When is pile-up important in the XMM-Newton EPIC cameras?

Context. Pile-up in X-ray charged couple device (CCD) detectors is defined as the reconstruction of independent events in the same detection cell as a single event during a read-out cycle. Pile-up can seriously compromise the spectral performance, modifying both the flux and the spectral shape of celestial sources.
Aims: In this paper we define rigorous metrics to characterise the effect of pile-up in terms of flux loss and spectral distortion.
Methods: We extend analytical formulae derived for pile-up on CCD detectors with the inclusion of the calibrated energy-dependence of the point spread function. We validated our analytical results through both Monte-Carlo simulations of the EPIC cameras on-board XMM-Newton and comparison with pile-up diagnostics in observed data.
Results: We estimate new count rate levels corresponding to a given degree of flux loss and spectral distortion for each EPIC imaging acquisition mode and provide guidance to observers wishing to estimate these values in their own observations.
Conclusions: We strongly recommend using these thresholds in planning future observations with the EPIC cameras.