Abstract
Context. Since its launch in November 2004, Swift has revolutionised our understanding of gamma-ray bursts. The X-ray telescope (XRT), one of the three instruments on board Swift, has played a key role in providing essential positions, timing, and spectroscopy of more than 300 GRB afterglows to date. Although Swift was designed to observe GRB afterglows with power-law spectra, Swift is spending an increasing fraction of its time observing more traditional X-ray sources, which have more complex spectra. Aims. The aim of this paper is a detailed description of the CCD response model used to compute the XRT RMFs (redistribution matrix files), the changes implemented to it based on measurements of celestial and on-board calibration sources, and current caveats in the RMFs for the spectral analysis of XRT data. Methods. The RMFs are computed via Monte-Carlo simulations based on a physical model describing the interaction of photons within the silicon bulk of the CCD detector. Results. 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 6 V on 2007 August 30 reduced the dark current, enabling the collection of useful science data at higher CCD temperature (up to −50 ◦ C). We also briefly describe the plan to recalibrate the XRT response files at this new voltage. Conclusions. We show that the XRT spectral response is described well by the public response files for line and continuum spectra in the 0.3−10 keV band in both PC and WT modes.
Highlights
Launched on 2004 November 20, the Swift gammaray burst satellite (Gehrels et al 2004) consists of three instruments: the wide-field of view, gamma-ray burst alert telescope (BAT; Barthelmy et al 2005) and two narrow field instruments (NFIs), the X-ray telescope (XRT; Burrows et al 2005) and the UV/optical telescope (UVOT; Roming et al 2005)
The aim of this paper is to describe in detail our CCD response model used to compute the redistribution matrix files (RMFs), the changes that have been made to the RMFs and auxiliary response files (ARFs) since the launch and the caveats to be aware of in the spectral analysis of XRT data when using the current response files distributed in the CALDB release on 2008-06-25
We showed that the v011 XRT response files, calibrated using data collected at Vss = 0 V, give good performance on continuum and line sources in both photon counting (PC) and windowed timing (WT) mode when compared to other X-ray instruments in the 0.3−10 keV energy band with a systematic error of less than 3% in both modes over 0.3−10 keV and better than 10% in absolute flux
Summary
Launched on 2004 November 20, the Swift gammaray burst satellite (Gehrels et al 2004) consists of three instruments: the wide-field of view, gamma-ray burst alert telescope (BAT; Barthelmy et al 2005) and two narrow field instruments (NFIs), the X-ray telescope (XRT; Burrows et al 2005) and the UV/optical telescope (UVOT; Roming et al 2005). – we discuss the in-flight operation of the XRT and its impact on the calibration of the XRT response, the calibration program, and in detail post-launch changes made to the CCD spectral model (low-energy response, line shoulder, shelf, RMF redistribution) and the ARFs based on celestial target calibration; Sect. – we present the spectroscopic performance of the XRT for several celestial targets compared to observations with other X-ray instruments, as well as caveats (line broadening due to the build-up of charge traps on the CCD and changes in the in-orbit operation of the CCD caused by raising the substrate voltage from 0 V to 6 V in 2007 August 30) for the spectral analysis when using the current RMFs and ARFs; Sect. We briefly describe the spectroscopic performance of the RMFs in both PC and WT modes prior to launch using ground calibration data; Sect. 3 – we discuss the in-flight operation of the XRT and its impact on the calibration of the XRT response, the calibration program, and in detail post-launch changes made to the CCD spectral model (low-energy response, line shoulder, shelf, RMF redistribution) and the ARFs based on celestial target calibration; Sect. 4 – we present the spectroscopic performance of the XRT for several celestial targets compared to observations with other X-ray instruments, as well as caveats (line broadening due to the build-up of charge traps on the CCD and changes in the in-orbit operation of the CCD caused by raising the substrate voltage from 0 V to 6 V in 2007 August 30) for the spectral analysis when using the current RMFs and ARFs; Sect. 5 – we present the main conclusions of the paper
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