Extracting Extended Source Spectra and Responses

Getting Started

Sample ObsID used: 1447 (ACIS-I, CAS A)

File types needed: evt2

Please ensure that you have set up ardlib to use the bad pixel file for your observation before following this thread.

unix% dmlist "$CALDB/docs/chandra/caldb_version/caldb_version.fits[cols caldb_ver,ciao_ver]" data 
...
    52 3.2.4     CIAO3.3.0.1  
    53 3.3.0     CIAO3.4      
    54 3.3.0.1   CIAO3.4 

Downloading acisspec

The most recent version of acisspec is v4.0 (07 February 2007):

unix% grep version_ `which acisspec`
version_str="4.0"
version_date="07 February 2007"

Please check that you are using the most recent version before continuing. If you do not have the script installed or need to update to a newer version, please refer to the Scripts page.

Defining Source & Background Regions

First we need to define regions from which the source and background spectra will be created:

unix% cat src.reg
# Region file format: CIAO version 1.0
ellipse(3664,4398,431.51973,335.50368,335.55502)

unix% cat bgd.reg
# Region file format: CIAO version 1.0
circle(3136,4770,146)

Make sure that you save the regions in CIAO format (Regions -> File Format -> CIAO) so that they are fully compatible with the analysis tools; see the Using CIAO Regions thread for more information.

The regions are shown displayed on the event file in Figure 1 . In this example, we chose the background region from a source-free portion of the same event file.

Running the Script

We need to set the soufile1 and bgfile1 parameters - including the regions with the appropriate DM filter - and the root parameter:

unix% punlearn acisspec 
unix% pset acisspec soufile1="acisf01447N002_evt2.fits[sky=region(src.reg)]"
unix% pset acisspec root=es_1447
unix% pset acisspec bgfile1="acisf01447N002_evt2.fits[sky=region(bgd.reg)]"

If you wish to include the ACIS dead area correction (not applied in this thread), set the pbkfile and dafile parameters as well:

unix% pset acisspec pbkfile=acisf059538423N002_pbk0.fits dafile=CALDB 

Now execute the script:

unix% acisspec
Primary source file: events or spectrum (acisf01447N002_evt2.fits[sky=region(src.reg)]): 
Root name for output files (es_1447): 
# mkwarf (CIAO 3.4): WARNING: Input image name was "EVENTS_IMAGE" instead of "WMAP".  Will attempt to use.

Couldn't determine chip position for pixel: (4316.500000,4052.500000) with value=1.000000. Skipping pixel
This sort of warning may appear many times

***ARDLIB warning: Filename es_1447_sou.wmap does not specify an extension. Assuming the first "interesting" extension.
***ARDLIB warning: Filename es_1447_sou.wmap does not specify an extension. Assuming the first "interesting" extension.
***ARDLIB warning: Filename es_1447_sou.wmap does not specify an extension. Assuming the first "interesting" extension.
# mkwarf (CIAO 3.4): WARNING: Input image name was "EVENTS_IMAGE" instead of "WMAP".  Will attempt to use.

***ARDLIB warning: Filename es_1447_bgd.wmap does not specify an extension. Assuming the first "interesting" extension.
***ARDLIB warning: Filename es_1447_bgd.wmap does not specify an extension. Assuming the first "interesting" extension.
***ARDLIB warning: Filename es_1447_bgd.wmap does not specify an extension. Assuming the first "interesting" extension.


Atten: grid input,"energy=0.3:11.0:0.01", is ignored as a weights file, "es_1447_sou.wfef", is found.

Total 85 regions to be processed:
--- Region #1 processed
--- Region #2 processed
--- Region #3 processed
(clip)
--- Region #83 processed
--- Region #84 processed
--- Region #85 processed



Atten: grid input,"energy=0.3:11.0:0.01", is ignored as a weights file, "es_1447_bgd.wfef", is found.


Total 18 regions to be processed:
--- Region #1 processed
--- Region #2 processed
--- Region #3 processed
(clip)
--- Region #16 processed
--- Region #17 processed
--- Region #18 processed

See the Common Warnings section for an explanation of the messages printed to the screen.

Check the parameter file that was used with plist acisspec.

There are a couple things to note about this run:

• The default values of the binwmap (det=8) and ewmap (300:2000) were used.

• The spectra were not grouped in this case. There are four parameters that can be used to specify grouping for the source (gtype, gspec) and background (bggtype, bggspec) spectra, if desired.

Using the Ouput

The script has created a number of output files:

unix% ls es_1447*
es_1447_bgd.pi          es_1447_bgd.wrmf        es_1447_sou.wmap
es_1447_bgd.warf        es_1447_sou.pi          es_1447_sou.wrmf
es_1447_bgd.wmap        es_1447_sou.warf

The BACKFILE, RESPFILE, and ANCRFILE header keywords have been updated in the source spectrum file:

unix% dmkeypar es_1447_sou.pi BACKFILE echo+
es_1447_bgd.pi

unix% dmkeypar es_1447_sou.pi ANCRFILE echo+
es_1447_sou.warf

unix% dmkeypar es_1447_sou.pi RESPFILE echo+
es_1447_sou.wrmf

This means that the background file and source-related responses will automatically be read in when es_1447_sou.pi is read into Sherpa. The RESPFILE and ANCRFILE header keywords are not updated in the background spectrum in order to allow for background subtraction in Sherpa.

For help on fitting this data in Sherpa, follow the Fitting PHA Data thread.

Common Warnings

There are several warning messages that you may see printed to the screen during the acisspec run:

• Originating library: ardlib

  ***ARDLIB warning: Filename es_1447_sou.wmap does not specify an extension. Assuming the first "interesting" extension.
  

  The warning may be ignored as it does not adversely affect the output; it is explained in this FAQ.

• Originating tool: mkwarf

  # mkwarf (CIAO 3.4): WARNING: Input image name was "EVENTS_IMAGE" instead of "WMAP".  Will attempt to use.
  

  There are two ways to create a WMAP, with dmextract or with dmcopy; acisspec uses the dmcopy method. In this process, the primary block of the WMAP file is name EVENTS_IMAGE, as opposed to WMAP. Since we know that sources.wmap8 was created as a WMAP image (i.e. the detector coordinate system was used), the warning message can be ignored.

• Originating tool: mkwarf

  Couldn't determine chip position for pixel: (4076.500000,4052.500000) with value=3.000000. Skipping pixel
  

  If the source is near a chip boundary, mkwarf may not be able to determine the chip location of every pixel in the WMAP. This has to do with the lack of SIM info in the image; details are available in this FAQ.

  In the vast majority of cases, the number of counts ignored (i.e. the sum of the "value" of each ignored pixel) is much smaller than the total signal in the WMAP. However, this may not be the case for small regions near chip boundaries. dmstat may be used to determine the sum of all pixels in the WMAP for comparison to the sum of the ignore pixel values.

• Originating tool: mkrmf

   Atten: grid input,"energy=0.3:11.0:0.01", is ignored as a weights file, "es_1447_sou.wfef", is found.
  

  The energy axis grid is ignored since the grid is read from the weights file. Since the value entered for the energy axis (axis1 in this case) must be syntactically correct for the tool to parse over it properly, acisspec uses "energy=0.3:11.0:0.0" as a placeholder.

Analysis Caveats

Users should be cautious about analyzing the data for sources near the edges of the ACIS CCDs.

1. For X-rays passing through the mirrors, the very bottom of each CCD is obscured by the frame store. As a result, some of the events in rows with CHIPY <= 8 are not detected. (The set of rows affected varies from CCD to CCD.) Since the CIAO tools do not compensate for this effect, the ARFs and exposure maps for sources in these regions may be inaccurate.

2. For sources within about thirty-two pixels of any edge of a CCD, the source may be dithered off the CCD during part of an observation. The aspect histogram, which is used to create ARFs and exposure maps, is designed to compensate for this effect.

3. A contaminant has accumulated on the optical-blocking filters of the ACIS detectors, as described in the ACIS QE Degradation why topic. Since there is a gradient in the temperature across the filters (the edges are colder), there is a gradient in the amount of material on the filters. (The contaminant is thicker at the edges.) Within about 100 pixels of the outer edges of the ACIS-I and ACIS-S arrays, the gradient is relatively steep. Therefore, the effective low-energy (' 1 keV) detection efficiency may vary within the dither pattern in this region. The ARF and instrument map tools are designed to read a calibration file which describes this spatial dependence.