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Background PSPC


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 tex2html_wrap_inline1948  counts s tex2html_wrap_inline1894  arcmin tex2html_wrap_inline1930 for 95 percent of the observing time with peaks of up to tex2html_wrap_inline2260  counts s tex2html_wrap_inline1894  arcmin tex2html_wrap_inline1930 . 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 tex2html_wrap_inline2266 and tex2html_wrap_inline2268 counts s tex2html_wrap_inline1894  arcmin tex2html_wrap_inline1930 depending on the exact pointing position in the sky. The median value is anticipated to be tex2html_wrap_inline2274  counts s tex2html_wrap_inline1894  arcmin tex2html_wrap_inline1930 . 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 tex2html_wrap_inline2280  counts s tex2html_wrap_inline1894  arcmin tex2html_wrap_inline1930 for viewing directions having a zenith angle of tex2html_wrap_inline2194 on the day side. The spectrum of the scattered solar X-rays is divided equally between the C band and the O K tex2html_wrap_inline2288 line at 0.525 keV. However, considerable hardening will occur during solar flares.

Michael Arida
Tue Jun 11 16:18:41 EDT 1996