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XMM­Newton CCF Release Note
XMM­CCF­REL­141
EPIC MOS pattern fractions
R. D. Saxton
19 December 2002
1 CCF components
Name of CCF VALDATE Blocks changed XSCS flag
EMOS1 QUANTUMEF 0014.CCF 2000­01­01 FRACTION CHANNEL NO
EMOS2 QUANTUMEF 0014.CCF 2000­01­01 FRACTION CHANNEL NO
2 Changes
In earlier releases the pattern fractions in channel space have been modelled as a fifth order
polynomial. This is adequate for RMF generation but does not model sharp discontinuities
in the curve, such as the Silicon edge, with sufficient accuracy for use in epatplot.
Here, the channel pattern fractions have been calculated by convolving the pattern frac­
tions in energy space with the redistribution function. The energy space pattern frac­
tions were measured during ground calibration runs at Orsay. To take into account pixel
thresholding a tapering function has been applied to the fractions below channel 26. The
calculation has been performed for MOS­1 and applied to MOS­2. It is not expected
that these fractions will vary significantly between the cameras. The calculated values
are relevant for all the MOS imaging modes but are not appropriate for Timing mode.
3 Scientific Impact of this Update
With this accurate modelling of the channel fractions, the task epatplot may be used to
investigate pile­up issues with a MOS observation.
The pattern fractions do affect the MOS RMF, calculated by rmfgen, particularly at low­
energies. However, the previous pattern fractions were rather similar at low­energies and
so there should be no significant effect on spectral fits.
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4 Estimated Scientific Quality
The point of this release is to provide a standard curve which can be compared with real
data to check whether an observation is affected by pile­up. A full frame, MOS­1, ob­
servation of the SNR G21.5­09, 0060/0122700101, was compared with the standard curve
using epatplot (Fig. 1.) This shows a good agreement indicating that this observation has
no pile­up problem.
Figure 2 shows the epatplot output for a small window mode, MOS­1, observation of
3C273, 0277/0136550101. This shows events extracted from a circle of radius 40 arcsec­
onds about the source and clearly diverges from the optimum curve above 1.5 keV. This
divergance is resolved by excising the central 15 arcseconds of the core of the source (Fig.
3).
Data from the same observation of 3C273 by MOS­2, extracted from an annulus of 10--60
arcseconds also matches the standard curve very well (Fig. 4).
5 Expected Updates
The pattern fractions in channel space for Timing mode have been set to be the same as
for full frame mode. In reality they differ markedly due to the Pattern 0, Timing mode
events actually being composed of Pattern 0,1 and 3 events. The curves will need to be
calculated and included in a future release of these CCF files.
6 Test procedures and results
The analyses of section 4 show that the pattern fractions are sufficiently accurate for
performing pile­up checks. The effect on the RMF should also be checked at a low
and high photon energy to ensure that no serious modification to the RMF has been
introduced.
The Previous and new RMFs are compared at 0.3 and 8.0 keV in Figs 5 and 6. In the
low energy curve it can be seen that the blue wing of the line is depressed in the new
version. At the higher energy there is little difference between the two versions. These
small differences will not significantly influence spectral fits.
References
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Figure 1: An epatplot comparison of the pattern fractions from a full frame observation of G21.5­09.
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Figure 2: A comparison of the pattern fractions of events extracted from a circle about a MOS­1, small
window mode observation of 3C273. Pile­up is clearly seen by the divergance of the data from the
standard curve.
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Figure 3: The pattern fractions of events extracted from a 15--60 arcsecond radius about a MOS1, small
window observation of 3C273.
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Figure 4: The pattern fractions of events extracted from a 10--60 arcsecond about a MOS­2, small window
observation of 3C273.
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Figure 5: A comparison of the redistribution function for MOS­1, at 0.3 kEV with the new (red) and old
(black) pattern fractions.
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Figure 6: A comparison of the redistribution function for MOS­1, at 8.0 kEV with the new (red) and old
(black) pattern fractions.
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