next up previous contents
Next: Count rate predictions for Up: Flux conversions Previous: General considerations

Count rate predictions for simple spectra

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:

  1. power law (Figure 12.3 gif)
  2. optically thin plasma (Figure 12.4 gif; cosmic abundance spectra computed following Raymond and Smith, 1977, ApJS, 35, 419)
  3. black-body (Figure 12.5 gif)
  4. two different model atmospheres (mainly applicable to white dwarfs; Figures 12.6 gif and 12.7 gif) - these are discussed in § 12.3.3 gif below.

For all spectral models, and for each of the scientific filters as listed in Table 5.3 gif, 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 tex2html_wrap_inline2416 varying from tex2html_wrap_inline2430 to tex2html_wrap_inline2656  cm tex2html_wrap_inline1930 . The absorption incorporated uses the cross-sections given by Morrison and McCammon (1983, ApJ, 270, 119) for energies above 0.03 keV ( tex2html_wrap_inline3112 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 tex2html_wrap_inline2420  ergs cm tex2html_wrap_inline1930  s tex2html_wrap_inline1894 in the energy range 0.02 - 2 keV. (Note that this is substantially larger than the actual pass-band of the WFC which is tex2html_wrap_inline3120  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 tex2html_wrap_inline2506 (in the 0.02 - 2 keV band) in units of tex2html_wrap_inline2420  ergs cm tex2html_wrap_inline1930  s tex2html_wrap_inline1894 , i.e., tex2html_wrap_inline3130 .

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 tex2html_wrap_inline3134 where tex2html_wrap_inline2206 is the photoelectric cross-section and tex2html_wrap_inline2416 the column density, the absorbed and unabsorbed source fluxes tex2html_wrap_inline3140 and tex2html_wrap_inline2506 are defined as follows:


respectively, where E1 and E2 are the energy limits of the passband.

next up previous contents
Next: Count rate predictions for Up: Flux conversions Previous: General considerations

Michael Arida
Tue Jun 11 16:18:41 EDT 1996