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 ( \sim 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 constrain to the absolute sky surface brightness in this energy range , so far affected by a \sim 40 % uncertainty .
Our measurement immediately 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 .