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تسجيل مستخدم جديدThe in-flight spectroscopic performance of the Swift XRT CCD camera during 2006-2007
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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.
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The Swift X-ray Telescope (XRT) focal plane camera is a front-illuminated MOS CCD, providing a spectral response kernel of 144 eV FWHM at 6.5 keV. We describe the CCD calibration program based on celestial and on-board calibration sources, relevant i
n-flight experiences, and developments in the CCD response model. We illustrate how the revised response model describes the calibration sources well. Loss of temperature control motivated a laboratory program to re-optimize the CCD substrate voltage, we describe the small changes in the CCD response that would result from use of a substrate voltage of 6V.
(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.
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
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