Chandra Calibration Update

Larry David

Most of the calibration work over the past year has focused on two key issues concerning the future of ACIS calibration: 1) planning how to monitor the ACIS gain without the External Calibration Source (ECS), and 2) improving the charge transfer inefficiency (CTI) correction for ACIS data taken at warm focal plane temperatures. Until now, the time-dependence of the ACIS gain has been monitored with the ECS, a radioactive ⁵⁵Fe source that uniformly illuminates all ACIS chips while the instrument is stowed during radiation belt passages. In general, the ACIS gain has experienced a secular decline over the course of the mission, with some modulation due to the solar cycle. Early in the mission, the gain was calibrated by co-adding three months of ECS data. As the ECS has continued to fade over the course of the mission due to its 2.7 year half-life, it became necessary to co-add six months of data. For the latest release of the time-dependent gain correction file (March 2023, CALDB 4.10.4), it was necessary to co-add one year’s worth of ECS data. This may well be the last time the ECS data is used to monitor the ACIS gain. In the future, the mid-energy gain will be monitored with periodic observations of Cas A, which has a very bright Si-Kα line at 1.8 keV. The calibration team is still discussing possible targets for monitoring the high-energy gain, but the most likely target is the Perseus cluster, which has a very bright Fe-Kα line at 6.7 keV.

The current focal plane temperature limits for ACIS observations to ensure the best calibration are −112 °C for ACIS-I and −111 °C for ACIS-S. At warmer temperatures, the gain (or computed photon energy) can be offset by up to 2% near the top of the front-illuminated (FI) chips, so only poor S/N observations are permitted at warmer temperatures. The primary cause for the gain offset is the fact that CTI increases with increasing temperature, which affects the detector gain and spectral resolution. There are three components to the CTI correction: 1) a temperature-dependent correction, 2) an energy-dependent correction, and 3) a spatially-dependent correction. The calibration team has found that all three components of the CTI correction need to be modified to improve the gain calibration at warm focal plane temperatures. Improved CTI correction products are presently being developed for the six imaging chips (ACIS-I0, -I1, -I2, -I3, -S2, and -S3) to allow ACIS observations at warmer temperatures. The remaining chips are primarily used for gratings observations—in which the photon energies are computed from the dispersion relation—and are thus fairly insensitive to the detector gain. The new CTI calibration products will require a significant re-formatting of the existing CALDB CTI files. A script will also be posted on the Chandra contributed software page when the new CALDB CTI files are released so users can apply the improved CTI corrections to their data.

The calibration team continues to monitor the build-up of contamination on the ACIS optical blocking filters through periodic “Big Dither” LETG/ACIS-S observations of Mkn 421, as well as through ACIS-I and ACIS-S observations of E0102−79 and Abell 1795. Recent observations of all three targets showed that the old ACIS contamination model was underpredicting the depth of the contaminant by about 10% since 2021. An updated ACIS contamination model was released in November 2022 (CALDB 4.10.2) to correct this problem.

The Chandra calibration team continues to support the International Astronomical Consortium for High Energy Calibration (IACHEC). The 15th annual meeting of the IACHEC took place on April 23–24. Many Chandra calibration scientists gave presentations at the meeting and chaired several of the calibration Working Groups.