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The PSPC has essentially no internally produced background. The charged particle induced background depends sensitively on the local particle environment along the orbit as well as the rejection efficiency of the PSPC veto electronics. The background is less than counts s arcmin for 95 percent of the observing time with peaks of up to counts s arcmin . The spectrum of the charged particle background in the detector is relatively flat with a small low pulse-height tail. Guest investigators should realise that the charged particle background will vary by factors of three or more during one orbit.
As far as diffuse celestial X-ray radiation is concerned, count rates vary between and counts s arcmin depending on the exact pointing position in the sky. The median value is anticipated to be counts s arcmin . Most of this flux appears in the C-band, i.e., at energies below 0.28 keV.
Scattered solar X-ray background is produced by atoms and molecules in the earth's atmosphere which scatter solar X-rays. Although the scattering cross sections are quite small, substantial amounts of flux may be scattered into the X-ray telescope under unfavourable viewing conditions. Clearly, this background is absent during night-time viewing; during day-time viewing the scattered flux depends, to zeroth order, on the Sun-lit column density of atmosphere. Since the atmospheric density decreases exponentially with altitude large changes in Sun-lit column density may occur quite rapidly (i.e., over time scales of a few minutes). Thus the expected background varies from zero (on the night side) to more than counts s arcmin for viewing directions having a zenith angle of on the day side. The spectrum of the scattered solar X-rays is divided equally between the C band and the O K line at 0.525 keV. However, considerable hardening will occur during solar flares.