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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.
