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XMM-Newton Guest Observer Facility


OM (GRISM Mode), Parameter Window


Prepare the Data
OM Artifacts and General Information
Reprocess the Data
Verify the Output

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 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.

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.

    - 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.

Reprocess the Data

The repipelining task for OM data taken in grism mode is omgchain. It produces images of the detected sources and background, extracts source spectra and region files, and makes source lists and postscript and PDF plots. At present, unlike omichain, omgchain does not allow for keywords to specify filters or exposures.

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).

Verifying the Output

Users are strongly encouraged to verify the consistency of the data prior to analysis. The correct correlation of zero and first orders is crucial for grism analysis. Users should download and inspect the rotated image with fv or ds9 and verify the identification of the orders by overlaying the *SPCREG*region file, as shown in Figure 1 (left); the *SPECLI* file also contains this informtaion. If users are interested in all source detections, the region file can also be overlayed and the full source list examined. Users should also examine the spectra plots automatically produced by omgchain (*SPECTR*.pdf), for both the source and background, making sure they are reasonable. For improved source detection, the parameter nsigma can be changed.
Figure 1. Left: The zoomed-in, repipelined, rotated image with the source regions overlayed (green). A source in the spectra list file is also shown (red). Right: the fluxed spectrum extracted from the source (red region).

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

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