[Artist's Concept of AXAF]

CXO Project Science:
Simulation of cosmic x-ray background

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X-ray background in HRC-I

Plot shows the photon spectrum of the cosmic x-ray background reaching the HRC-I focal plane. For the HRC-I on the HRMA axis, plus symbol denotes results with all focus-structure shielding; dashed line, for none. For the ACIS-S on the HRMA axis (the more benign Next-in-Line position), square symbol denotes results with all focus-structure shielding; dotted line, for none. In either case, the only pronounced difference due to the focus-structure shielding is for energies less than about 50 keV.

[X-ray background in HRC-I]

Tables compare results, integrated over 3 bands, for full and for no tantalum on the focus structure. It shows both average flux, in terms of both photons and counts, using the most recent model for the HRC high-energy quantum efficiency.

HRC-I x-ray background, with HRC-I on HRMA axis
In its observing position, the change in the contribution of the cosmic x-ray background to the HRC-I background, due to removal of the focus-structure structure tantalum, is from 0.0083 ct cm^-2 s^-1 to 0.0103 ct cm^-2 s^-1 (23% increase).

E(keV) <50 50-90 >90 All Units
With Ta 0.019 0.200 0.464 0.683 ph cm^-2 s^-1
Without Ta 0.037 0.212 0.477 0.726 ph cm^-2 s^-2
Change 91.1 6.0 2.8 6.2 % photons
With Ta 0.0012 0.0059 0.0013 0.0083 ct cm^-2 s^-1
Without Ta 0.0027 0.0062 0.0014 0.0103 ct cm^-2 s^-2
Change 128.8 5.5 6.9 23.1 % counts
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HRC-I x-ray background, with ACIS-S on HRMA axis
In its Next-in-Line (NiL) position, the change in the contribution of the cosmic x-ray background to the HRC-I background, due to removal of the focus-structure structure tantalum, is from 0.0338 ct cm^-2 s^-1 to 0.0425 ct cm^-2 s^-1 (26% increase).

E(keV) <50 50-90 >90 All Units
With Ta 0.212 0.434 0.927 1.572 ph cm^-2 s^-1
Without Ta 0.296 0.463 0.953 1.712 ph cm^-2 s^-2
Change 39.8 6.6 2.8 8.8 % photons
With Ta 0.0181 0.0126 0.0031 0.0338 ct cm^-2 s^-1
Without Ta 0.0256 0.0137 0.0032 0.0425 ct cm^-2 s^-2
Change 41.5 8.1 3.6 25.6 % counts
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