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Cosmic-Ray Afterglows

Summary

Updated algorithm and tool

acis_find_afterglow, which incorporates the most recent afterglow and hot-pixel identification algorithms was incorporated into version 8.4.2 of the standard pipeline processing software (December 2011).

The tool acis_find_afterglow searches for afterglows using a short, sliding time window (see the parameter expnowindow). 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 (see the parameter probthresh).

acis_find_afterglow should be used for only timed mode observations. The afterglow and hot-pixel tools cannot be used for continuous-clocking mode datasets.

acis_find_afterglow should be executed after destreak. The output file produced by acis_find_afterglow, which includes any newly-identified afterglows and hot pixels as well as the bad pixels and columns in the input badpixfile, should be reprocessed with acis_build_badpix to ensure that pixels adjacent to newly-identified afterglows and hot pixels are handled properly and that the list of bad pixels is in the proper order. For examples of how to use acis_find_afterglow see the ahelp file and the Reprocessing Data to Create a New Level=2 Event File thread.

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. A selected sample of afterglows is shown below. 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 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.

A variety of afterglows

[Version: postscript]

A variety of afterglows

The pulse height (PHA) v. relative frame number (EXPNO) 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 pulse height typically decreases from frame to frame, it can increase toward the end of an afterglow when the pulse height is relatively low.

The first afterglow-identification algorithm, which was implemented in the CIAO tool acis_detect_afterglow, was used for pipeline 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 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 (see Examine the Afterglow Events below). The fraction of the source events that are discarded depends on the brightness and variability of a source.

In an attempt to minimize the loss of source events, another afterglow-identification algorithm was developed and implemented in the CIAO tool acis_run_hotpix, now deprecated. (Note that acis_run_hotpix is a wrapper around the tools acis_find_hotpix , acis_classify_hotpix, acis_build_badpix. This tool searched for detector pixels that have an unusually large number of events. If the median number of frames from one event on the pixel to the next is relatively low, then the pixel is marked as having an afterglow. Otherwise, the pixel is marked as being hot. Hot pixels and afterglows 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 ensure that events associated with a dithered source are not discarded, but may prevent the identification of hot pixels and afterglows within the dither pattern of the source. The tool acis_run_hotpix has been used for pipeline processing (and reprocessing) since the fall of 2004. While it is relatively gentle on astrophysical sources and can identify afterglows with "frame gaps," it does let some afterglows "slip through the cracks." The afterglow detection efficiency, which depends on the number of events in an afterglow, declines quickly as the number of events in an afterglow drops below 8.

Like the tool acis_run_hotpix, acis_find_afterglow searches for afterglows and hot pixels and is designed to ensure that events associated with dithered sources are not discarded. Unlike acis_run_hotpix, acis_find_afterglow performs a search for afterglows using a short, sliding time window instead of using the data for the entire duration of an observation (i.e. the search is three dimensional instead of two dimensional). As a result, acis_find_afterglow identifies essentially all of the afterglows that have four or more events. Furthermore, the background estimation algorithm, which is used for both cosmic-ray afterglows and hot pixels, has been enhanced to avoid contamination from bright point sources.

Examine the Afterglow Events

The following example shows how to display the afterglow events. Although this technique will work for any of the afterglow-identification tools, the image shown here illustrates how acis_detect_afterglow can misidentify source events as afterglows. After the event data file acisf00459_000N002_evt1.fits was processed using acis_detect_afterglow, the data were filtered to exclude events for which none of the afterglow status bits (16-19, counting from the right, starting at zero) are marked as bad (1). The non-afterglow bits are filtered using the wildcard "x", which lets either a good or bad value pass. This filter retains the events in which any of the afterglow flags have been set to 1:

unix% dmcopy \
      "acisf00459_000N002_evt1.fits[exclude status=xxxxxxxxxxxx0000xxxxxxxxxxxxxxxx]" \
      acis_459_afterglows_evt1.fits

Comparing the event files

Comparing the event files

Displaying the files acis_459_afterglows_evt1.fits (left) and acisf00459_000N002_evt1.fits (right) in ds9 shows that the source is clearly visible in the afterglow image (left), meaning that many source photons have been marked as afterglows.

Resetting the Status Bits

Before using acis_find_afterglow, the afterglow status bits in the input data file must be reset so that they may be recomputed. This step is included in the Reprocessing Data to Create a New Level=2 Event File thread and in the chandra_repro script.