Background

The SIXA detectors will encounter cosmic background from the diffuse x-ray background and scattered solar x-rays, as well as non-X-ray background from charged particles, cosmic-ray-induced secondary radiation from the spacecraft structure, and induced radioactivity. The main contributors to the non-X-ray background are expected to be induced radioactivity and bremsstrahlung from charged particles. The total non-X-ray background is estimated to be $\sim 10^{-3}$ counts cm^-2 s^-1 keV^-1, or $\sim1.3\times10^{-2}$ counts s^-1 pixel^-1, from 0.5 - 20 keV.

The diffuse x-ray background in SIXA may be estimated using the effective area shown in Figure 58 and a model xrb spectrum. We use the ROSAT spectrum of Wang and McCray (1993) below 3 keV, and the HEAO-1 spectrum of Boldt (1987) above 3 keV, and find a value of $\sim5$ c s^-1 deg^-2 or $\sim1.7\times10^{-2}$ c s^-1 pixel^-1, on-axis. Readers should be aware, however, that the soft component of the xrb is dominated by galactic contributions and can vary with viewing direction by as much as a factor of 5. The total xrb in SIXA may therefore vary by a factor of 2-3.