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In order to help guest observers with their feasibility estimates a limited comparison of the ROSAT XMA+PSPC combination to the performance of the Einstein Observatory IPC is presented. Only expected source count rates are discussed. Such a comparison is not trivial since the band passes of the two systems differ, with ROSAT being more sensitive at lower energies and Einstein being more sensitive at higher energies. Therefore, generally speaking, ROSAT will give significant improvements in sensitivity for soft X-ray sources.
The relative count rates between ROSAT and Einstein depend highly on the details of the assumed incident spectrum. To present specific examples we consider a thermal source characterised by a temperature T and column density as well as power law spectra characterised by a photon index and absorption column density .
In the case of power law spectra (cf., Figure 10.21 ) the ROSAT PSPC is about as sensitive as the Einstein IPC for very hard spectra, while for softer spectra with photon indices of two or higher and/or spectra with relatively little interstellar absorption column density (i.e., cm ) the PSPC will record count rates three times larger than the Einstein IPC. Similarly, Figure 10.22 shows the ratio of the PSPC and IPC count rates as a function of T and . At low temperature, where the emission is dominated by soft photons, the PSPC is about six times more sensitive than the IPC; this increase in sensitivity is due to both the increase in effective area and change of band pass. At high temperature the PSPC/IPC ratio depends sensitively on as can be seen from Figure 10.22 . For small values of most of the detected photons will be recorded in the carbon window and therefore the PSPC/IPC ratio will be approximately equal to the ratio in effective area of the two instruments. For large values of most of the recorded photons will be in the hard band and thus the PSPC/IPC ratio can drop below unity since for these photons the Einstein mirror has more effective area than the ROSAT XMA.