The X-ray telescope on board the Swift satellite for gamma-ray burst astronomy has been exposed to the radiation of the space environment since launch in November 2004. Radiation causes damage to the detector, with the generation of dark current and
charge trapping sites that result in the degradation of the spectral resolution and an increase of the instrumental background. The Swift team has a dedicated calibration program with the goal of recovering a significant proportion of the lost spectroscopic performance. Calibration observations of supernova remnants with 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 Swift XRT gain file and in corresponding improvements to the Swift XRT HEAsoft software. We provide illustrations of the impact of the enhanced energy resolution, and show that we have recovered most of the spectral resolution lost since launch.
(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 observa
tions, 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.
We present science highlights and performance from the Swift X-ray Telescope (XRT), which was launched on November 20, 2004. The XRT covers the 0.2-10 keV band, and spends most of its time observing gamma-ray burst (GRB)afterglows, though it has also
performed observations of many other objects. By mid-August 2007, the XRT had observed over 220 GRB afterglows, detecting about 96% of them. The XRT positions enable followup ground-based optical observations, with roughly 60% of the afterglows detected at optical or near IR wavelengths. Redshifts are measured for 33% of X-ray afterglows. Science highlights include the discovery of flaring behavior at quite late times, with implications for GRB central engines; localization of short GRBs, leading to observational support for compact merger progenitors for this class of bursts; a mysterious plateau phase to GRB afterglows; as well as many other interesting observations such as X-ray emission from comets, novae, galactic transients, and other objects.
The Swift X-ray Telescope focal plane camera is a front-illuminated MOS CCD, providing a spectral response kernel of 135 eV FWHM at 5.9 keV as measured before launch. We describe the CCD calibration program based on celestial and on-board calibration
sources, relevant in-flight experiences, and developments in the CCD response model. We illustrate how the revised response model describes the calibration sources well. Comparison of observed spectra with models folded through the instrument response produces negative residuals around and below the Oxygen edge. We discuss several possible causes for such residuals. Traps created by proton damage on the CCD increase the charge transfer inefficiency (CTI) over time. We describe the evolution of the CTI since the launch and its effect on the CCD spectral resolution and the gain.
