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

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 +/- 0.06 and a normalization of 2.46 +/- 0.09 photons cm(-2) s(-1) sr(-1) keV(-1) at 3 keV (similar to11.6 photons cm(-2) s(-1) sr(-1) keV(-1) at 1 keV), corresponding to a 2-10 keV flux of (2.24 +/- 0.16) x 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 similar to40% 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 +/- 7% (1sigma), suggesting the existence of a new population of faint sources, largely undetected within the current sensitivity limits of the deepest X-ray surveys.