The 2–8 keV cosmic X-ray background spectrum as observedwith 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 ± 0.06 and a normalization of 2.46 ± 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 ± 0.16) 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. CITE) and with an independent analysis of XMM-Newton EPIC/MOS data (Lumb et al. CITE), 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 ± 7% (1σ), suggesting the existence of a new population of faint sources, largely undetected within the current sensitivity limits of the deepest X-ray surveys.