Create a New ACIS Bad Pixel File: Identify ACIS Hot Pixels and Cosmic Ray Afterglows

About the ACIS Hot Pixel Tools

acis_run_hotpix is a script which executes, in this order, acis_build_badpix, acis_find_hotpix, acis_classify_hotpix, and acis_build_badpix a second time.

Collectively, these tools

1. search for pixels where the bias value is too low or too high (acis_build_badpix),

2. search for suspicious pixels where there is an unusually large or unusually small number of events (acis_find_hotpix),

3. classify the events on suspicious pixels as being associated with cosmic ray afterglows, hot pixels or astrophysical sources (acis_classify_hotpix), and

4. write the bad pixels (acis_classify_hotpix) and their neighbors (acis_build_badpix) to the output bad pixel file.

Note that the bad pixels in the input bad pixel file are not all necessarily included in the output bad pixel file. All the bad pixels are recalculated and flagged as appropriate in the output file.

Get Started

Sample ObsID used: 459 (HETG/ACIS-S, 3C 273)

File types needed: evt1; bpix1; msk1; bias0; pbk0

The event file (acis_459_reset_evt1.fits) was created by running the Remove the acis_detect_afterglow Correction; it is not the evt1 file from standard processing. Also, it is now possible to to obtain the correct bias and parameter block files from ChaSeR or WebChaSeR; they are included in the secondary data products.

In this thread, we assume that all relevant files are in the same working directory.

Check the mode of the observation:

unix% dmkeypar acis_459_reset_evt1.fits READMODE echo+
TIMED

unix% dmkeypar acis_459_reset_evt1.fits DATAMODE echo+
FAINT

This is a TIMED FAINT mode observation, so acis_run_hotpix may be used.

We also need to check the processing version:

unix% dmkeypar acis_459_reset_evt1.fits ASCDSVER echo+
CIAO 3.4

Since this data has already had acis_detect_afterglow correction removed, the ASCDSVER keyword value is the CIAO software version that was used. If you have not removed the correction yet, you should see something like this:

unix% dmkeypar acisf00459_000N002_evt1.fits ASCDSVER echo+
R4CU5UPD11.1

The observation was processed before the tool was included pipeline, i.e. the ASCDSVER is lower than DS 7.4.0. Note that the version naming convention changed after version R4CU5UPD14 to the "DS" system, starting with DS 6.0.0. Before continuing, remove any afterglow correction that was applied via acis_detect_afterglow (e.g. during SDP); see the Remove the acis_detect_afterglow Correction for instructions.

Create a New Bad Pixel File

The acis_run_hotpix tool is run to create the new bad pixel file:

unix% punlearn acis_run_hotpix
unix% pset acis_run_hotpix infile=acis_459_reset_evt1.fits
unix% pset acis_run_hotpix outfile=acis_459_new_bpix1.fits
unix% pset acis_run_hotpix badpixfile=acisf00459_000N002_bpix1.fits
unix% pset acis_run_hotpix biasfile=@bias_files.lis 
unix% pset acis_run_hotpix maskfile=acisf00459_000N002_msk1.fits
unix% pset acis_run_hotpix pbkfile=acisf063875928N002_pbk0.fits
unix% acis_run_hotpix
Input event list (acis_459_reset_evt1.fits): 
Output bad pixel file (acis_459_new_bpix1.fits): 
Input mask file ( <filename> | default | none | NONE ) (acisf00459_000N002_msk1.fits): 
Input bad pixel file (acisf00459_000N002_bpix1.fits): 
Input bias images (@bias_files.lis): 

The badpixfile parameter is set to the observation-specific bad pixel file from the Archive, not a bad pixel file from the CALDB. The latter should not be used when running this tool.

Note that for most observations there will be more than one bias file. All the files must be input to the biasfile parameter, either as a list or as a stack (see ahelp stack for more information). Here we used:

unix% more bias_files.lis 
acisf063874723N002_1_bias0.fits
acisf063874723N002_2_bias0.fits
acisf063874723N002_3_bias0.fits
acisf063874723N002_4_bias0.fits
acisf063874723N002_5_bias0.fits

The contents of the parameter file may be checked using plist acis_run_hotpix.

Applying the File

Now that we have a new bad pixel file, it needs to be applied to the event file. To do so, run acis_process_events as shown in the Create a New Level=2 Event File thread. The bad pixel file is used in the badpixfile parameter:

unix% pset acis_process_events badpixfile=acis_459_new_bpix1.fits

Also, set this as the the observation-specific bad pixel file for any subsequent analysis tasks; see the Use Observation-specific Bad Pixel Files thread for instructions.

Warning message: "File /tmp/hotpixel_list.fits does not have any rows"

Some datasets will return this warning when acis_run_hotpix is run:

# acis_classify_hotpix (CIAO 3.4): WARNING: File /tmp/hotpixel_list.fits does not have any rows.

It just means that no hot pixels were identified in the observation, which is a normal occurrence.

Caveats

There are a few caveats associated with acis_run_hotpix.

• acis_run_hotpix will not find afterglows and hot pixels if they are embedded in a bright source.

• The tool can misidentify some legitimate X-ray events as bad. The worst case scenario is a very faint source that has a flare whose duration is much less than the period of the dither (nominally 707 s in the Z direction). The dither why topic has more information.