Modelling the spectral response of the Swift-XRT CCD camera

Experience learnt from in-flight calibration

O. Godet, A. P. Beardmore, A. F. Abbey, J. P. Osborne, G. Cusumano, C. Pagani, M. Capalbi, M. Perri, K. L. Page, David Nelson Burrows, S. Campana, J. E. Hill, Jamie A. Kennea, A. Moretti

Research output: Contribution to journalArticle

35 Citations (Scopus)

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.

Original languageEnglish (US)
Pages (from-to)775-797
Number of pages23
JournalAstronomy and Astrophysics
Volume494
Issue number2
DOIs
StatePublished - Feb 1 2009

Fingerprint

spectral sensitivity
CCD cameras
flight
telescopes
files
calibration
charge coupled devices
modeling
x rays
time measurement
afterglows
matrix
counting
photons
matrices
energy
electric potential
dark current
gamma ray bursts
spectral analysis

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

Godet, O., Beardmore, A. P., Abbey, A. F., Osborne, J. P., Cusumano, G., Pagani, C., ... Moretti, A. (2009). Modelling the spectral response of the Swift-XRT CCD camera: Experience learnt from in-flight calibration. Astronomy and Astrophysics, 494(2), 775-797. https://doi.org/10.1051/0004-6361:200811157
Godet, O. ; Beardmore, A. P. ; Abbey, A. F. ; Osborne, J. P. ; Cusumano, G. ; Pagani, C. ; Capalbi, M. ; Perri, M. ; Page, K. L. ; Burrows, David Nelson ; Campana, S. ; Hill, J. E. ; Kennea, Jamie A. ; Moretti, A. / Modelling the spectral response of the Swift-XRT CCD camera : Experience learnt from in-flight calibration. In: Astronomy and Astrophysics. 2009 ; Vol. 494, No. 2. pp. 775-797.
@article{6a17f83eb4ac46fd9099e92b11063d66,
title = "Modelling the spectral response of the Swift-XRT CCD camera: Experience learnt from in-flight calibration",
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.",
author = "O. Godet and Beardmore, {A. P.} and Abbey, {A. F.} and Osborne, {J. P.} and G. Cusumano and C. Pagani and M. Capalbi and M. Perri and Page, {K. L.} and Burrows, {David Nelson} and S. Campana and Hill, {J. E.} and Kennea, {Jamie A.} and A. Moretti",
year = "2009",
month = "2",
day = "1",
doi = "10.1051/0004-6361:200811157",
language = "English (US)",
volume = "494",
pages = "775--797",
journal = "Astronomy and Astrophysics",
issn = "0004-6361",
publisher = "EDP Sciences",
number = "2",

}

Godet, O, Beardmore, AP, Abbey, AF, Osborne, JP, Cusumano, G, Pagani, C, Capalbi, M, Perri, M, Page, KL, Burrows, DN, Campana, S, Hill, JE, Kennea, JA & Moretti, A 2009, 'Modelling the spectral response of the Swift-XRT CCD camera: Experience learnt from in-flight calibration', Astronomy and Astrophysics, vol. 494, no. 2, pp. 775-797. https://doi.org/10.1051/0004-6361:200811157

Modelling the spectral response of the Swift-XRT CCD camera : Experience learnt from in-flight calibration. / Godet, O.; Beardmore, A. P.; Abbey, A. F.; Osborne, J. P.; Cusumano, G.; Pagani, C.; Capalbi, M.; Perri, M.; Page, K. L.; Burrows, David Nelson; Campana, S.; Hill, J. E.; Kennea, Jamie A.; Moretti, A.

In: Astronomy and Astrophysics, Vol. 494, No. 2, 01.02.2009, p. 775-797.

Research output: Contribution to journalArticle

TY - JOUR

T1 - Modelling the spectral response of the Swift-XRT CCD camera

T2 - Experience learnt from in-flight calibration

AU - Godet, O.

AU - Beardmore, A. P.

AU - Abbey, A. F.

AU - Osborne, J. P.

AU - Cusumano, G.

AU - Pagani, C.

AU - Capalbi, M.

AU - Perri, M.

AU - Page, K. L.

AU - Burrows, David Nelson

AU - Campana, S.

AU - Hill, J. E.

AU - Kennea, Jamie A.

AU - Moretti, A.

PY - 2009/2/1

Y1 - 2009/2/1

N2 - 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.

AB - 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.

UR - http://www.scopus.com/inward/record.url?scp=59649106826&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=59649106826&partnerID=8YFLogxK

U2 - 10.1051/0004-6361:200811157

DO - 10.1051/0004-6361:200811157

M3 - Article

VL - 494

SP - 775

EP - 797

JO - Astronomy and Astrophysics

JF - Astronomy and Astrophysics

SN - 0004-6361

IS - 2

ER -