As emphasised above, the
source coordinates are most crucially under the observer's control and
have a profound influence on the accuracy of the wavelength scale as
recorded in the RGS response matrix, or RMF, that is produced automatically
by rgsproc using the task rgsrmfgen although not for pipeline PPS products.
There are different RMFs for RGS1 and RGS2 and for each order.
Many parts of the instrument model held
in the CCF are also used to describe the geometry and
radiation transfer properties of each component of the instrument from
telescope, through grating assembly to detector and these are not
normally subject to change. An RMF is used to connect an input wavelength or energy
grid representing cosmic physical units
to the output instrumental BETA_CORR channels in which the
observed selected data have been cast. While the spacing of the output grid is
fixed by a requirement to sample sufficiently the instrumental resolution
and must agree with the relevant accumulated spectrum,
the input grid is another quantity under the user's express control via
the rgsrmfgen parameters emin, emax and ebins.
Although wavelength units are natural for grating spectrometers,
the OGIP RMF standard requires energy units, given here in keV.
Here is
an example of an independent run of rgsrmfgen from the command line to
generate the 2nd order RGS2 matrix for Mkn421 :
rgsrmfgen spectrumset=P0136540101R2S002SRSPEC2003.FIT \
evlist=P0136540101R2S002EVENLI0000.FIT \
emin=0.4 emax=2.5 rows=5000 rmfset=RGS2.o2.rmf
Despite the elongated appearance of spectral lines in plots like Fig. 36, the point-source monochromatic response is in fact reasonably symmetric as shown in Fig. 38. Although a typical line covers several pixels, it is possible to locate the line centroid to a fraction of a pixel. In this case, it is necessary to generate a response matrix covering the line and some surrounding continuum with a much more dense grid than usual in order to take full advantage of all the information available in the data and calculate reliable errors.
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