Comparison of the rest-frame 2-keV flux as inferred from the longest versus the second longest observation (left panel), and from the observation characterised by the smallest versus the second smallest off-axis angle Θ. The dispersions with respect to the 1:1 relation are a combination of the contribution due to the variability in the X-ray band and to observational issues, especially to flux calibration, related to different off-axis angle of the source in different observations.