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The region task will automatically determine the optimal source and background extraction regions for a detected source in an XMM observation. The source positions are drawn from the EPIC FITS observation source list (a PPS product, created by the task xmmid). The source and background extraction regions produced conform to the ASC region specification [1]. The regions are defined in sky coordinates. region will attempt to maximise the area of the extraction region for each source, whilst ensuring that the extraction regions of neighbouring sources do not overlap, and that the background extraction region for a given source excludes neighbouring sources.
Nearest-neighbourhood is not a two-way mapping, ie. it is possible that, whilst source B is the nearest neighbour of source A, the converse is not true. To maximise the area of the the source extraction regions overlaps are tackled in order of increasing nearest neighbour distance, ie. the closest pair is dealt with first. This prevents the source extraction regions being reduced to a smaller size than necessary. A second pass is needed to catch any situations where simple considerations of the nearest-neighbour are insufficient.
The source and background extraction regions are determined as follows:
region can also be used to define the region used for extracting the global background lightcurves and spectra (which are PPS products). If the parameter opmode is specified as global the background region file produced will cover the whole field-of-view but with the detected point sources excluded.
The radiusmode parameter chooses the algorithm used to define the source extraction radii (before the regions are analysed for overlap). The available modes currently available are:
region needs to read the event list which will be filtered through the region files, in order to read the keywords which define the instrument and coordinate system. This information is drawn from the event list rather than from the source list, in order to allow non-XMM source lists to be used.
XMM-Newton SOC/SSC -- 2003-01-10