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I discuss the constraints that can be derived on the abundance of high redshift (z>6) (mini)quasars from the unresolved soft X-Ray background. Furthermore, I will show how existing Lyα surveys can be used to probe the very faint MB≳-21 mag (i.e., ≳7–8 mag fainter than the SDSS quasars) end of the z≥,4.5 quasar luminosity function. [ABSTRACT FROM PUBLISHER]

Most of the soft and a growing fraction of the harder X-ray background has been resolved into emission from point sources, yet the resolved fraction above 7 keV has only been poorly constrained. We use of XMM–Newton observations of the Lockman Hole and a photometric approach to estimate the total flux attributable to resolved sources in a number of different energy bands. We find the resolved fraction of the X-ray background to be ∼90 per cent below 2 keV but it decreases rapidly at higher energies with the resolved fraction above ∼7 keV being only ∼50 per cent. The integrated X-ray spectrum from detected sources has a slope of Γ∼ 1.75, much softer than the of the total background spectrum. The unresolved background component has the spectral signature of highly obscured active galactic nuclei. [ABSTRACT FROM AUTHOR]

We present the Chandra Multiwavelength Project (ChaMP) X-ray point source number counts and cosmic X-ray background (CXRB) flux densities in multiple energy bands. From the ChaMP X-ray point source catalog, ~5500 sources are selected, covering 9.6 deg2 in sky area. To quantitatively characterize the sensitivity and completeness of the ChaMP sample, we perform extensive simulations. We also include the ChaMP+CDFs (Chandra Deep Fields) number counts to cover large flux ranges from 2 × 10-17 to 2.4 × 10-12 (0.5-2 keV) and from 2 × 10-16 to 7.1 × 10-12 (2-8 keV) ergs cm-2 s-1. The ChaMP and the ChaMP+CDFs differential number counts are well fitted with a broken power law. The best-fit faint and bright power indices are 1.49 ± 0.02 and 2.36 ± 0.05 (0.5-2 keV), and 1.58 ± 0.01 and 2.59img1.gif (2-8 keV), respectively. We detect breaks in the differential number counts that appear at different fluxes in different energy bands. Assuming a single power-law model for a source spectrum, we find that the same population(s) of soft X-ray sources causes the break in the differential number counts for all energy bands. We measure the resolved CXRB flux densities from the ChaMP and the ChaMP+CDFs number counts with and without bright target sources. By adding the known unresolved CXRB to the ChaMP+CDF resolved CXRB, we also estimate total CXRB flux densities. The fractions of the resolved CXRB without target sources are 78% ± 1% and 81% ± 2% in the 0.5-2 and 2-8 keV bands, respectively, somewhat lower than but generally consistent with earlier numbers because of their large errors. These fractions increase by ~1% when target sources are included., Astronomy