Update to the ACIS Contamination Model

ACIS observations of astronomical sources as well as its own external calibration source (ECS) have shown that molecular contamination has been building up on the optical/UV blocking filters since launch. Early LETG/ACIS observations of Ton S180 in Nov. 1999 and 3C 273 in Jan. 2000 showed that there was a slight reduction in the low energy ACIS QE1. A nearsimultaneous LETG/HRC-S observation of 3C273 did not show a similar problem with the low energy HRC-S QE. It was not clear that the low energy QE was systematically decreasing with time until a deep LETG/ACIS-S observation of Mkn 421 in 2002 showed an even further reduction in the low energy QE and the presence of absorption edges produced by C, O and F. Since ACIS operates at a focal plane temperature of -120 ◦ and the HRC operates at roughly room temperature, it was concluded that the reduction in the low energy QE of ACIS was due to out gassed material condensing on the cold ACIS filters. The composition of the molecular contaminant on the ACIS filters is discussed in detail in Marshall et al. (2004). During the first few weeks after launch, the charge transfer inefficiency (CTI) of the ACIS CCDs increased significantly due to radiation damage incurred during passage through the Earth’s radiation belts near perigee. Since that time, ACIS is stowed during each perigee pass, i.e., the HRC is at the focal point. When ACIS is stowed, it is illuminated by its ECS, which consists of a bare Fe55 source, an Al fluorescence source and a Ti fluorescence. To monitor the build-up of contamination on the ACIS filters, data is collected from the ECS before and after each perigee pass. To provide sufficient statistics, the ECS data must be co-added into 3 month intervals. The ECS has a strong Mn-K line at 5.9 keV and Mn-L and F-K lines that comprise most of the line emission around 660 eV (see Fig. 1). This figure shows a calorimeter resolution spectrum of an Fe55 source that was kindly provided to the Chandra calibration team by Dan McCammon2. With the energy resolution of the ACIS CCDs, the Mn-L and F-K lines are blended into a single feature near 660 eV. Prior to 2009, only a CCD resolution spectrum of the ECS was available to the calibration team and it was assumed that the blended feature near 660 eV was primarily due to emission from Mn-L and Fe-L emission lines3. Since the MnK α line at 5.9 keV is not affected by absorption, the optical depth of the contaminant can be monitored by measuring the flux ratio between the blended lines at 660 eV and the Mn-K α line. Since the individual lines near 660 eV cannot be resolved with the ACIS CCDs, a flux weighted opacity must be computed based on a model for the line energies and relative line strengths. Prior to 2009, this was done assuming the emission was dominated by Mn-L and Fe-L emission lines. Since that time, the emission lines and line strengths shown in Fig. 1 have been used. The ECS data also provides information on the optical depth at the Al-K α (1.49 keV) fluorescence line. The optical depth at Ti-K α (4.51 keV) is simply too low to be measured reliably.