  Motivated by a paper (Kirsch et al. 2005) on possible use of the Crab Nebula
as a standard candle for calibrating X-ray response functions, we examine
consequences of intrinsic departures from a single (absorbed) power law upon
such calibrations. We limit our analyses to three more modern X-ray
instruments-the ROSAT/PSPC, the RXTE/PCA, and the XMM-Newton/EPIC-pn (burst
mode). The results indicate a need to refine two of the three response
functions studied. We are also able to distinguish between two current
theoretical models for the system spectrum.
