The evolution of the ACIS contamination layer over the 16-year mission of the Chandra X-ray Observatory

The Chandra X-ray Observatory (CXO) was launched 16 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. The calibration team within the Chandra X-ray Center (CXC) generates calibration files that describe the additional absorption produced by the contamination layer as a function of time, position, and energy. We have verified the accuracy of this contamination file for the on-axis aimpoints using the standard model spectrum for the Supernova Remnant 1E 0102.2-7219 in the Small Magellanic Cloud developed by the International Consortium for High Energy Calibration (IACHEC), but we show the model is less accurate for the off-axis positions after 2013. In 2015, the ACIS Detector Housing heater was turned on to increase the temperature of the OBF in the hope that the accumulation rate of the contamination layer would decrease. We show that the accumulation rate of the contaminant is unchanged since the DH heater was turned on.