\sigma$ in the 2-10 keV band) in seven EPIC performance/verification phase observations. The large collecting area of EPIC allows us to explore the spectral properties of these faint hard X-ray sources with 2  < F2-10  < 80  $\times$ 10 -14 erg cm -2 s -1 even though the length of the exposures are modest (~20 ks). Optical identifications are available for 21 sources of our sample. Using a simple power law plus Galactic absorption model we find an average value of the photon index $\Gamma$ ~ 1.6-1.7, broadly consistent with recent measurements made at similar fluxes with ASCA and with Chandra stacked spectral analyses. We find that 31 out of 41 sources are well fitted by this simple model and only eight sources require absorption in excess of the Galactic value. Interestingly enough, one third of these absorbed sources are broad line objects, though with moderate column densities. Two sources in the sample are X-ray bright optically quiet galaxies and show flat X-ray spectra. Comparing our observational results with those expected from standard synthesis models of the cosmic X-ray background (CXB) we find a fraction of unabsorbed to absorbed sources larger than predicted by theoretical models at our completeness limit of F2-10 ~ 5  $\times$ 10 -14 erg cm -2 s -1. The results presented here illustrate well how wide-angle surveys performed with EPIC on board XMM-Newton allow population studies of interesting and unusual sources to be made as well as enabling constraints to be placed on some input parameters for synthesis models of the CXB.


Key words: galaxies: active -- X-rays: galaxies -- X-rays: diffuse background

Offprint request: E. Piconcelli, piconcel@tesre.bo.cnr.it

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