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On-orbit measurements have been made of the HRI background, which is decomposed into several components:
The internal background is believed to mainly come from residual radioactivity in the MCP glass and the surrounding detector structure. This was measured on the ground and in orbit, and is counts s over the entire device. This appears to be a stable component of the background and is uniform over the detector.
The externally induced background from trapped particles, cosmic rays, and scattered solar radiation was about 0.5 counts s minimum on the Einstein HRI. This was occasionally two to three times greater for a small number of HRI observations. For ROSAT the trapped particle fluxes are higher than for Einstein due to the higher inclination of the orbit and the higher altitude, although there are parts of the ROSAT orbit where the particle flux is as low as that for Einstein. Based on the on-orbit calibration data that has been analyzed thus far, the external background rate for the ROSAT HRI will vary from a minimum of 2 counts s to about 10 counts s at which point the HRI high voltage is reduced to protect the detector from overload.
The X-ray background (XRB) contribution consists of a galactic component and an extragalactic component. Since the overall XRB flux in the ROSAT energy band is not well known, it is not possible to accurately calculate the contribution of these sources to the detector counting rate. An estimate for this contribution can be made based on an assumed XRB model; a detailed discussion of this is given in § 11.1 . An estimate from the on-orbit calibration data corrected for the internal and particle induced rates is counts s over the detector.
Thus the total ROSAT HRI background rate is estimated to be at least counts s over the entire field of view. Table 7.2 summarizes the background sources, where the expected values are listed as well and the ranges of these values. The table also gives the total background in various units.
|Internal||1.5 (1.0 - 2.0)||counts s|
|External||3.0 (2.0 - 10)||counts s|
|XRB||1.0 (0.5 - 2.0)||counts s|
|Total||5.5 (3.5 - 14)||counts s|
|counts s cm|
|counts s arcmin|
|counts s arcsec|