EDP Sciences
Free access
Volume 419, Number 3, June I 2004
Page(s) 837 - 848
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20034421

A&A 419, 837-848 (2004)
DOI: 10.1051/0004-6361:20034421

The 2-8 keV cosmic X-ray background spectrum as observed with XMM-Newton

A. De Luca1, 2 and S. Molendi1

1  Istituto di Astrofisica Spaziale e Fisica Cosmica, Sezione di Milano "G. Occhialini" - CNR v.Bassini 15, 20133 Milano, Italy
2  Università di Milano Bicocca, Dipartimento di Fisica, P.za della Scienza 3, 20126 Milano, Italy

(Received 30 September 2003 / Accepted 17 November 2003 )

We have measured the spectrum of the Cosmic X-ray Background (CXB) in the 2-8 keV range with the high throughput EPIC/MOS instrument onboard XMM-Newton. A large sample of high galactic latitude observations was used, covering a total solid angle of 5.5 square degrees. Our study is based on a very careful characterization and subtraction of the instrumental background, which is crucial for a robust measurement of the faintest diffuse source of the X-ray sky. The CXB spectrum is consistent with a power law having a photon index $\Gamma=1.41$  $\pm$ 0.06 and a normalization of 2.46  $\pm$ 0.09 photons cm -2 s -1 sr -1 keV -1 at 3 keV (~11.6 photons cm -2 s -1 sr -1 keV -1 at 1 keV), corresponding to a 2-10 keV flux of (2.24  $\pm$ 0.16)  $\times$ 10-11 erg cm -2 s -1 deg -1 (90% confidence level, including the absolute flux calibration uncertainty). Our results are in excellent agreement with two of the most recent CXB measurements, performed with BeppoSAX LECS/MECS data (Vecchi et al. 1999) and with an independent analysis of XMM-Newton EPIC/MOS data (Lumb et al. 2002), providing a very strong constraint to the absolute sky surface brightness in this energy range, so far affected by an ~40% uncertainty. Our measurement implies that the fraction of CXB resolved by the recent deep X-ray observations in the 2-10 keV band is of 80  $\pm$ 7% (1 $\sigma$), suggesting the existence of a new population of faint sources, largely undetected within the current sensitivity limits of the deepest X-ray surveys.

Key words: X-rays: diffuse background -- cosmology: diffuse radiation -- surveys -- instrumentations: detectors

Offprint request: A. De Luca, deluca@mi.iasf.cnr.it

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