**XMM-Newton Science Analysis System
**

eregionanalyse (especget-1.54) [xmmsas_20190531_1155-18.0.0]

###

Encircled energy factor

The fraction of counts contained within the source region assuming
a point source located at the centre of the region. The PSF
used to calculate the fraction is defined by the `psfmodel`
parameter at a fixed photon energy of 1.5 keV. This is currently only
implemented for circular source regions, other shapes will give the
wrong factor.

XMM-Newton SOC/SSC -- 2019-06-02