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XMM­Newton CCF Release Note
XMM­CCF­REL­208
OM Grisms Calibration: correcting count rate in extracted
spectrum
A. Talavera
June 12, 2006
1 CCF components
Name of CCF VALDATE EVALDATE List of Blocks changed XSCS flag
OM GRISMCAL 0004 2000­01­01T00:00:00 --- Header keywords NO
--- FLUX GRISM1 NO
2 Changes
2.1 UV Grism Flux Calibration
SAS task omgrism extracts the spectrum from a grism rotated image. The measured count rate is
normalized by dividing it by the spectrum sampling step. The new wavelength scale introduced with
OM GRISMCAL 0003 for the UV grism implied a change in the wavelength step. The normalization
was fixed in the code prior to omgrism version 1.9, which shall be released with SAS 7.0. This means
that UV grism data processed with SAS 6.5 had a wrong normalization in their extracted spectrum.
This was compensated by the ISF because it was derived from extracted spectra having the wrong
normalization and therefore the final spectra in flux units are correct.
omgrism will produce from now on spectra with a proper normalization. Instead of re­deriving
a new ISF, it is easy to demonstrate that the previous one can be corrected simply by multiplying
it by the ratio between the current wavelength step and the one used before in the code. Such a
corrected ISF (for the UV grism ­ GRISM1) is implemented in the new OM GRISMCAL 0004.CCF.
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2.2 Keywords in the FITS Header
To avoid usage of this new CCF with previous versions of omgrism we have added the ALGOID
keyword in its FITS header.
The units of the ISF (FLUX GRISMn extensions) were defined wrongly in the header of the
CCF. They have been changed to erg/cm2/count.
3 Scientific Impact of this Update
As already mentioned, the extracted spectra in count rate units produced with SAS 6.5 are wrong.
The error varies from 13% at 1800 š A decreasing to 2.5% at 3600 š A. However the fluxed spectrum is
correct.
When using the new version of omgrism within SAS 7.0 the extracted count rates will be correct
and the fluxed spectra will also be correct due to the change made in the ISF.
4 Estimated Scientific Quality
The accuracy of the grism calibration both in wavelength and flux does not change with respect to
the previous issue of this CCF.
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.

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5 Expected Updates
Future updates will be issued as necessary from the monitoring of the calibration with time.
The current flux calibration does not include coincidence losses in the detector, nor time depen­
dent sensitivity variations.
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.
6 Test procedures
As it was done with previous versions of this CCF, several standard stars have been processed to
verify that both the new version of and the arithmetic correction applied to the UV ISF were
correct.
7 Summary of the test results
The SAS extracted and calibrated spectra are both correct now.
For more details please refer to XMM­CCF­REL­199.
References