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 or a single power-law with photon index and an exponential roll-off at 41 keV. The measured flux in the 2-10 keV energy band is 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 1.5-200 keV band, the CXRB spectrum can be well described by two smoothly-joined power laws with the energy break at keV corresponding to a peak located at 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).
Highlights
The cosmic X-ray background (CXRB) is usually defined as the integrated emission of all the extragalactic sources in the X-ray energy band (∼2−100 keV)
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
Combining Swift-XRT with Swift-Burst Alert Telescope (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 29.0 ± 0.5 keV corresponding to a νFν peak located at 22.4 ± 0.4 keV
Summary
The cosmic X-ray background (CXRB) is usually defined as the integrated emission of all the extragalactic sources in the X-ray energy band (∼2−100 keV). There is a general consensus on the sources from which the CXRB originates, and the background paradox can be considered solved (Setti & Woltjer 1989); the spectrum of the X-ray integrated emission is still very important in the study of the statistical properties of those sources that are too faint to be detected individually by currently operating telescopes, as highly absorbed AGNs and very high red shift quasars. We present a new measurement of the CXRB spectrum in the 1.5−7 keV energy band, obtained by the analysis of the archival data of the X-ray telescope (XRT) on board the Swift satellite (Gehrels et al 2004), a mission dedicated to the study of gamma-ray bursts (GRBs) and their afterglows. Following-up gamma-ray burst afterglows, the Swift-XRT obtains deep exposures on random positions of the sky, totally uncorrelated with already known bright X-ray sources, providing us with a simple and direct measurement of the CXRB spectrum
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