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The Chandra X-ray Observatory (CXO) was launched over 21 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. In this paper, we present a revision to the time dependence of the accumulation rate. In a previous paper, we described a model in which the accumulation rate started to decrease in 2017. We present new data which show that the accumulation rate has been roughly constant and linear from 2017 until the present. We show that the current calibration file underestimates the additional absorption of the contamination layer 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) and spectral data from the the cluster of galaxies known as Abell 1795. We present a revised model that produces consistent line normalizations and fluxes in 0.5–1.0 and 1.0-2.0 keV bands for these two sources over the course of the mission. This revised model is expected to be released in December 2020 in the next release of the CXO The Chandra X-ray Observatory (CXO) was launched over 21 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. In this paper, we present a revision to the time dependence of the accumulation rate. In a previous paper, we described a model in which the accumulation rate started to decrease in 2017. We present new data which show that the accumulation rate has been roughly constant and linear from 2017 until the present. We show that the current calibration file underestimates the additional absorption of the contamination layer 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) and spectral data from the the cluster of galaxies known as Abell 1795. We present a revised model that produces consistent line normalizations and fluxes in 0.5–1.0 and 1.0-2.0 keV bands for these two sources over the course of the mission. This revised model is expected to be released in December 2020 in the next release of the CXO Calibration Databasealibration Database
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