A component can be specified as a background model by appending the string b / to the component name. A background model is convolved with the instrumental redistribution matrix but not with the effective area. This provides a means for including a description of detector-instrumental and particle backgrounds. If a response (RMF) matrix but not an auxiliary (ARF) file has been input, then the effective-area curve is calculated by assuming that the rows of the RMF sum to one. This will lead to errors at the top and bottom of the energy range of the detector.