Photon pileup occurs when two or more photons are absorbed by a CCD within the same spatial and temporal detection cell. The CCD will be unable to resolve the individual photon events and instead record a single event with a pulse-height that is roughly the sum of the individual event pulse-heights. Hence, pileup results in a reduction of the observed count rate and a PHA spectrum that is skewed towards higher energies.
The pileup_warning column gives the observed per-frame count-rate for the 3-by-3 pixel region centered on the detection. It may be used as an estimate for the pileup-fraction in the region for weak to moderate levels of pileup (0 up to 10 percent) according to the following table:
|pileup_warning||pileup fraction (%)|
Here, pileup fraction is defined to be the fraction of event-producing charge-clouds that are due to photon pileup.
It is important to appreciate the fact that pileup is a non-linear phenomenon, and regions that suffer from heavy pileup may have pileup_warning values that that vastly underestimate the amount of pileup. An example of this is the so-called pileup crater that results when many photons pileup to produce a charge cloud whose spatial extent (grade) no longer resembles that of an X-ray. In such a case, very few X-rays would be detected resulting in a small value for the pileup_warning.
For a justification of these values and the assumptions involved, see Pileup Fractions and Count Rates. More extensive discussions of pileup may be found in Event Pileup in Charged-coupled Devices (Davis, J.E., 2001, ApJ, 562, 575), and The Chandra ABC Guide to Pileup.