ChandraMultiwavelength Project X‐Ray Point Source Number Counts and the Cosmic X‐Ray Background

We present the Chandra Multiwavelength Project (ChaMP) X-ray point sourcenumber counts and the cosmic X-ray background (CXRB) flux densities in multipleenergy bands. From the ChaMP X-ray point source catalog, ~5,500 sources areselected covering 9.6 deg^2 in sky area. To quantitatively characterize thesensitivity and completeness of the ChaMP sample, we perform extensivesimulations. We also include the ChaMP+CDFs (Chandra Deep Fields) number countsto cover large flux ranges from 2x10^{-17} to 2.4x10^{-12} (0.5-2 keV) and from2x10^{-16} to 7.1x10^{-12} (2-8 keV) erg/cm^2/sec. The ChaMP and the ChaMP+CDFsdifferential number counts are well fitted with a broken power law. The bestfit faint and bright power indices are 1.49^{+0.02}_{-0.02} and2.36^{+0.05}_{-0.05} (0.5-2 keV), and 1.58^{+0.01}_{-0.01} and2.59^{+0.06}_{-0.05} (2-8 keV), respectively. We detect breaks in thedifferential number counts and they appear at different fluxes in differentenergy bands. Assuming a single power law model for a source spectrum, we findthat the same population(s) of soft X-ray sources causes the break in thedifferential number counts for all energy bands. We measure the resolved CXRBflux densities from the ChaMP and the ChaMP+CDFs number counts with and withoutbright target sources. Adding the known unresolved CXRB to the ChaMP+CDFresolved CXRB, we also estimate total CXRB flux densities. The fractions of theresolved CXRB without target sources are 78^{+1}_{-1}% and 81^{+2}_{-2}% in the0.5-2 keV and 2-8 keV bands, respectively, somewhat lower, though generallyconsistent with earlier numbers since their large errors. These fractionsincrease by ~1% when target sources are included.