XMM-Newton Science Analysis System


omfchain (omfchain-1.43.2) [xmmsas_20230412_1735-21.0.0]

Comments

• 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 under-sampled 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 differs from that taken from the current calibration file (CCF), than the OMLCBUILD output count rates could vary from exposure to exposure, even if the source brightness remains constant.

  This effect could be increased for bright sources 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, although for faint source one it is possible to use smaller values (this would increase the signal-to-noise ratio). The value of 6 pixels is currently used as the default value for the chain in order to get an optimal signal-to-noise ratio for most of the sources. One may change this parameter by using the command `omfchain srcradius=...`. If srcradius is set to a negative value, it will be regarded as measured in unbinned pixels and will be fixed for all the exposures (its default value is negative).

  When changing the srcradius parameter one should also properly establish the background extraction area setting bkginner and bkgouter parameters (see the description of the task 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 (it would be constant for the option bkgfromimage=yes). One should also examine the image of the Fast-mode window, for example, with the ftool, fv or ds9. Check whether the source is well centred or close to an edge. If the latter, inspect the light curves, especially from different 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. Currently, 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.

  The visual inspection may also reveal that a bright source of interest is affected by modulo-8 noise (a rectangular pattern around the source). Obviously, the standard (calibrated) point-spread function is not applicable to these kind of images (the source counts could be found at any pixel within the Fast-window; and a large fraction of them fall outside of this window). For this kind of images the determining of background from the Fast-window becomes impossible, and it would be appropriate to consider the entire Fast-window as the source extraction region. Since the sources generating modulo-8 noise patterns are quite bright, the background becomes negligeable and, thus, could be set to zero. For this purpose one can set the source extraction region to the entire Fast window by setting the parameter srcradius to, say, 12 pixels (half of the Fast-window x- or y-size). The light-curve obtained in this way would be more reliable.