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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:
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.