An "afterglow" occurs when a cosmic ray interacts with a front-illuminated CCD to produce a large amount of charge. Most of the charge is clocked off of the CCD in a single frame. However, a small amount can be captured in charge traps, which release the charge relatively slowly. As a result, a sequence of events can appear in a single pixel over a few to a few dozen frames for timed-exposure mode observations.
The tool acis_find_afterglow - added in CIAO 4.4 - searches for afterglows using a short, sliding time window. If there is a statistically significant excess of events compared to the expected number of background events, then the excess is identified as an afterglow unless the excess seems to be associated with a source that is periodically dithered across the pixel.
acis_find_afterglow, which incorporates the most recent afterglow and hot-pixel identification algorithms, supersedes the tools acis_detect_afterglow and acis_run_hotpix and will be incorporated into a future update of the standard pipeline processing software.
Afterglows have been observed in continuous-clocking mode observations. However, the existing afterglow identification tools cannot be used to search for afterglows in continuous-clocking mode datasets.
Further details are available in the Cosmic-Ray Afterglow why topic.