A new measurement of the cosmic X-ray background

¹ INAF, Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy e-mail: alberto.moretti@brera.inaf.it
² Department of Astronomy & Astrophysics, Pennsylvania State University, 525 Davey Lab, University Park, PA 16802, USA
³ INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica Sezione di Palermo, via U. La Malfa 153, 90146 Palermo, Italy
⁴ ASI Science Data Center, via G. Galilei, 00044 Frascati, Italy
⁵ University of Leicester, LE1 7RH, UK
⁶ Stanford Linear Accelerator Center, 2575 Sand Hill Road, Menlo Park, CA 94025, USA
⁷ KIPAC, 2575 Sand Hill Road, Menlo Park, CA 94025, USA
⁸ Università degli Studi di Milano-Bicocca, Dipartimento di Fisica, Piazza delle Scienze 3, 20126 Milano, Italy
⁹ NASA/Goddard Space Flight Center, Greenbelt Road, Greenbelt, MD20771, USA

¹⁰ Universities Space Research Association, 10211 Wincopin Circle, Suite 500, Columbia, MD, 21044-3432, USA


Received: 21 October 2008
Accepted: 31 October 2008

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).