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General Assumptions

 

In order to estimate the HRI response to a cosmic X-ray source, several performance characteristics of the combined HRI and ROSAT telescope are needed. In the sections that follow we provide estimates of minimum source count rate detection and expected count rates from sources. The assumptions that were used in providing these estimates are:

• The HRI/Telescope effective area is that shown in Figure 11.1  and listed in Table 11.1 . The effective area is the product of the XMA area, the transmission of the Filter and Shield of the HRI, and the CsI coated MCP quantum efficiency.
• The HRI point source detection cell size is 5'' on a side. This is based on the combination of the XMA point spread function, the HRI detector resolution, and the estimated aspect uncertainty. The aspect uncertainty is the largest contributor to the choice of detection cell size and it remains an estimate at this time.
• 50 percent of the source photons fall within the detection cell. We have used the half power point for the mirror of 3'' diameter and the aspect smearing to make this estimate. It is likely to be conservative.
• The HRI internal background rate is about 1.5 counts s . This background is mainly due to radioactive isotopes in the MCP glass.
• The HRI external background from trapped particles,cosmic rays, and solar x-rays varies with the orbital position and viewing direction. As discussed in § 7.8.3  the typical ROSAT rate is about 3.0 counts s , but may be as much as 3 to 4 times greater for short intervals of an observation.
• The X-ray background is computed from the spectrum shown in Figure 11.2 , which is a compilation of Wisconsin and LLL measurements (ApJ 193, L133 and 172, L67) of the diffuse galactic background, and an extrapolation of a power law fit to the extragalactic background (e.g., Schwartz 1978 IAU/COSPAR Proceedings). In estimating the HRI count rate we integrate this spectrum over the total effective area of the XMA + HRI. We note that the Filter and Shield greatly reduce the HRI sensitivity at low energies and that there is a major dip in effective area at the carbon edge. It should also be noted that the galactic component of the x-ray background varies with the viewing direction. We have taken a conservative estimate of the galactic background in the sense that we use the high emission estimates in calculating the HRI count rates. In some cases the HRI background count rates due to galactic emission may be up to a factor of 5 lower than the value of 1.0 counts s used here.

 

Energy Lambda Effective Energy Lambda Effective

(keV) (Å) (keV) (Å)

0.10 124.0 1.0 1.10 11.3 94.5

0.20 62.0 20.2 1.20 10.3 89.7

0.28 44.3 43.5 1.30 9.5 84.7

0.28 44.1 0.0 1.40 8.9 76.1

0.30 41.3 1.0 1.50 8.3 56.5

0.40 31.0 10.6 1.60 7.8 50.0

0.50 24.8 19.3 1.70 7.3 37.2

0.60 20.7 7.6 1.80 6.9 28.0

0.63 19.7 11.9 1.90 6.5 19.4

0.70 17.7 35.5 2.00 6.2 11.2

0.80 15.5 59.9 2.10 5.9 2.7

0.90 13.8 82.6 2.20 5.6 0.6

1.00 12.4 83.2 2.30 5.4 0.4

3.40 5.2 0.2

None of these assumptions is absolutely correct, but they provide a typical description of the XMA+HRI performance which may be useful in preparing ROSAT observation proposals.