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XMM­Newton CCF Release Note
XMM­CCF­REL­199
OM Grisms Calibration: Update
A. Talavera, C. H. James
October 19, 2005
1 CCF components
Name of CCF VALDATE EVALDATE List of Blocks changed XSCS flag
OM GRISMCAL 0003 2000­01­01T00:00:00 --- FLUX GRISM1 NO
--- FLUX GRISM2 NO
--- WAVELENGTH GRISM1 NO
2 Changes
The first issue of the OM grisms calibration was based on spectra processed with IDL routines. In
the release of this calibration we announced that a new issue based on data fully processed with
SAS would be the next step.
Here it is. The main change in this version is that the calibration of the grisms is now derived
from spectra extracted by SAS. The absolute flux calibration for both grisms has been updated.
Also the wavelength scale for the UV grism.
2.1 Wavelength calibration
Due to a change in the algorithm that computes the reference point for the wavelength scale in SAS
processing, a shift is produced in the wavelength scale. This a#ects mainly the UV grism spectra
because its zero order (the reference point) can be very bright and a#ected by several e#ects.
We have re extracted the spectra of the F­type stars used in the previous calibration, and we
have defined a new dispersion relation as
1

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L = 991.778 + 1.8656X + 0.0007713X 2
for GRISM1 (UV), where L is the wavelength in š A and X the distance in pixels to the centroid
of the zero order in the undistorted, rotated grism image.
The dispersion relation for GRISM2 (Visible) has not been modified.
2.2 Flux calibration
Selected spectra of the spectrophotometric standard stars GD153 and HZ2 have been reprocessed
with SAS 6.5, using the newly derived wavelength scale for the UV grism and the same than before
for the Visible one. With these new spectral extractions we have defined new Inverse Sensitivity
Functions (ISF) for each grism.
The ISF is defined as:
CalibratedF lux = Observed spectrum count rateâISF
therefore
ISF = Standard f lux/OM spectrum count rate
Both ISFs have been obtained by averaging the ISFs computed individually for the standard
stars GD153 and Hz2.
The flux calibration is defined in the ranges 1800 ­ 3600 š A, and in 3000 ­ 6000 š A, for GRISM1 and
GRISM2 respectively, in 50 š A steps. The standard deviation of the average of individual functions
obtained from each star is provided as the error of the ISF.
3 Scientific Impact of this Update
These calibrations allow SAS, and individual users to convert the count rates measured in images
obtained with the OM grisms into a flux distribution as a function of wavelength. Since these new
calibrations are fully obtained through SAS, we have now a fully coherent system.

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4 Estimated Scientific Quality
4.1 Wavelength calibration
The internal accuracy of the dispersion relations is 2 š A in GRISM1 and 10 š A in GRISM2.
When the dispersion relation is applied to any extracted spectrum, the main source of error is
the determination of the reference point (the zero order centroid). This can introduce shifts in the
wavelength scale of up to 10 š A in GRISM1 and up to 20 š A in GRISM2.
4.2 Flux calibration
A first level verification of the flux calibration is obtained by applying the ISF to the spectra of
the standard stars used to derive it. This provides a consistency test. The fluxes obtained for the
standard stars GD153 and HZ2 agree within better than 5% for GRISM1 and GRISM2.
5 Expected Updates
Now that the wavelength and flux calibrations of the OM grisms have been coherently derived from
spectra obtained throughout SAS, we have accomplished a major update.
Future updates will be issued as necessary from the monitoring of the calibration with time. We
have not detected up to now a loss of sensitivity that would lead us to a time dependent correction.
The flux calibration we present here (as it was the case with the previous one) does not include
coincidence losses in the detector, an e#ect very important, well understood and corrected in OM
aperture photometry performed with SAS. We assume that spectra obtained with the OM grisms
have in general low count rates and therefore are not a#ected by coincidences.
The spectra used in the derivation of the calibration were extracted from sources located at
the standard boresight used in XMM observations. Therefore, possible variations across the detec­
tor (particularly in wavelength due to grisms distortions), when we work on field or multi­object
spectroscopy, are not included.
A separate document will provide a detailed explanation of all steps followed in the derivation
of the calibrations as well as extensive testing.

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6 Test procedures
In order to test the validity of the calibration, not only the standard stars used to derive it, GD153
and HZ2, but several more, as HZ43, GD50, BD+33 2642, G93­48, LTT9491 and BPM16274, have
been reprocessed with the last SAS version and this calibration.
The SAS extracted and calibrated spectra have been compared with standard fluxes or values
in the literature for these stars.
7 Summary of the test results
Fluxes obtained with SAS and this calibration agree in general within 10% with the ones given in
the literature. For standard stars we have agreements in the range of 5% in the UV grism and 10%
in the Visible grism. In very noisy spectra, or in the shortest wavelength of the UV and the longest
one in the Visible, discrepancies can reach 20% level.
Figures 1 and 2 show respectively the comparison of catalogue fluxes and OM extracted data
calibrated with the ISF's in this CCF for the standard stars GD153 and HZ2. All spectra shown
have been extracted automatically with omgchain using default parameters. Note that large emis­
sion/absorption features are due to field sources whose zero order contaminates the spectrum or the
background extraction region.
References

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Figure 1: Comparison between catalogue and XMM­Newton OM Grism spectra of the standard
white dwarf GD153

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Figure 2: Comparison between catalogue and XMM­Newton OM Grism spectra of the standard
white dwarf HZ2