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XMM­Newton CCF Release Note
XMM­CCF­REL­100
PSF of the X­ray telescopes
R.D. Saxton
December 19, 2001
1 CCF components
Name of CCF VALDATE List of Blocks
changed
CAL VERSION XSCS flag
XRT1 XPSF 0005.CCF 2000­01­01 KING PARAMS SAS v5.3 NO
XRT2 XPSF 0005.CCF 2000­01­01 KING PARAMS SAS v5.3 NO
XRT3 XPSF 0003.CCF 2000­01­01 KING PARAMS SAS v5.3 NO
2 Changes
A new extension, KING PARAMS, has been added to the CCF files XRT XPSF to characterise the
point spread function (PSF) of the telescopes as a King function. This follows work performed on
the two MOS telescopes in Milan [1] and on the PN, in Leicester, using a similar method. This
parameterisation was originally encoded into the XRTn XENCIRCEN files but has been recoded
here to make them more accessible for the SAS.
The KING PARAMS extension contains the columns ENERGY, THETA, PARAMS which give
the King model parameters, core radius and slope, as a function of photon energy and off­axis angle
[2].
3 Scientific Impact of this Update
This note describes the first release of the King function parameters which describe the PSF. These
parameters are used by the SAS to calculate the encircled energy and correct the EPIC effective
area files (ARF) for losses due to photons scattered out of the source box. The encircled energy
values themselves were previously released in the XRTn XENCIRCEN files [3]. The structure of
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XMM­Newton CCF Release XMM­CCF­REL­100 Page: 2
these files, however, didn't allow the off­axis dependency of the PSF to be fully characterised. The
present implementation remedies this problem and allows the SAS to calculate the encircled energy
more efficiently.
4 Estimated Scientific Quality
The parameters have been calculated by fitting a King function to the radial profiles of in­orbit
celestial point sources. These fits have been made as a function of photon energy and off­axis angle
on available calibration sources. For the MOS telescopes the work is reasonably complete except for
higher energy photons at large off­axis angles (see Figure 18 of [1]). The parameterisation of the PN
PSF has only been performed on­axis so far. In reality the encircled energy of the XRT telescopes
is rather insensitive to the off­axis angle (see Figures 33­36 of [1]). This means that the use of the
on­axis PN PSF is unlikely to produce large errors in the returned encircled energy fractions for
sources at off­axis angles Ÿ 10 arcminutes. The PSF is currently unmeasured for off­axis angles
greater than 12 arcminutes.
References
[1] S. Ghizzardi. In­flight calibration of the on­axis and near off­axis PSF for the Mos­1 and Mos­2
cameras. EPIC­MCT­TN­011.
[2] Christian Erd, Phillipe Gondoin, David Lumb, Rudi Much, Uwe Lammers, Giuseppe Vacanti
and Richard Saxton. Calibration Access and Data Handbook. XMM­PS­GM­20, issue 2.2.
[3] D. Lumb. EPIC Spectral Response Distribution. XMM­CCF­REL­84.