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THE XMM-NEWTON ABC GUIDE, STREAMLINED
OM (IMAGING Mode), Command Line

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Contents

Prepare the Data
OM Artifacts and General Information
Reprocess the Data
Verify the Output
Make a PHA File

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Prepare the Data

Please note that the two tasks in this section (cifbuild and odfingest) must be run in the ODF directory. These are the only tasks with that requirement, and after this section, we will work exclusively in our reprocessing directory.

Many SAS tasks require calibration information from the Calibration Access Layer (CAL). Relevant files are accessed from the set of Current Calibration File (CCF) data using a CCF Index File (CIF). To set the environment parameter and make the ccf.cif file, type

   cd ODF
   setenv SAS_ODF /full/path/to/ODF/directory/
   setenv SAS_ODFPATH /full/path/to/ODF/directory/
   cifbuild

To use the updated CIF file in further processing, you will need to reset the environment variable SAS_CCF:

   setenv SAS_CCF /full/path/to/ODF/ccf.cif

The task odfingest extends the Observation Data File (ODF) summary file with data extracted from the instrument housekeeping data files and the calibration database. It is only necessary to run it once on any dataset, and will cause problems if it is run a second time. If for some reason odfingest must be rerun, you must first delete the earlier file it produced. This file largely follows the standard XMM naming convention, but has SUM.SAS appended to it. After running odfingest, you will need to reset the environment variable SAS_ODF to its output file. To run odfingest and reset the environment variable, type

   odfingest
   setenv SAS_ODF /full/path/to/ODF/full_name_of_*SUM.SAS

You will likely find it useful to alias these environment variable resets in your login shell (.cshrc, .bashrc, etc.).

OM Artifacts and General Information

Before proceeding with the pipeline, it is appropriate to discuss the artifacts that often affect OM images. These can affect the accuracy of a measurement by, for example, increasing the background level. Some of these can be seen in Figure 1.

    - Stray light. Background celestial light is reflected by the OM detector housing onto the center on the OM field of view, producing a circular area of high background. This can also produce looping structures and long streaks.
    - Modulo 8 noise. In the raw images, a modulo 8 pattern arises from imperfections in the event centroiding algorithm in the OM electronics. This is removed during image processing.
    - Smoke rings. Light from bright sources is reflected from the entrance window back on the detector, producing faint rings located radially away from the center of the field of view.
    - Out-of-time events. Sources with count rates of several tens of counts/sec show a strip of events along the readout direction, corresponding to photons that arrived while the detector was being read out.

Users should also keep in mind some differences between OM data and X-ray data. Unlike EPIC and RGS, there are no good time intervals (GTIs) in OM data; an entire exposure is either kept or rejected. Also, OM exposures only provide direct energy information when in grism mode, and the flat field response of the detector is assumed to be unity.

If you simply want a quick look at your data, sky images and source lists are in *SIMAGE*.FTZ and *SWSRLI*.FTZ, respectively. Further, there are low resolution sky images for each filter; they follow the nomenclature:

    PjjjjjjkkkkOMX000RSIMAGbb000.QQQ

where

jjjjjj - Proposal number
kkkk - Observation ID
b - Filter keyword: B, V, U, M (UVM2), L (UVW1) and S (UVW2)
QQQ - File type (e.g., PNG, FTZ)

So for example, P0123700101OMX000RSIMAGV000.FTZ is the final low resolution sky image in the V filter.

To see what files have been summed to make the final image, search for the keyword XPROC0 in the FITS header. For our example image, this would be

   XPROC0  = 'ommosaic imagesets=''product/P0123700101OMS004SIMAGE1000.FIT produc&'
   CONTINUE  't/P0123700101OMS415SIMAGE1000.FIT product/P0123700101OMS416SIMAGE10&'
   CONTINUE  '00.FIT product/P0123700101OMS417SIMAGE1000.FIT product/P0123700101O&'
   CONTINUE  'MS418SIMAGE1000.FIT'' mosaicedset=product/P0123700101OMX000RSIMAGV0&'
   CONTINUE  '00.FIT exposuremap=no exposure=1000 # (ommosaic-1.11.7) [xmmsas_200&'
   CONTINUE  '61026_1802-6.6.0]'

The source list file (*SWSRLI*.FTZ) also contains useful information. Some column names are listed in Table 1.

Reprocess the Data

To reprocess the data in all exposures and filters, make a new working directory and call omichain from inside it.

cd ..
mkdir PROC
cd PROC
omichain

This produces numerous files, including images and regions for each exposure and each filter. Luckily, omichain will let you specify exposures, filters, and other parameters, so if you are interested only in, say, the sources detected in the mosaicked V band image, we could run omichain with the appropriate flags:

   omichain filters=V processmosaicedimages=yes omdetectnsigma=2.0 omdetectminsignificance=3.0 

where

filters - list of filters to be processed
processmosaicedimages - process the mosaicked sky images?
omdetectnsigma - number of σ above background required for a pixel to be
   considered part of a source
omdetectminsignificance - minimum significance of a source to be included in the
   source list file

The output files can be used immediately for analysis, though users are strongly urged to examine the output for consistancy first (see the next section). The chains apply all necessary corrections, so no further processing or filtering needs to be done. Please note that the chains do not produce output files with exactly the same names as those in the PPS directory. (They also produce some files which are not included in the PPS directory at all.) Table 2 lists the file ID equivalences between repipelined and PPS files.

Verifying the Output

While the output from the chains is ready for analysis, OM does have some peculiarities, as discussed above. While these usually have only an aesthetic effect, they can also affect source brightness measurements, since they increase the background. In light of this, users are strongly encouraged to verify the consistency of the data prior to analysis. There are a few ways to do this. Users can examine the combined source list with fv, which will let them see if interesting sources have been detected in all the filters where they are visible. Users can also overlay the image source list on to the sky image with ds9 or gaia by using slconv to change source lists into region files. The task slconv allows users to set the regions radii in arcseconds to a constant value or scale them to header keywords, such as RATE. By default, ds9 region files have suffixes of .reg; gaia region files have suffixes of .gaia. In the example below, we make a region file from the source list for the mosaicked, V-band sky image.

To make a ds9 region file from a source list, type

   slconv srclisttab=P0123700101OMS000RSISWSV.FIT radiusexpression=5 \
      outfileprefix=Vband_mosaic outputstyle=ds9 

where

srclisttab - source list file name
radiusexpression - constant or expression (possibly involving keywords)
   used to determine the radii of the plotted circles
outputstyle - output format; either ds9 or gaia
outfileprefix - prefix of output file name

The mosaicked, V-band sky image, P0123700101OMS000RSIMAGV.FIT, with the region file from slconv overlayed, is shown in Figure 1. There clearly are spurious detections; these can be removed by hand, or by rerunning omichain with a different background-level threshold or source significance.

Make a PHA File

If we want to analyze the spectrum from OM either by itself or together with that from RGS or EPIC in XSpec, Sherpa, or other familiar software package, we will first need to transform the OM data into OGIP II format. To do this, we must first identify the RA and Dec of the source that we are interested in. This can be done by displaying the EPIC event file in ds9 and loading the OM region file to make sure there is something there; see Figure 2. It can be seen that the sources are identified with circles of various colors, with the colors indicating the quality flags. The flags have colors as defined here. Sources are numbered, so it is easy to find the correct entry in the OM source list.

For our example, we will consider the OM source that corresponds to the X-ray source that we made an EPIC spectrum for. Loading the EPIC data mos1_filt_time.fits and the combined image source region file P0123700101OMCOMBOBSMLI0000.reg, we see that we need source number 192, which has RA=163.165152 and Dec=57.408701. (We will ignore the quality flag warning.)

Now we can do the transform:

   om2pha srclist=P0123700101OMCOMBOBSMLI0000.FIT ra=163.165152 dec=57.408701 output=om_src.pi

where

srclist - source list file name
ra - source's right ascension in degrees
dec - source's declination in degrees
output - output file name

The output FITS file will have information in the header that directs the analysis sofware to look for the canned response files which are available here. We will download om_effarea_v2.0.tgz, which contains the responses for all the OM filters, to make sure we will have what we need. No further processing, or header editing, is required. From here, we can proceed straight to analyzing our spectrum in XSpec or Sherpa.

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This file was last modified on Tuesday, 19-Nov-2013 17:08:40 EST
Curator:Michael Arida (ADNET); michael.arida@nasa.gov