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

Abstract

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 sourcesare selected, covering 9.6deg 2 in sky area. To quantitativelycharacterize 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 (0.5Y2 keV) and from 2 ; 10 � 16 to 7:1 ; 10 � 12 (2Y8 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 (0.5Y2 keV), and 1:58 � 0:01 and 2:59 þ0:06 � 0:05 (2Y8 keV), respectively. We detect breaks in the differential number counts that appear at differentfluxes in different energybands. Assuming a single power-law model for asource spectrum,wefind 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. ByaddingtheknownunresolvedCXRBtotheChaMP+CDFresolvedCXRB,wealsoestimate totalCXRBfluxdensities.Thefractionsof theresolvedCXRBwithouttargetsourcesare78% � 1%and81% � 2% in the 0.5Y2 and 2Y8 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. Subject headingg cosmology: observations — methods: data analysis — surveys — X-rays: diffuse background — X-rays: general