Within SAS, a tool is available to check the pile-up fraction in the pn data: epatplot. This tool compares the observed pattern distributions (derived from a pn calibrated and concatenated even list) with the calibrated models. The latter are instrument mode dependent.
An example of the epatplot output (as in the SASv5.3 implementation) is shown in Fig.1
Fig.1: the SAS v5.3 epatplot output in the case of a strong pile-uped source
The upper panel shows the energy spectra. Different colors correspond to different pattern choices: singles (red), doubles (blue), triples (green), quadruples (ultramarine). The lower panel is the key element for the evaluation of the pile-up fraction: it shows the observed distribution of pattern as a function of energy (PI channel in Analog-Digital Units), using the same color code. A continuous line, superimposed to the histograms, represents the expected distribution. In the example above, the remarkable difference between observation and model indicate that the pn data are substantially affected by pile-up along the whole on energy bandpass.
The only way to reduce pile-up in a given dataset is to excise the core of the PSF, up to a radius where the pile-up fraction becomes negligible. The effect of a 4 arc-minutes excision on the same observation is shown in Fig.2. The agreement between observed and theoretical distribution is much better. Spectra extracted from annuli around the source, excluding the innermost 4' are pile-up free.
Fig.2: the same as Fig.1, if 4' around the PSF core are excised.
The user must be aware that differences between the observed and expected pile-up distribution can occur both in the softest and hardest energy band, due to the effects unrelated to pile-up:
Two more caveats shall be born in mind: