next up previous contents
Next: FEASIBILITY: XMA + HRI Up: FEASIBILITY: XMA+PSPC Previous: Time variability studies

Comparison to the Einstein IPC.


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 tex2html_wrap_inline2416 as well as power law spectra characterised by a photon index tex2html_wrap_inline2288 and absorption column density tex2html_wrap_inline2416 .

In the case of power law spectra (cf., Figure 10.21 gif) 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., tex2html_wrap_inline2724  cm tex2html_wrap_inline1930 ) the PSPC will record count rates three times larger than the Einstein IPC. Similarly, Figure 10.22 gif shows the ratio of the PSPC and IPC count rates as a function of T and tex2html_wrap_inline2416 . 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 tex2html_wrap_inline2416 as can be seen from Figure 10.22 gif. For small values of tex2html_wrap_inline2416 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 tex2html_wrap_inline2416 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.

 fig10-21 figure1147

 fig10-22 figure1155

next up previous contents
Next: FEASIBILITY: XMA + HRI Up: FEASIBILITY: XMA+PSPC Previous: Time variability studies

Michael Arida
Tue Jun 11 16:18:41 EDT 1996