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XMM-Newton CCF Release Note
XMM-CCF-REL-171
RGS Instrumental Flourine Absorption
A.M.T. Pollock
June 24, 2004
1 CCF components
Name of CCF VALDATE List of Blocks
changed
XSCS ag
RGS1 EXAFS 0004 1999-01-01T00:00:00 MgF2-P-CCD1 NO
MgF2-P-CCD2
MgF2-P-CCD3
MgF2-P-CCD4
MgF2-P-CCD5
MgF2-P-CCD6
MgF2-P-CCD7
MgF2-P-CCD8
MgF2-P-CCD9
RGS2 EXAFS 0004 1999-01-01T00:00:00 MgF2-P-CCD1 NO
MgF2-P-CCD2
MgF2-P-CCD3
MgF2-P-CCD4
MgF2-P-CCD5
MgF2-P-CCD6
MgF2-P-CCD7
MgF2-P-CCD8
MgF2-P-CCD9
2 Changes
When statistics are high enough, RGS spectra show a weak but denite instrumental absorbtion
feature near 17.9  A due to MgF 2
in the CCDs. Data have been incorporated in the instrument model
in order to prevent the feature from being mistaken as of cosmic origin. The RGS EXAFS CCFs
have been modied accordingly.
1

XMM-Newton CCF Release XMM-CCF-REL-171 Page: 2
In general, elemental X-ray absorptions are stored in the XMM ABSCOEF CCF that contains the
Henke scattering factors tabulated on a xed energy grid. In the CAL these data are accessed when
computing the transmission probabilities which enter the formula for the RGS quantum eфciencies as
described in the CAL Handbook. It later became clear that certain RGS edge absorption features
could not be modelled accurately enough using the Henke data so the EXAFS structures were
introduced to improve the Henke data where necessary. The EXAFS do not contain the scattering
factors but the mean absorption lengths in cm tabulated against energy in eV. For this MgF 2 layer,
the tabulation is the same for all CCDs and covers the range 652:8  E(eV)  729:2 in 206 steps of
0.4eV.
The SAS recognizes materials in the EXAFS table names that match the naming scheme
CompoundName-[P|A]-CCD[1-9]
where CompoundName is one of the following hardcoded composite material names :
char *absMaterial[] = { "MgF2", "Al", "Al2O3", "SiO2", "Si", "H2O", "Si" };
bool passive[] = { true, true, true, true, true, true, false };
These names appear as part of the case-senstive extension names so, for example, \MgF2-P-CCD1"
is needed to dene the EXAFS data for Fluorine in the passive MgF 2 layer on CCD1. \P" stands
for \Passive" as opposed to \A" for \Active". These details also need to be added to the CAL
Handbook.
3 Scientic Impact of this Update
Little or no risk of confusing an instrumental feature with absorption in the intergalactic medium,
for example.
4 Estimated Scientic Quality
Systematic ux uncertainties near the Flourine edge reduced to the few percent level that applies
through most of the RGS waveband.
5 Test procedures & results
The equivalent width of the instrumental Fluorine feature is quite low and thus not easy to see
in single observations of even strong sources. For example, Fig. 1 shows RGS 1st-order spectra of

XMM-Newton CCF Release XMM-CCF-REL-171 Page: 3
Mkn421 during one of its brighter episodes along with power-law models made with and without the
deeper feature in the new CCF. In RGS2, the feature is nearly masked by noise. Several detector
defects are also visible.
Greater detail can be seen in Fig. 2 which shows a similar comparison of rgsfluxer uxed spectra
calculated with old and new CCFs for all Mkn421 data except those obtained during the cool-
ing campaign in November 2002. The new CCFs successfully remove the instrumental absorption
feature.
6 Expected Updates
None expected.

XMM-Newton CCF Release XMM-CCF-REL-171 Page: 4
Figure 1: Details of models of RGS rst-order spectra of Mkn421 in 0259 0136540101. Identical
RGS1 and RGS2 data were analysed with response matrices with and without improved instrumental
absorption by MgF 2
provided by the new RGS EXAFS CCFs. Several features due to bad pixels
have not changed.

XMM-Newton CCF Release XMM-CCF-REL-171 Page: 5
Figure 2: Details of the RGS spectrum of Mkn421 with and without the improved description of
instrumental absorption by MgF 2
provided by the new RGS EXAFS CCFs. At the top in green
the old CCF with a clear feature at 17.9  A, due to an inadequate description of the F-edge. Several
features due to bad pixels have not changed. The red line shows the result with the new CCFs with
no feature. The lower plot shows the dierence between the two with error bars in yellow. The new
EXAFS tables range between 17 and 19  A, at the ends of which there is no jump to be seen. This
means that Mg absorption was also properly included in the CCFs.