The Chandra ABC Guide to Pileup

The purpose of this document is to provide a Chandra-ACIS specific overview of pileup - i.e., the phenomena of two or more photon events overlapping in a single detector frame and being read as a single event. We discuss the definition of pileup and describe its effects on detected spectra and variability. We outline methods for avoiding pileup, and also discuss when not to avoid pileup. We describe several ways the degree of pileup can be estimated when planning an observation. Methods of mitigating the effects of pileup in real data are outlined. This document is meant as a ‘living resource’. As knowledge of how to detect, assess, and correct for the effects of pileup improves, we will update and expand the procedures described below.