Volume 548, December 2012
|Number of page(s)||11|
|Published online||28 November 2012|
Spectrum of the unresolved cosmic X-ray background: what is unresolved 50 years after its discovery
INAF, Osservatorio Astronomico di Brera, via Brera 28,
2 INAF, Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
3 INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica, via E. Bassini 15, 20133 Milano, Italy
4 DiSAT, Università dell’Insubria, via Valleggio 11, 22100 Como, Italy
5 INFN, Sezione di Milano Bicocca, P.za Della Scienza 3, 20126 Milano, Italy
6 INAF, Osservatorio Astronomico di Brera, via Bologna, via Ranzani 1, 40127 Bologna, Italy
Accepted: 10 October 2012
Aims. We study the spectral properties of the unresolved cosmic X-ray background (CXRB) in the 1.5–7.0 keV energy band with the aim of providing an observational constraint on the statistical properties of those sources that are too faint to be individually probed.
Methods. We made use of the Swift X-ray observation of the Chandra deep field South complemented by the Chandra data. Exploiting the lowest instrument background (Swift) together with the deepest observation ever performed (Chandra) we measured the unresolved emission at the deepest level and with the best accuracy available today.
Results. We find that the unresolved CXRB emission can be modeled by a single power law with a very hard photon index Γ = 0.1 ± 0.7 and a flux of 5-2.6+3.2 erg s-1 cm-2 deg-2 in the 2.0–10 keV energy band (1σ error). Thanks to the low instrument background of the Swift-XRT, we significantly improved the accuracy with respect to previous measurements.
Conclusions. These results point towards a novel ingredient in AGN population synthesis models, namely a positive evolution of the Compton-thick AGN population from local Universe to high redshift.
Key words: X-rays: diffuse background / X-rays: general / galaxies: active
© ESO, 2012
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