Quiescent Particle Background

Figure 3: Image showing the QPB pattern in the 7.9-8.2 keV band. The lower panel shows the FWC spectrum in the central hole (red), and outside the central hole (black).

The shape of the QPB spectrum in the continuum is relatively uniform over a given chip; the spectral shape and the normalizations vary from chip to chip. (Stand by for the publication demonstrating that!) The strength of the lines does vary across chips, and different species vary in different ways (see Kuntz & Snowden 2008). Both the continuum and the lines of the QPB spectrum for the pn vary across the detector, sometimes quite dramatically (see Figure 3 and Lumb et al. 2002).

For spectroscopic investigations, the background is summed over different regions, so the variation is not a major issue for regions that cover a large fraction of the FOV. Imaging studies are often in relatively narrow energy bands. Thus the number of counts available from which to determine the background may be limited, and if the band contains a component that varies strongly with location, artifacts may occur. Imaging with the pn around 8.0 keV may be particularly problematic.