SIMBAD references

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 2x10^–16 to 10^–13 ergs.cm^–2.s^–1. Over this flux range, the contribution to the X-ray background is 1.1x10^–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 1.4x10^–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 0.5x10^–11 ergs.cm^–2.s^–1.deg^–2 could still be present in an unresolved component.