We present the X–ray source number counts in two energy bands ( 0.5–2 and 2–10 keV ) from a very large source sample : we combine data of six different surveys , both shallow wide field and deep pencil beam , performed with three different satellites ( ROSAT , Chandra and XMM–Newton ) .
The sample covers with good statistics the largest possible flux range so far : [ 2.4 \times 10 ^ { -17 } -10 ^ { -11 } ] erg s ^ { -1 } cm ^ { -2 } in the soft band and [ 2.1 \times 10 ^ { -16 } -8 \times 10 ^ { -12 } ] erg s ^ { -1 } cm ^ { -2 } in the hard band .
Integrating the flux distributions over this range and taking into account the ( small ) contribution of the brightest sources we derive the flux density generated by discrete sources in both bands .
After a critical review of the literature values of the total Cosmic X–Ray Background ( CXB ) we conclude that , with the present data , the 94.3 _ { -6.7 } ^ { +7.0 } \% and 88.8 _ { -6.6 } ^ { +7.8 } \% of the soft and hard CXB can be ascribed to discrete source emission .
If we extrapolate the analytical form of the Log N–Log S distribution beyond the flux limit of our catalog in the soft band we find that the flux from discrete sources at \sim 3 \times 10 ^ { -18 } erg s ^ { -1 } cm ^ { -2 } is consistent with the entire CXB , whereas in the hard band it accounts for only 93 \% of the total CXB at most , hinting for a faint and obscured population to arise at even fainter fluxes .