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Дата изменения: Fri Jan 10 23:33:22 2003
Дата индексирования: Sat Dec 22 10:29:24 2007
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XMM-Newton Science Analysis System


omfchain (omfchain-1.22.13) [xmmsas_20030110_1802-5.4.1]

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• If the chain runs to completion, a number of important output files will appear. The key product file names start with `P' while those beginning with `F' are intermediate images and time-series.

• Since the fast mode window is necessarily small, the source PSF fills much of the window (clearly evident for bright sources). The source may also be significantly offset from the centre. As such, the source and background extraction apertures may be truncated by the window. They may also be contaminated by flux from other potential sources in the window - where detected, these other sources zones are masked out. Moreover, in general the background annulus will be contaminated by flux from the target. To determine source and background levels under such circumstances, the background is assumed constant over the small window and a pair of simultaneous equations involving the background level, source flux and PSF fractions appropriate to the two regions, are solved for each time bin. This procedure naturally corrects for PSF beyond the window limits and for masked-out zones.

  The absolute level of the resulting light curve is dependent on the precision of the current PSF. Relative variations between time bins may also be introduced due to migration of the source centroid within the window when part of the source is beyond the window boundary. It is not possible to fully correct for tracking drift, even when tracking history is available, when the source is near the edge. It is generally advisable to adopt a fixed extraction radius (not less than 6 full-resolution pixels) for a given set of fast mode exposures. This is currently used as the default value for the chain, enhancing S/N for weaker sources.

  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 (this is the default action of omfchain). If changing srcradius parameter one should also remember that the radii of the background annulus scale with it according to the bkginner and bkgouter parameters (see description of these 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 plausible - watch out for frequent drop-outs, isolated high or substantial negative values. Establish that the background light curve is reasonable. It is useful also to 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 different exposures, to see if the separate exposure-specific light curves are consistent (bearing in mind any intrinsic source variability, gaps between exposures and, of course, filter employed). If the image appears blurred or has darker strips along the edges, look at the tracking history file to see whether tracking was stable. 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. It is possible to change the detection efficiency via the boxscale, smoothsize, nsigma, and contrast parameters (see the description of omdetect.