Five data sets used in this work. The XRT unresolved spectrum (black) has been fit by a sum of 5 different power law: (i) Chandra sources, not detected by XRT (blue); (ii) X-ray emission of optical/IR sources (violet); (iii) XRT sources to model PSF residuals (red); (iv) the CXRB spectrum to model stray-light contamination (green). The XRT unresolved spectrum has been offset up by a factor 8 for clarity.