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## Are there any differences between "standard" and SAS-generated
EPIC response matrices?

The difference between the "standard matrices", available in the
XMM-Newton calibration pages,
and those generated via the SAS tasks
arfgen
and rmfgen is now virtually negligible, at least as long as one
is concerned with spectral fitting of point-like, on-axis
sources. The user is referred to the SAS v5.3
Technical Note and the results of the
SAS
Science Validation.
There is, however, a case where the usage of SAS-generated response matrices
is largely to be preferred. If one has a spectrum with very good statistics
in an emission line with centroid energy larger than 2.5 keV (*e.g.*:
the Iron K-alpha line), one should be careful about using the "standard"
matrices. These are binned at en energy resolution of 50 eV. XSPEC does
not perform
any interpolations in the energy space, and therefore the line energy
can be obtained with an accuracy ~25 eV. A more accurate determination
of the line energy can be obtained by running
arfgen
and rmfgen to create more finely spaced matrices:

rmfgen spectrumset=${myspectrum} rmfset=${myrmf} withenergybins=yes
nenergybins=7475 energymin=0.05 energymax=15.0
arfgen spectrumset=${myspectrum} arfset=${myarf} withrmfset=yes
rmfset=${myrmf}

The redistribution matrix,
binned at 2eV, takes about 25 MB of disk space. On the other hand,
the ARF is
small.

Pages maintained by SAS librarian.

Any question about **SAS** should be addressed to the
XMM-Newton help desk.

Updated on:
November 18, 2006