Fig. 8. Hardness ratio plot of point sources in the NGC 253 disk. The square marks the position of X33 in the HR diagram. The filled hexagon marks the hardness ratios of the entire NGC 253 disk emission (cf. Table 7). Crosses indicate the measurements and errors for different sources in the disk of NGC 253. One group (marked with diamonds at the center of the crosses) are bright point sources which were detected with the PSPC and HRI, the second group (left of the hexagon) represents sources only detected with the PSPC and not visible during the ROSAT HRI observations