Journal Article

A new method for determining the sensitivity of X-ray imaging observations and the X-ray number counts

A. Georgakakis, K. Nandra, E. S. Laird, J. Aird and M. Trichas

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 388, issue 3, pages 1205-1213
Published in print August 2008 | ISSN: 0035-8711
Published online July 2008 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2008.13423.x
A new method for determining the sensitivity of X-ray imaging observations and the X-ray number counts

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We present a new method for determining the sensitivity of X-ray imaging observations, which correctly accounts for the observational biases that affect the probability of detecting a source of a given X-ray flux, without the need to perform a large number of time-consuming simulations. We use this new technique to estimate the X-ray source counts in different spectral bands (0.5–2, 0.5–10, 2–10 and 5–10 keV) by combining deep pencil-beam and shallow wide-area Chandra observations. The sample has a total of 6295 unique sources over an area of 11.8 deg2 and is the largest used to date to determine the X-ray number counts. We determine, for the first time, the break flux in the 5–10 keV band, in the case of a double power-law source count distribution. We also find an upturn in the 0.5–2 keV counts at fluxes below about 6 × 10−17erg s−1cm−2. We show that this can be explained by the emergence of normal star-forming galaxies which dominate the X-ray population at faint fluxes. The fraction of the diffuse X-ray background resolved into point sources at different spectral bands is also estimated. It is argued that a single population of Compton thick active galactic nuclei (AGN) cannot be responsible for the entire unresolved X-ray background in the energy range 2–10 keV.

Keywords: methods: data analysis; methods: miscellaneous; methods: statistical; surveys; X-rays: diffuse background; X-rays: galaxies

Journal Article.  6008 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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