How is the background map produced?
The HRI background template is constructed using an internal detector background map (from ground-based data), a soft X-ray background map (for unresolved celestial objects), a bright earth background map (from in-flight data), and a flat-field charged particle background map (assumed to be flat). The soft X-ray and bright earth background maps reflect the vignetting of the mirror and also have ten percent fluctuations due to the non-uniform response of the HRI. The contribution of the internal detector map in a given sequence is determined by multiplying it by the live time of the observation. The contributions of the bright earth, soft x-ray, and charged particle maps are determined by fitting the maps to the data within the central 12' after all point sources have been removed.
As the relative levels of the background components have not yet been optimized using flight data, the background map may show a distinct peak (which comes from the region in which the quantum efficiency is highest). Several PCV observations are subject to this effect because the map normalization produced a bright-earth component with too large a weight compared to the flat-field particle background component. Changes will be made to the background map generation algorithm once sufficient sky data have been studied. In the meantime, users should be aware that background-subtracted data may show (spurious) fluctuations of approximately ten percent, on a scale of several arc minutes.
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