THE XMM-NEWTON ABC GUIDE, STREAMLINED
OM (IMAGING Mode), Command Line
Prepare the Data
OM Artifacts and General Information
Reprocess the Data
Verify the Output
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.).
- 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:
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)
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.
|RA||RA of the detected source|
|DEC||Dec of the detected source|
|RATE||extracted count rate|
|RATE_ERR||error estimate on the count rate|
|SIGNIFICANCE||Significance of the detection (in σ)|
|MAG||Brightness of the source in magnitude|
|MAGERR||uncertainty on the magnitude|
- cd ..
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=3.0where
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.
|EVLIST||none||Fast mode events list|
|FIMAG_||FIMAG_||combined full-frame image|
|FSIMAG||FSIMAG||combined full-frame sky image|
|HSIMAG||HSIMAG||full-frame HIRES sky image mosaic|
|IMAGE_||IMAGE_||image from any filter or Grism|
|IMAGE_||IMAGEF||Fast mode image|
|LSIMAG||LSIMAG||full-frame LORES sky image mosaic|
|OBSMLI||OBSMLI||combined observation source list|
|REGION||SWSREG||sources region file|
|REGION||SFSREG||Fast mode sources region file|
|REGION||SGSREG||Grism ds9 regions|
|RIMAGE||GIMAGE||Grism rotated image|
|RSIMAG||RSIMAG||default mode sky mosaic|
|SIMAGE||SIMAGE||sky aligned image|
|SIMAGE||SIMAGF||Fast mode sky aligned image|
|SIMAGE||none||Grism sky aligned image|
|SPCREG||SPCREG||Grism ds9 spectrum regions|
|SPECLI||SPECLI||Grism specra list|
|SPECTR||SPECTR||source extracted spectra|
|SWSRLI||SFSRLI||Fast mode sources list|
|SWSRLI||SGSRLI||Grism sources list|
|TIMESR||TIMESR||Fast mode source timeseries|
|TSHPLT||TSHPLT||tracking history plot|
|TSTRTS||TSTRTS||tracking star timeseries|
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=ds9where
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
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