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Title:
A new method for determining the sensitivity of X-ray imaging observations and the X-ray number counts
Authors:
Georgakakis, A.; Nandra, K.; Laird, E. S.; Aird, J.; Trichas, M.
Affiliation:
AA(Astrophysics Group, Blackett Laboratory, Imperial College, Prince Consort Road, London SW7 2AZ), AB(Astrophysics Group, Blackett Laboratory, Imperial College, Prince Consort Road, London SW7 2AZ), AC(Astrophysics Group, Blackett Laboratory, Imperial College, Prince Consort Road, London SW7 2AZ), AD(Astrophysics Group, Blackett Laboratory, Imperial College, Prince Consort Road, London SW7 2AZ), AE(Astrophysics Group, Blackett Laboratory, Imperial College, Prince Consort Road, London SW7 2AZ)
Publication:
Monthly Notices of the Royal Astronomical Society, Volume 388, Issue 3, pp. 1205-1213. (MNRAS Homepage)
Publication Date:
08/2008
Origin:
MNRAS
Astronomy Keywords:
methods: data analysis , methods: miscellaneous , methods: statistical , surveys , X-rays: diffuse background , X-rays: galaxies
DOI:
10.1111/j.1365-2966.2008.13423.x
Bibliographic Code:
2008MNRAS.388.1205G

Abstract

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 5-10keV) by combining deep pencil-beam and shallow wide-area Chandra observations. The sample has a total of 6295 unique sources over an area of 11.8deg2 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 5-10keV band, in the case of a double power-law source count distribution. We also find an upturn in the 0.5-2keV counts at fluxes below about 6 × 10-17ergs-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 2-10keV.
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