Recovering Swift-XRT energy resolution through CCD charge trap mapping

The X-ray telescopeon board the Swiftsatellite for gamma-ray burstastronomy has been exposed to the radiation of the space environmentsince launch in November 2004. Radiation causes damage to the detector, with the generation of dark currentand charge trapping sites that result in the degradation of the spectral resolutionand an increase of the instrumental background. The Swiftteam has a dedicated calibration program with the goal of recovering a significant proportion of the lost spectroscopic performance. Calibration observations of supernova remnantswith strong emission lines are analysed to map the detector charge traps and to derive position-dependent corrections to the measured photon energies. We have achieved a substantial recovery in the XRT resolution by implementing these corrections in an updated version of the SwiftXRT gain file and in corresponding improvements to the SwiftXRT HEAsoft software. We provide illustrations of the impact of the enhanced energy resolution, and show that we have recovered most of the spectral resolutionlost since launch.