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The PSPC offers spectral resolution through event pulse heights as well as through the use of the boron filter. Because of the good counter resolution, events occurring in the carbon band can be separated from those at higher energies without significant pulse height smearing. Approximately four different colors can be determined over the whole PSPC band pass. An example of measured PSPC pulse height spectra is given in Figure 10.14 ; the upper panel contains the extremely soft spectrum of a presumed LMXB in the LMC which is detected only in carbon band; the medium panel shows the pulse height spectrum of an active nearby star with emission both in the C and M bands; while the lower panel shows the pulse spectrum of a cut-off source (LMX-1).
The boron filter allows a further subdivision of the carbon band by comparison of the count rate with and without filter. This filter transmits most of the photons at higher energies while absorbing most photons between 0.18 and 0.28 keV. Therefore the filter/open count rate ratio is sensitive to the spectral intensity between 0.1 and 0.18 keV. For various values of we show the filter/open filter ratio versus photon index for power law spectra (Figure 10.15 ), the filter/open ratio versus temperature for thermal spectra (Figure 10.16 ), and blackbody spectra (Figure 10.17 ). In the case of power law spectra the boron filter will be useful for column densities below cm . For given the ratio of filter/open count rate is sensitive to the photon index. For thermal spectra the filter/open ratio is very sensitive to temperature in the temperature range between K and K while for very high temperatures the filter/open ratio is sensitive to . For blackbody spectra the filter/open ratio is rather sensitive to for column densities above cm while for lower column densities the temperature dependence in the range between 0.1 and 0.3 keV is very pronounced.