**XMM-Newton Science Analysis System
**

region (region-7.15.3) [xmmsas_20230412_1735-21.0.0]

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Specifying source region shapes and sizes

The basic source region shape is a circle. There are three ways to assign the radius, specified via the parameter radiusstyle:

- radiusstyle=`userfixed'. The user should supply the radius via the parameter fixedradius, at present in units of arcseconds.
If outunit=`xy', the smaller of the XY-system pixel dimensions in arcseconds provides the conversion factor to convert this radius to sky-image pixel coordinates.

- radiusstyle=`enfrac'. In this choice, the task uses for the source region radius that value which encloses energyfraction of the total flux under the Point Spread Function (PSF).
Note that in this case, regardless of the value of outunit, the initial circle must be drawn in the DETXY system, since the PSF is returned in this system. However if outunit=`xy' this involves a reprojection onto another image plane. In theory, locii which appear circular in the DETXY projection plane appear in general as ellipses in another projection plane (and pixellization scheme). At present the effect of this reprojection on the source region contours is ignored and circles are retained. The scalar conversion between circle radii in the two coordinate systems is performed by taking the smaller of the XY-system pixel dimensions in DETXY coordinates as the conversion factor. In most cases the effect of this approximation is expected to be insignificant. This issue may be dealt with more rigorously in a later version of the task.

- radiusstyle=`contour'. In this choice, the task calculates an elliptical locus which approximates the contour of the PSF at a given flux value. This procedure is described more fully in the following subsection (3.4.1). Note that in this choice the distortion caused by transform from the instrument-centric PSF frame to a sky-centric XY frame is taken into account.

**Subsections**
XMM-Newton SOC -- 2023-04-16