What is the origin and nature of the cosmic X-ray background (here after CXRB)? Can detected discrete X-ray sources account for the CXRB? Since Giacconi et al. (1962) discovered homogeneous and isotropic cosmic X-ray background (here after CXRB), these questions have been investigated using data from several X-ray satellites but still remain technical issues such as the incompleteness of the used data. The CXRB consists of resolved and unresolved components. The resolved CXRB originates in discrete sources, while the unresolved CXRB originates in diffuse components and faint sources that are below current observational flux limits. The resolved CXRB can be directly measured from number counts of the discrete X-ray sources. <br />
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In this talk, we will present the total CXRB flux density and the resolved fraction of the total CXRB using the Chandra multivelength project (ChaMP) X-ray point source catalog and the Chandra Deep Fields (CDFs) data. The ChaMP X-ray point source catalog contains ~6,800 X-ray sources detected in 149 Chandra observations covering wide sky area (~10 deg2) and provides X-ray photometric data in 8 different energy bands as well as X-ray spectral hardness ratios and colors. To quantitatively characterize the sensitivity and completeness of the ChaMP sample, we performed extensive simulations. In addition, the origin and cause of the break in the X-ray point source number counts will be discussed in this talk. <br />
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