James Theiler, Jeff Bloch
Los Alamos National Laboratory
The ALEXIS (Array of Low-Energy X-ray Imaging Sensors) satellite scans nearly half of the sky every fifty seconds, and downlinks time-tagged photon data twice a day. The standard science quicklook processing produces over a dozen sky maps at each downlink, and these maps are automatically searched for potential transient point sources. In order to assess the statistical significance of a potential point source, one has to have some idea of the background level at that point. In some cases the background can be estimated a priori, from a good understanding of the various noise sources in the detector and on the sky. But in practice this is rarely reliable, and it is more common to estimate the background by looking at count rates in the vicinity of the candidate point source. This implicitly assumes that the background is uniform, at least over the spatial area that includes the point source location and the background estimation region. For the ALEXIS data, due to effects ranging from the uneven exposure of the scanning telescopes' coverage to the spatial structure inherent in the diffuse X-ray sky, the background exhibits striking nonuniformity A method is presented for estimating the background at a given location on a sky map by interpolating the estimated background from a set of concentric annuli which surround this location. If the background is nonuniform but smoothly varying, this can provide a more accurate estimate than can be obtained by using a single annulus. This work is supported by the United States Department of Energy.