  (Abbreviated) We show that the XRT spectral response calibration was
complicated by various energy offsets in photon counting (PC) and windowed
timing (WT) modes related to the way the CCD is operated in orbit (variation in
temperature during observations, contamination by optical light from the sunlit
Earth and increase in charge transfer inefficiency). We describe how these
effects can be corrected for in the ground processing software. We show that
the low-energy response, the redistribution in spectra of absorbed sources, and
the modelling of the line profile have been significantly improved since launch
by introducing empirical corrections in our code when it was not possible to
use a physical description. We note that the increase in CTI became noticeable
in June 2006 (i.e. 14 months after launch), but the evidence of a more serious
degradation in spectroscopic performance (line broadening and change in the
low-energy response) due to large charge traps (i.e. faults in the Si crystal)
became more significant after March 2007. We describe efforts to handle such
changes in the spectral response. Finally, we show that the commanded increase
in the substrate voltage from 0 to 6V on 2007 August 30 reduced the dark
current, enabling the collection of useful science data at higher CCD
temperature (up to -50C). We also briefly describe the plan to recalibrate the
XRT response files at this new voltage.
