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XMM-Newton Science Analysis System


omfchain (omfchain-1.34.3) [xmmsas_20080701_1801-8.0.0]

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• If the chain runs to completion, a number of important output files will appear. The chain product filenames have the extension `.FIT'. The names of the files start with `P' for the products and with `F' for the intermediate images and timeseries.
• The use of the parameters for the source and background extraction areas requires some care. Although the source extraction region is simply a circle, the undersampled nature of the OM PSF means that the circle has a small radius and thus encompasses only a few pixels. Pixels on the circle edge therefore contribute a substantial fraction to the area, and $\pi r^2$ may not be a good approximation to the extraction area. Furthermore, the background extraction region is an annulus, possibly with circular areas excluded (see the OMREGION task description), and these excluded circles are not necessarily totally enclosed by the annulus. In addition, the FAST-mode OM window is small, and a part of the source and background areas falls out of this window. The source and background counts should be scaled to the CAL circular area in order to correct them for the coincidence loss. This scaling is sensitive to the currently used OM PSF. If the source and background areas are affected by the source proximity to the edge of the OSW or if the real OM PSF differes from that, taken from the current calubration file (CCF), than the OMLCBUILD output count rates could vary from exposure to exposure for the same source. The effect could be increase for bright sorces due to the 8-pixel pattern noise. In order to avoid these variations and to obtain a uniform combined light curve, it is better to use a fixed radius for the source extraction area for all the exposures in the ODF. This radius should not be less than 6 pixels. This is currently used as the default value for the chain in order to get better signal-to-noise ratio. One may change this parameter by using the command `omfchain srcradius=...`. If srcradius is set to a negative value, it will be fixed while processing all the exposures (its default value is negative). Changing srcradius parameter one should also properly esablish the background extraction area setting bkginner and bkgouter parameters (see description of the parameters).
• A few output files (P* files) from omfchain should be checked. A useful first step is to inspect the output PDF-light curve plots to check that the data are sensible - watch out for frequent drop-outs, isolated high or substantial negative values. Establish that the background light curve is reasonable. One should also examine the image of the Fast mode window, for example, with the ftool, fv. Check whether the source is well centred or close to an edge. If the latter, inspect the light curves, especially from fifferent exposures, to see if the separate exposure-based light curves are plausibly consistent (bearing in mind any intrinsic source variability). If the image appears blurred or odd in any way, look at the tracking history file to see whether tracking was reliable. You may also want to overlay the image source list onto skymap usingimplot, e.g.

  implot set= sky_image_file.FIT withsrclistset=y srclistset=osw_source_list_file.FIT device=your_device (e.g. /XW) itv=1 radius=3 maxsrc=10 colour=7.

  While often there will be only one object in the image, one should check for spurious detections, which could affect the analysis of the target source and also for missed sources or prominent objects in the window, which may contaminate the target. At the current time, bright contaminating sources are not treated properly, and, if present, affect the light curve of the target (and vice versa) - this will be rectified in a future version of the relevant SAS tasks. It is possible to change the detection efficiency via the boxscale, smoothsize, nsigma, and contrast parameters.