Swift XRT calibration note-energy scale offsets

Observational evidence has been obtained which suggests the CCD bias level, which is mode dependent and subtracted onboard during XRT observations, can vary considerably during a typical orbital snapshot on an astrophysical target. The variation seems to be at least partly related to changes in the CCD temperature and/or scattered optical light from the sunlit Earth, and it can affect both of the main science modes: windowed timing (WT) and photon counting (PC). When the bias changes during an observation, it can cause energy scale offsets which could give rise to spectral features (such as an edge at ∼ 0.5 keV) in the observed spectra (for example, figure 1).