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Дата изменения: Fri Jan 10 22:46:38 2003
Дата индексирования: Sat Dec 22 07:48:15 2007
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Поисковые слова: magnetic north

XMM-Newton Science Analysis System


region (region-6.2.2) [xmmsas_20030110_1802-5.4.1]

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Description

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:

1. The source celestial coordinates (RA/Dec) are read from the source list.
2. For each source, the celestial coordinates are transformed to a working coordinate system (arcseconds, tangent plane projection relative to the reference RA/Dec. of the event file).
3. For each source, the source extraction radius is calculated for each source, using the position in the instrument field-of-view. The extraction radius is defined as that at which a user-defined fraction of the PSF is enclosed.
4. The separation to the nearest-neighbour is calculated for each source.
5. In order of increasing nearest-neighbour separation the sources are checked for overlap with their nearest neighbour. If no overlap occurs no action is taken, otherwise the radius is adjusted (see detailed algorithm) and the source flagged as having been adjusted.
6. In a second pass all pairs of sources are checked for overlap. If overlap is detected the size of the larger of the two extraction regions is reduced to eliminate the overlap. (Not implemented in version 2.0).
7. The background extraction region is defined as an annulus around the source. The inner radius is the same as the adjusted source extraction radius, the outer radius is three times the inner radius. The background extraction region is checked for overlap with nearby sources. If overlap is found the original source extraction region of the intersecting source is added as an exclusion clause to the background extraction region definition.
8. The source and background regions will be clipped to the edge of the field-of-view.
9. The source and background regions are transformed (scaling + translation) into the same units as the sky positions of the events in the reference event list.

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:

fixed

A fixed radius is used for all source extraction regions. The radius is defined by the radius parameter.

enfrac

A radius is used which encloses a specified fraction of the source energy. The encircled energy fraction is defined by the energyfraction parameter.

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.