A new measurement of the cosmic X-ray background

A. Moretti, C. Pagani, G. Cusumano, S. Campana, M. Perri, A. Abbey, M. Ajello, A. P. Beardmore, D. Burrows, G. Chincarini, O. Godet, C. Guidorzi, J. E. Hill, J. Kennea, J. Nousek, J. P. Osborne, G. Tagliaferri

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Abstract

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. 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. 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 Γ = 1.47 ± 0.07 or a single power-law with photon index Γ = 1.41 ± 0.06 and an exponential roll-off at 4.1 keV. The measured flux in the 2-10 keV energy band is 2.1.8 ± 0.13 × 10-11 erg cm-2 s-1 deg-2 in the 2-10 keV band. Combining Swift-XRT with Swift-BAT (15-200 keV) we find that, in the 15-200 keV band, the CXRB spectrum can be well described by two smoothly-joined power laws with the energy break at 29.0 ± 0.5 keV corresponding to a vFv peak located at 22.4 ± 0.4 keV. 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).

Original languageEnglish (US)
Pages (from-to)501-509
Number of pages9
JournalAstronomy and Astrophysics
Volume493
Issue number2
DOIs
StatePublished - Jan 2009

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

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    Moretti, A., Pagani, C., Cusumano, G., Campana, S., Perri, M., Abbey, A., Ajello, M., Beardmore, A. P., Burrows, D., Chincarini, G., Godet, O., Guidorzi, C., Hill, J. E., Kennea, J., Nousek, J., Osborne, J. P., & Tagliaferri, G. (2009). A new measurement of the cosmic X-ray background. Astronomy and Astrophysics, 493(2), 501-509. https://doi.org/10.1051/0004-6361:200811197