Chandra Multiwavelength Project X-Ray Point Source Number Counts and the Cosmic X-Ray Background

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 2.59^+0.06_-0.05 (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.