Our methodology for producing model QPB spectra (Kuntz & Snowden 2008) and other background components is based as much as possible on “first principles”. We attempt to model as many of the aspects as possible using a wide range of inputs, e.g., FWC data, data from the unexposed corners of archived observations, and ROSAT All-Sky Survey data.
We avoid the use of blank-sky data for a number of reasons. First, the blank-sky data include unknown levels of residual SP contamination and SWCX contamination. Second, the cosmic X-ray background varies strongly across the sky, so what might be a good background in one region may be quite inappropriate in another. A significant fraction of the authors' scientific interest lies in the study of the diffuse X-ray background so we need a method that will not throw out our “signal”. Other methods, such at that of Arnaud et al. (2001), Read & Ponman (2003), and Nevalainen, Markevitch, & Lumb (2005), have relied more on blank-field data for their subtractions. A comparison between the methods used in the study of clusters of galaxies using Nevalainen et al. (2005) results is shown in Figure 2. As expected, the results of the two methods are in reasonably good agreement with the size of the uncertainties at large annuli being smaller using the XMM-ESAS method. At smaller annuli the background is small compared to the signal so any differences in the fit results are expected to be minor. Also included in Figure 2 are the results from the Chandra data analyzed by Vikhlinin et al. (2005). The discrepancy between XMM-Newton and Chandra was determined to arise from the Chandra calibration, which has been subsequently corrected, although inconsistencies still exist.