Absolute measurement of the unresolved cosmic X-ray background in the 0.5-8 kev band with Chandra

We present the absolute measurement of the unresolved 0.5-8 keV cosmic X-ray background ( CXB) in the Chandra Deep Fields ( CDFs) North and South, the longest observations with Chandra ( 2 and 1 Ms, respectively). We measure the unresolved CXB intensity by extracting spectra of the sky and removing all point and extended sources detected in the CDF. To model and subtract the instrumental background, we use observations obtained with ACIS in stowed position, not exposed to the sky. The unresolved signal in the 0.5-1 keV band is dominated by diffuse Galactic and local thermal-like emission. We find unresolved intensites in the 0.5-1 keV band of ( 4: 1 +/- 0: 3) x 10(-12) ergs cm(-2) s(-1) deg(-2) for CDF-N and ( 5.0 +/- 0: 4) x 10(-12) for CDF-S. In the 1-8 keV band, the unresolved spectrum is adequately described by a power law with a photon index Gamma = 1.5(-0.4)(+0.5) and normalization 2.6 +/- 0.3 photons s(-1) keV(-1) sr(-1) at 1 keV. We find unresolved CXB intensities of ( 1.04 +/- 0.14) x 10(-12) ergs cm(-2) s(-1) deg(-2) for the 1-2 keV band and ( 3.4 +/- 1.7) x 10(-12) ergs cm(-2) s(-1) deg(-2) for the 2-8 keV band. Our detected unresolved intensities in these bands significantly exceed the expected flux from sources below the CDF detection limits, if one extrapolates the log N/log S curve to zero flux. Thus, these background intensities imply either a genuine diffuse component or a steepening of the log N/log S curve at low fluxes, most significantly for energies < 2 keV. Adding the unresolved intensity to the total contribution from sources detected in these fields and wider field surveys, we obtain a total intensity of the extragalactic CXB of ( 4.6 x 0.3) x 10(-12) ergs cm(-2) s(-1) deg(-2) for 1-2 keV and ( 1.7 x 0.2) x 10(-11) ergs cm(-2) s(-1) deg(-2) for 2-8 keV. These totals correspond to a CXB power-law normalization ( for Gamma = 1.4) of 10.9 photons cm(-2) s(-1) keV(-1) sr(-1) at 1 keV. This corresponds to resolved fractions of 77% +/- 3% and 80% +/- 8% for 1-2 and 2-8 keV, respectively.