The 2-8 keV X-Ray Number Counts Determined from Chandra Blank Field Observations

We present the 2-8 keV number counts from the 1 Ms Chandra observation of the Chandra Deep Field-North. We combine these with the number counts from a 78 ks exposure of the Hawaii Survey field SSA22 and with the number counts obtained in independent analyses of the Chandra Deep Field-South and the Hawaii Survey field SSA13 to determine the number counts from 2 × 10-16 to 10-13 ergs cm-2 s-1. Over this flux range, the contribution to the X-ray background is 1.1 × 10-11 ergs cm-2 s-1 deg-2. When the contributions above 10-13 ergs cm-2 s-1 from BeppoSAX or ASCA observations are included, the total rises to 1.4 × 10-11 ergs cm-2 s-1 deg-2. However, there appears to be substantial field-to-field variation in the counts in excess of the statistical uncertainties. When the statistical and flux-calibration uncertainties (in both the background and source measurements) are taken into account, as much as 0.5 × 10-11 ergs cm-2 s-1 deg-2 could still be present in an unresolved component.