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Last modified: 23 February 2010

URL: http://cxc.harvard.edu/ciao4.2/why/afterglow.html

Cosmic-Ray Afterglows


Summary

At present, the tool acis_run_hotpix is used in the pipeline to identify cosmic-ray afterglows. The results obtained using this tool should be sufficient unless a user is interested in searching for sources that have a small number of counts (i.e. less than about 10 counts). At this point, the statistical significance of the afterglows is too low for the tool to identify them. While it is possible to change the statistical threshold by using the probthresh parameter, such a change may increase the chances that real x-ray events are discarded.

In these situations, it may be advantageous to use the tool acis_detect_afterglow - in addition to acis_run_hotpix - to try to eliminate afterglows that have only a few events. Some care is still required to ensure that acis_detect_afterglow does not eliminate a significant number of real x-ray events.

There are examples in the ahelp files that show how to use these tools. There is also a CIAO analysis thread for acis_run_hotpix:


A cosmic-ray "afterglow" is produced when a large amount of charge is deposited on a CCD by a cosmic ray. 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 detector pixel over a few frames as the trapped charge is released. A selected sample of afterglows is plotted in Figure 1. Notice that the events in an afterglow need not occur in consecutive frames. There can be gaps of a few frames between events. Also note that while the summed pulse heights of the events typically decrease from frame to frame, the pulse heights can increase toward the end of an afterglow when the pulse heights are relatively low.

[Pulse height v. relative frame number for a selected sample of afterglows]

Figure 1. The pulse height v. relative frame number for a selected sample of afterglows. Note that events need not occur in consecutive frames. There can be gaps of a few frames without events in the middle of an afterglow. While the summed pulse height typically decreases from frame to frame, it can increase toward the end of an afterglow when the pulse height is relatively low. [PostScript of Figure 1]

To date, two algorithms have been used by the CXC to identify cosmic-ray afterglows. The first algorithm was implemented in the CIAO tool acis_detect_afterglow and used for standard data processing from the summer of 2000 to the fall of 2004. This algorithm searches for occasions when events are detected in two or more consecutive frames on the same CCD pixel. While the events are flagged as potential cosmic-ray afterglows and excluded from Level 2 event-data files, the corresponding pixels are not included in the observation-specific bad-pixel file. This algorithm finds many afterglow events, but at the expense of discarding x-ray events associated with real astrophysical sources. The fraction of the source events that are discarded depends on the brightness and variability of the source.

In an attempt to minimize the loss of source events, another algorithm was developed and implemented in the CIAO tool acis_run_hotpix. (Note that acis_run_hotpix is a wrapper around the tools acis_find_hotpix, acis_classify_hotpix and acis_build_badpix.) This second algorithm searches for detector pixels that have an unusually large number of events. Suspicious pixels are added to the observation-specific bad-pixel file only if the neighboring pixels do not have a significant excess of events. This condition helps insure that events associated with dithered sources are not discarded. Events associated with afterglows are flagged and excluded from Level 2 event files. The newer algorithm has been used for pipeline processing (and reprocessing) since the fall of 2004. While it is relatively gentle on astrophysical sources, it does let some afterglows slip through the cracks. The afterglow detection efficiency drops sharply when the number of events in an afterglow is less than about 10. The reason for this drop in efficiency is that the statistical significance of the afterglows, as computed by acis_run_hotpix, is too low for the afterglows to be identified.

To try to catch some of the afterglows that have fewer events, a third algorithm is being developed. This new algorithm is expected to identify afterglows with as few as four events.


Last modified: 23 February 2010