THE XMM-NEWTON ABC GUIDE, STREAMLINED
OM (FAST Mode), GUI
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 make the ccf.cif file, first make sure the environment variables are set:
cd ODF setenv SAS_ODF /full/path/to/ODF/directory/ setenv SAS_ODFPATH /full/path/to/ODF/directory/
Next, call cifbuild from the SAS GUI. A window with the parameter options will appear; the defaults should be fine, so just click "Run".
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.cifThe 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 environment variable, call odfingest from the SAS GUI. As before, a pop-up window with the parameter options will appear; the defaults should be fine, so just click "Run". (It is safe to ignore the warnings.)
To change the environmental variable, type
setenv SAS_ODF /full/path/to/ODF/full_name_of_*SUM.SASYou 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.
cd .. mkdir PROC cd PROCThen, close and re-open the SAS GUI, so that it will place the output files in the new directory, and call omfchain. The default parameters are fine for most data sets, so just click "Run".
There are two types of output files: those that start with F are intermediate images or time series files; those that start with P are products. The processed image in sky-coordinates from one exposure, P0411081601OMS006SIMAGE1000.FIT, and the background-subtracted light curve and various statistics associate with it, F0411081601OMS006TIMESR1000.PS, are shown in Figure 1.
The background light curve in Figure 1 (right) is constant because omfchain runs with the parameter bkgfromimage=yes by default, so that the background light curve is found by using the imaging-mode data, instead of the fast-mode window. This is preferable for even only moderately bright sources (count rate > 0.6 ct/s), as the fast-mode window is small and any background measurement that uses it will likely be contaminated with source photons. This is less of a concern if your source is faint, in which case the background can by found from data in the fast-mode window by setting bkgfromimage to no.
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