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This section provides a set of graphs which can be used to estimate the WFC count rate in each of the scientific filters as a function of spectral parameters and for a range of different spectral models. The models included are:
); cosmic
abundance spectra computed
following Raymond and Smith, 1977, ApJS, 35, 419)) and 12.7 ) -
these are discussed in § 12.3.3 below.
For all spectral models, and for each of the scientific filters as listed in
Table 5.3 , the graphs show the expected WFC
count rate as a function of temperature (or energy index in the case of power
law) for a set of column densities 
varying from 
to
cm 
. The absorption incorporated uses the cross-sections given
by Morrison and McCammon (1983, ApJ, 270, 119) for
energies above 0.03 keV ( 
Å), extrapolated
to lower energies using the cross-sections given by
Cruddace et al. (1974, ApJ, 187, 497).
The plots are normalized to give the
count rates for a nominal unabsorbed flux of
ergs cm s 
in the energy range 0.02 - 2 keV.
(Note that this is substantially larger
than the actual pass-band of the WFC which is 
keV,
depending on filter choice. We use this band to facilitate comparison with
XRT predictions, or with EXOSAT CMA results.) Thus the estimated count
rate for the models is calculated by multiplying the plotted conversion
factor (CF) by the estimated unabsorbed source flux 
(in the
0.02 - 2 keV band) in units of ergs cm s
, i.e., 
.
The unabsorbed flux in this context means the
flux in the passband assuming no interstellar absorption, i.e., if the source
has an assumed spectrum S(E) and interstellar absorption is modelled as
where 
is the photoelectric cross-section
and the column density, the absorbed and unabsorbed source fluxes
and are defined as follows:
respectively, where E1 and E2 are the energy limits of the passband.