Chandra Calibration Update

Larry David

The calibration team has released time-dependent ACIS gain correction files (tgain) every six months for the past few years by co-adding data from the External Calibration Source (ECS). ACIS is exposed to the ECS whenever it is in the stowed position, which occurs during each radiation belt passage. Early in the mission, tgain files were released every three months, but due to the fading of the ECS, which has a half-life of 2.7 years, it became necessary to co-add more data. The continued fading of the ECS has necessitated a new procedure as to how the tgain file is generated. Future tgain files will be generated once per year using both Cas A and ECS data. Cas A exhibits many strong emission lines that enable an accurate gain calibration in the 1-3 keV range. Due to the lack of a strong high energy line in Cas A, we will continue to use the Mn-Kα line at 5.9 keV in the ECS. The Mn-Kα line is the brightest line in the ECS. The next tgain file will be derived from data accumulated up to the end of January, 2023.

The calibration team last released an update to the ACIS contamination model in the Fall of 2020. Every year, the calibration team carries-out a set of calibration observations to monitor the build-up of contamination, including: Big Dither LETG/ACIS-S observations of Mkn 421, several ACIS-I and ACIS-S observations of E0102-79, and a raster scan of Abell 1795 on both ACIS-I and ACIS-S. A comparison of all the data collected over the past 18 months shows that the CALDB version of the ACIS contamination continues to due a fairly good job at predicting the optical depth of the contaminant. The optical depth of the contaminant at 1.5 keV (where ACIS still has a significant effective area) predicted by the current contamination model is within 2% of the measured value. At 0.66 keV (where there is much less effective area) the optical depth of the model can underestimate the measured optical depth by up to 10%. The calibration team is awaiting the results of the Abell 1795 raster scan before making a decision as to whether an update to the contamination model is warranted.

Due to the increasing operating temperature of ACIS, a significant fraction of the present ACIS calibration effort is dedicated to calibrating the response of the CCDs at temperatures above the nominal operating temperature of -120°C. The response of the CCDs is mainly driven by how the charge transfer inefficiency (CTI) which varies with temperature. The CTI for the FI chips is much more temperature sensitive than the CTI for the BI chips. For this reason, the calibration team is primarily focused on calibrating the FI chips used for most imaging observations (ACIS-I, S2). Gratings data are less sensitive to running at higher ACIS operating temperatures, since the photon energies are computed from the dispersion relation and not the detector gain. The three main calibration products (i.e., gain, QE, and spectral response) all degrade with increasing temperature. To complicate matters further, the CTI increases at a non-linear rate with temperature at warmer temperatures. The calibration team is in the process of generating a set of QEU maps and spectral response files in two degree centigrade intervals, spanning the temperature range form -120°C to -108°C. Initially, CALDB products will be released for the ACIS-I and S2 chips. The calibration team will then investigate how the response of the BI chips (S1 and S3) varies with temperature.

The calibration team continues to monitor the gain and QE of the two HRC detectors with periodic gratings and imaging observations of a soft source (the white dwarf HZ43) and a hard source (the supernova remnant G21.5-09). Due to the continued degradation in the gain and QE, the operating high voltage of both the HRC-I and HRC-S were increased during the past year. The gain and QE of the two detectors were restored to their values of approximately three to four years ago after the high voltage increase. Since the high voltage increase, the decline in gain and QE have continued. Updated gain, QE, and QEU maps for the HRC-S and a QE file for the HRC-I were released in late 2021 to correct HRC data taken after the high voltage increase.