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Proceedings Paper

The complicated evolution of the ACIS contamination layer over the mission life of the Chandra X-ray Observatory
Author(s): Paul P. Plucinsky; Akos Bogdan; Herman L. Marshall; Neil W. Tice
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Paper Abstract

The Chandra X-ray Observatory (CXO) was launched almost 19 years ago and has been delivering spectacular science over the course of its mission. The Advanced CCD Imager Spectrometer (ACIS) is the prime instrument on the satellite, conducting over 90% of the observations. The CCDs operate at a temperature of -120°C and the optical blocking filter (OBF) in front of the CCDs is at a temperature of approximately −60°C. The surface of the OBF has accumulated a layer of contamination over the course of the mission, as it is the coldest surface exposed to the interior to the spacecraft. We have been characterizing the thickness, chemical composition, and spatial distribution of the contamination layer as a function of time over the mission. All three have exhibited significant changes with time. There has been a dramatic decrease in the accumulation rate of the contaminant starting in 2017. The lower accumulation rate may be due to a decrease in the deposition rate or an increase in the vaporization rate or a combination of the two. We show that the current calibration file which models the additional absorption of the contamination layer is significantly overestimating that additional absorption by using the standard model spectrum for the supernova remnant 1E 0102.2-7219 developed by the International Astronomical Consortium for High Energy Calibration (IACHEC). In addition, spectral data from the cluster of galaxies known as Abell 1795 and the Blazar Markarian 421 are used to generate a model of the absorption produced by the contamination layer. The Chandra X-ray Center (CXC) calibration team is preparing a revised calibration file that more accurately represents the complex time dependence of the accumulation rate, the spatial dependence, and the chemical composition of the contaminant. Given the rapid changes in the contamination layer over the past year, future calibration observations at a higher cadence will be necessary to more accurately monitor such changes.

Paper Details

Date Published: 6 July 2018
PDF: 15 pages
Proc. SPIE 10699, Space Telescopes and Instrumentation 2018: Ultraviolet to Gamma Ray, 106996B (6 July 2018); doi: 10.1117/12.2312748
Show Author Affiliations
Paul P. Plucinsky, Harvard-Smithsonian Ctr. for Astrophysics (United States)
Akos Bogdan, Harvard-Smithsonian Ctr. for Astrophysics (United States)
Herman L. Marshall, MIT Kavli Institute for Astrophysics and Space Research (United States)
Neil W. Tice, MIT Kavli Institute for Astrophysics and Space Research (United States)


Published in SPIE Proceedings Vol. 10699:
Space Telescopes and Instrumentation 2018: Ultraviolet to Gamma Ray
Jan-Willem A. den Herder; Shouleh Nikzad; Kazuhiro Nakazawa, Editor(s)

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