This is true for most sources. However, the optimal channel selections should be checked for each observation. An example is given in Figure 4 . Here we show three plots. The next step is to produce simulated data files.
Required inputs are parameters of the spectral model and the response matrices.
It is then possible to compare the distribution of simulated hardness ratios with that of the real data. Figure 5 shows the results of 4 simulations for 61 Cyg. The models used in the simulations were Raymond-Smith plasmas with temperatures of 1.0, 0.3, 0.4, and 0.5keV. One hundred fake data files were produced for each model. The hardness ratios of an 0.4 keV plasma group around 0.85, as might be expected from the fit to the HRI spectra and the PSPC results. However, the hardness ratio is also consistent with an 0.5 and 1keV plasma.