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XMM­Newton CCF Release Note
XMM­CCF­REL­116
PSF of the X­ray telescopes
R. D. Saxton
14 May 2002
1 CCF components
Name of CCF VALDATE List of Blocks
changed
CAL VERSION XSCS flag
XRT3 XPSF 0004.CCF 2000­01­01 KING PARAMS NO
2 Changes
A new parameterisation of the EPIC­pn (XRT3) telescope point spread function (PSF)
has been produced by Simona Ghizzardi. This follows earlier work performed, using the
same method, on the XRT1 and XRT2 PSFs [1].
The new values replace an earlier parameterisation of the PSF, which was only valid on­
axis. They are stored in the KING PARAMS extension of the CCF tabulated against
ENERGY and THETA (off­axis angle) [2].
3 Scientific Impact of this Update
The PSF is described by a King function whose parameters, core radius and index, are
themselves functions of energy and off­axis angle. The current work has used many bright
point sources both on and off axis to determine the energy dependent PSF. This should
produce more reliable results than the previous implementation which was based on a few
on­axis sources.
The parameterisation is:
core—radius = 6.636 ­ 0.305 * energy ­ 0.175 * offax
­ 0.0067 * energy * offax
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Figure 1: Comparison of the SAS PSF with a composite PN radial profile
index = 1.525 ­ 0.015 * energy ­ 0.012 * offax
­ 0.0010 * energy * offax
where energy is the photon energy in keV and offax is the off­axis angle in arcminutes.
4 Estimated Scientific Quality
The PSF has been compared against a composite radial profile in Figure 1. This radial
profile has been created for the PN camera by combining many bright sources observed
on­axis. The core of the radial profile has been taken from long observations of non piled­
up sources whereas the wings are taken from observations of very bright sources but at
radii where pile­up ceases to be a problem.
It can be seen that the parameterised PSF is a good representation at both low and high
photon energies out to three arcminutes.
2

5 Expected Updates
None are forseen.
6 Test procedures
The changes introduced here directly affect the encircled energy correction which is applied
by the SAS task arfgen. The output of arfgen should be checked to ensure that this
correction is as expected. Corrections should be measured for a number of energies,
off­axis angles and circle radii.
7 Test results
The EEF correction at 1.5 keV for an on­axis PN source within a circle of radius 20
arcseconds is 72.1% which is as expected from an integration of the PSF. The correction
at 12.1 keV at 10 arcminutes off­axis for a 40 arcsecond radius circle is 82.5% which is
also as expected.
References
[1] S. Ghizzardi, ''In­flight calibration of the on­axis and near off­axis PSF for the Mos­1
and Mos­2 cameras'', EPIC­MCT­TN­011.
[2] C. Erd, P. Gondoin, D. Lumb, R. Much, U. Lammers, G. Vacanti and Richard Saxton.
Calibration Access and Data Handbook. XMM­PS­GM­20, issue 2.2.
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