A broad diversity of source types is found in deep Chandra and XMM-Newton surveys. This is apparent in even basic flux-flux plots such as that shown in Figure 4; at the faintest X-ray flux levels in the Chandra Deep Fields, the extragalactic sources range in optical flux by a factor of 10,000.
Figure 4. I-band magnitude versus 0.5-2 keV flux for extragalactic X-ray sources in the CDF-N (triangles) and CDF-S (squares). Sources with redshifts of 0-0.5, 0.5-1, 1-2, and 2-6 are shown as violet, blue, green, and red symbols, respectively (symbol sizes also increase with redshift). Small black dots indicate sources without spectroscopic redshifts. The slanted, dotted lines indicate constant values of log(fX / fI); the respective log(fX / fI) values are labeled. Note the broad range of optical magnitudes at faint X-ray fluxes and that a large fraction of the optically faint X-ray sources lack spectroscopic redshifts.