**XMM-Newton Science Analysis System
**

arfgen (arfgen-1.104) [xmmsas_20230412_1735-21.0.0]

##

Encircled energy correction

In the case of a point source,
arfgen
can correct for the flux that is outside
the region(s) over which the spectrum was accumulated.
These regions are read from
the spectrum's DSS information.
The correction works by taking a model of the PSF from the CAL,
centred at the source position
and calculating the total fraction of that PSF
within the regions of interest.

To enable the encircled energy correction for point sources,
the parameter extendedsource
must be set to false,
as well as modelee being set to true.

The encircled energy correction is energy dependent, so ideally
the EE should be computed for each
and every point on the energy grid.
However, given that the EE calculation
is computationally intensive, the option to resample the energy
grid is provided by the parameter eegridfactor. The resultant
EE correction vector is interpolated back onto the original energy
grid, and each ARF element is corrected using the corresponding
EE correction factor. See section 10
for further information.

By default arfgen
uses a fomulation of the PSF as a
function of energy and off-axis angle, known as the ELLBETA model,
to calculate the
encircled energy correction. This model is valid for all extraction
region shapes as it works by summing up contributions to the PSF
over a grid of pixels which fall within the region. The resolution
of this grid (and of the bad pixel calculation) is set by the
parameter badpixelresolution which has a default of 0.5
arcseconds. The PSF model may be overridden using the
psfmodel parameter.

XMM-Newton SOC -- 2023-04-16