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region (region-7.15.3) [xmmsas_20230412_1735-21.0.0]


Local-background region shapes and sizes

Local-background regions are in all cases annuli. The fundamental choice here is between annuli concentric about the optic axis of the particular EPIC telescope or annuli concentric about the source location itself. This choice is made via the parameter fovbkgannulus and is described as follows.

So far it has been assumed that the annuli are circular. However, if the region units are selected to be `xy', then locii of equal radius (in DETXY coordinates) from the OA will in general appear as ellipses in the XY projection and pixellization scheme. If fovbkgannulus=`yes' in this case then the annuli must follow this distortion in order to preserve the approximate uniformity of background around the annulus. The task therefore distorts the circular DETXY annuli into appropriate ellipses if outunit=`xy' (not the default), fovbkgannulus=`yes' (not the default) and nobkgellipse=`no' (default), but not otherwise.

All source-extraction regions are excised from any given local-background region: thus in most general terms a local-background extraction region looks like an elliptical annulus with holes at source locations.

XMM-Newton SOC -- 2023-04-16