Verifying the ACIS contamination model with 1E0102.2-7219

Abstract

The low-energy sensitivity of the Advanced CCD Imaging Spectrometer (ACIS) instrument on board the <i>Chandra</i> X-ray Observatory (CXO) has been continuously degrading since launch due to the accumulation of a layer of contamination on the ACIS optical blocking filter (OBF). This contamination layer, the result of out-gassing and off-gassing within the observatory, introduces a new, energy dependent absorption into the ACIS response. The thickness of this layer has been increasing with time and its spatial distribution across the OBF has been continually changing with time. We utilize multiple observations of the SMC Supernova Remnant 1E0102.2-7219 to verify the models for the spectral, temporal, and spatial dependence of the contamination layer. We also use this source to investigate cross-calibration between the front illuminated (FI) and back illuminated (BI) CCDs. 1E0102.2-7219 has a soft, line-dominated spectrum which is very sensitive to the additional absorption of the contamination layer. The extensive calibration observations of 1E0102.2-7219 over the course of the mission at several different locations on the ACIS Imaging (I) and Spectroscopy (S) arrays allows for a verification of the temporal and spatial dependence of the contamination model.