as you noticed the norm factor in the xspec model acisabs does not
affect the estimate of the transmission of the contaminant.
This factor is frozen by default in the xspec model acisabs
(as recommended in the web site) :
However, the idl tool acisabs.pro (equivalent approach now
available from the same web site)
can be used to plot the observed decay with time of the ratio
of observed X-ray transmissions at 0.67keV and 5.895keV.
This decay does depend on the the assumed value for the norm parameter ***.
Finally, a cautionary note:
Several of the functions called by
the xpsec model acisabs are part of a library of functions included
in the recent XANADU release (XSPEC version 11.2).
Please make sure you are running the latest version of XSPEC 11.2.
*** This norm factor is essentially the ratio of (S1/S2)*[T_OBF(E1)/T_OBF(E2)]
where S1 and S2 are the relative stengths of the calibration line complexes ( with no filters
present) at 0.67keV and 5.895keV .
T_OBF(E1) and T_OBF(E2) are the transmissions of the OBFs
at energies 0.67keV and 5.895keV, respectively.
Since for ACIS-S the ratio T_OBF(E1)/ T_OBF(E2) ~ 0.71 (uncontaminated filters)
the implied ratio of the calibration line strengths is S1/S2 = norm/0.71 ~ 0.01
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