THE XMM-NEWTON ABC GUIDE, STREAMLINED
OM (GRISM Mode), Parameter Window
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, navigate into the ODF directory using the icons in the User Account Window of the Hera interface. Then, in the Tool Parameter Window, in the Task Name box, enter cifbuild and click "Get".
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. To run odfingest, in the Tool Parameter Window, in the Task Name box, enter odfingest and click "Change".
Hera automatically resets the relevant environmental parameters to the output of these tasks, so we can continue merrily on our way.
- 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.
To reprocess the data, go up one directory in the tree and make a new working directory using the buttons in the upper left corner of the User Account Window. When you are in the working directory, call omichain. As in the previous section, just enter the task name in Task Name box and click "Change". The default parameters are fine for most datasets, so click "Run omgchain".
There are two types of output files: those that start with g are intermediate or auxiliary files and source lists; those that start with p are products.
The processed image, rotated to align with the columns of the image (p0125320801OMS005RIMAGE0000.FIT), is shown in Figure 1 (left). Two region files are overlayed: p0125320801OMS005REGION0001.ASC, which correspond to the sources detected in this rotated image (green), and p0125320801OMS005SPCREG0001.ASC, which corresponds to the sources in the spectra list file (red) and indicates the locations of the zero and first orders. The task omgchain automatically extracted the spectrum of the red region (p0125320801OMS005SPECTR0000.FIT); this is shown in Figure 1 (right).
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