low galactic neutral hydrogen column density, $N_{\rm H}=8\times 10^{19} {{\rm cm}}^{-2}$, and a lack of bright stars [@rosati2002].

OPTICAL IDENTIFICATIONS IN THE CDFS\[optid\]
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Our primary optical imaging was obtained using the FORS1 camera on the ANTU (UT-1 at VLT) telescope. The R band mosaics cover $\sim$360 arcmin$^2$ to depths between 26 and 26.7 (Vega magnitudes). This data does not cover the full CDFS area and must be supplemented with other observations (see Figure \[position\]). The ESO Imaging Survey (EIS) has covered this field to moderate depths (5 $\sigma$ limiting AB magnitudes of 26.0, 25.7, 26.4, 25.4, 25.5 and 24.7 in U$^\prime$, U, B, V, R and I, respectively) in several bands [@arnouts2001; @vandame2001]. The EIS data has been obtained using the Wide Field Imager (WFI) on the ESO-MPG 2.2 meter telescope at La Silla. The positioning of the X-ray sources is better than 0.5[@giacconi2002] and we readily identify likely optical counterparts in 85% of the cases.

Figure \[fxR\] shows the classical correlation between the R-band magnitude and the soft X-ray flux of the CDFS sources. The objects are marked according to their classification (see below). By comparison with the deepest ROSAT survey in the Lockman Hole [@lehmann2001], the Chandra 