What are the systematic errors associated with the background modeling of pcabackest?
The PCA team published an accounting of systematic errors in the background model in Jahoda et al (2005) ApJ S, 163, 401. A more recent analysis by the PCA team from 2006 is also available. Generally speaking, the systematic uncertainties are less than 1-2% of the background level in the 2-20 keV band, although this figure depends on the particular PCUs and energy bands being considered. More detailed information can be found in the above two papers. My spectrum shows an unexpected feature centered around 4.5-5 keV; could this be a feature in the response matrix? Yes, there is a known feature there due to the Xenon L edge at 4.78 keV. If you plot the ratio of your residual to the model and see a wiggle of ~3-4 %, (looking in fact like like a ~ shape) then this is probably due to this edge. Some people choose to fit a gaussian absorption feature here to get a better fit for their model, or to simply ignore those channels. Others use grppha to add a systematic error of ~ 2% to each chann
