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The expected PSPC count rates depend sensitively on the assumed incident spectrum which is unknown for most sources. In general, the convolution of effective area and incident spectrum is non-trivial because of substantial changes in effective area as a function of energy. In order to facilitate count rate calculations, the count rate curves are provided for power law spectra, thermal line spectra and blackbody spectra as a function of interstellar absorption column density .
In Figures 10.2 - 10.7, the energy-to-counts conversion factor (ECF) is shown, i.e., the factor by which the unabsorbed source flux (in units of ergs cm s in the 0.1 - 2.4 keV band) has to be multiplied in order to obtain the expected PSPC on-axis count rate. Figure 10.2 shows the ECF for power law spectra with photon index for various N values between cm and cm . Whereas for small values of N a distinct variation of ECF vs. occurs, the ECF is basically constant (with respect to ) at values around counts cm erg for larger values of typically found for AGNs. Figure 10.3 shows the equivalent curve for thermal line spectra. For such spectra the ECF does depend sensitively on the assumed temperature (in addition to ) with the largest ECF values found at temperatures K. Lastly, Figure 10.4 shows the ECF for blackbody spectra; here the maximum ECF is found at .
Figures 10.5 - 10.7 show the ECF for the PSPC used with the boron filter for power law, thermal line and blackbody spectra for various values of . Although the general shapes of the curves remain unchanged, the absolute values change significantly due to the reduced transmission of the boron filter.
Figures 10.8 - 10.13 show the ECF for the PSPC if only the hard pulse height channels are used for power law, thermal line and blackbody spectra for various values of . Because the PSPC background is dominated by soft events (cf., Chapter 7 ), sources with hard spectrum can be more easily detected in the higher energy band; the curves provided in Figures 10.8 - 10.13 allow to assess (in conjunction with Figure 12.1 ) whether a restriction to the higher energies increases sensitivity or not.
The curves shown in Figures 10.2 - 10.13 should be used as follows:
The expected count rate from a coronal source with ergs s at a distance of 50 pc and cm is to be estimated. The temperature of the source is assumed to be K. With these parameters an unabsorbed flux of ergs cm s is calculated. From Figure 10.3 a ECF of 1.25 is estimated, hence a count rate of 4.25 counts s is expected. Similarly, using the boron filter, a ECF of 0.6 is found from Figure 10.6 , and consequently the count rate is expected to be 2.0 counts s . Note that no corrections for dead time, vignetting or scattering out of the detect cell have been applied. Therefore the curves in Figures 10.2 - 10.7 apply only to on-axis observations of point sources with sufficiently large detect cell sizes.
Example 2: A power law source with photon index 1.7 and an intensity (at 1 keV) of 0.01 keV cm s keV is to be observed through an absorbing column density of cm . With this spectrum one obtains an unabsorbed flux of ergs cm s in the pass band 0.1-2.4 keV. From Figure 10.2 , one finds a ECF of 0.55, hence a count rate of 2.4 counts s is expected. Similarly, for the boron filter one calculates an expected count rate of 1.2 counts s with a ECF of 0.28 taken from Figure 10.5 .