A new measurement of the cosmic X-ray background

Aims We present a new measurement of the cosmic X-ray background (CXRB) in the 1.5-7 keV energy band, performed by exploiting the Swift X-ray telescope (XRT) data archive. We also present a CXRB spectral model in a wider energy band (1.5-200 keV), obtained by combining these data with the recently published Swift-BAT measurement. <BR />Methods From the XRT archive we collect a complete sample of 126 high Galactic latitude gamma-ray burst (GRB) follow-up observations. This provides a total exposure of 7.5 Ms and a sky-coverage of ~7 square degrees which represents a serendipitous survey, well suited for a direct measurement of the CXRB in the 1.5-10 keV interval. Our work is based on a complete characterization of the instrumental background and an accurate measurement of the stray-light contamination and vignetting calibration. <BR />Results We find that the CXRB spectrum in the 1.5-7 keV energy band can be equally well fitted by a single power-law with photon index Gamma=1.470.07 or a single power-law with photon index Gamma=1.410.06 and an exponential roll-off at 41 keV. The measured flux in the 2-10 keV energy band is 2.18 0.13 10<SUP>-11</SUP> erg cm<SUP>-2</SUP> s<SUP>-1</SUP> deg<SUP>-2</SUP> in the 2-10 keV band. Combining Swift-XRT with Swift-BAT (15-200 keV) we find that, in the 1.5-200 keV band, the CXRB spectrum can be well described by two smoothly-joined power laws with the energy break at 29.00.5 keV corresponding to a nu F<SUB>nu</SUB> peak located at 22.40.4 keV. <BR />Conclusions Taking advantage of both the Swift high energy instruments (XRT and BAT), we produce an analytical description of the CXRB spectrum over a wide (1.5-200 keV) energy band. This model is marginally consistent with the HEAO1 measurement (~10% higher) at energies higher than 20 keV, while it is significantly (30%) higher at low energies (2-10 keV).