Calibration Update

Larry David

The Chandra calibration team has focused on three main ACIS calibration issues over the past year: 1) releasing a set of temperature-dependent response matrices, 2) correcting the recent decline in the ACIS gain during the current solar maximum, and 3) monitoring the build-up of contamination on the ACIS filters. The energy resolution of the ACIS CCDs degrades with increasing focal plane temperature due to the increase in charge transfer inefficiency (CTI) with temperature (i.e., lines become broader as the focal plane temperature increases). In CALDB 4.12.0, a set of temperature-dependent rmfs were released for the primary front-illuminated (FI) chips used for imaging (all four CCDs on ACIS-I and the S2 CCD). The rmfs were calibrated in two degree temperature bins for each of the five CCDs using data from the ECS (external calibration source) accumulated over the course of the Chandra mission. The new temperature-dependent rmfs produce a significant improvement in fitting emission line spectra, especially data taken at warm focal plane temperatures.

The current solar maximum has led to a decrease in the ACIS detector gain, which is sensitive to the charged particle background. Background events fill in the radiation-induced traps in the ACIS CCDs and reduce the CTI. Thus, CTI is lower during periods of high background and higher during periods of low background. The rapid decline in the quiescent background rate prior to the current solar maximum led to a greater CTI than predicted by the ACIS calibration products in the current version of the CALDB and resulted in the detector gain being undercorrected. To address this issue, the calibration team released an updated set of ACIS time-dependent gain correction files (tgain) on August 19 in CALDB 4.12.2.

The calibration team continues to track the build-up of contamination on the ACIS filters with imaging observations of the rich cluster Abell 1795 and gratings observations of the blazar Mkn 421. Gratings observations over the course of the mission have shown that the contaminant is composed of three elements (C, O, and F). The build-up rate and spatial distribution of each element is accounted for separately in the ACIS contamination model. Observations over the past year have shown that the optical depth at the O-K edge is currently increasing more rapidly than predicted by the current version of the ACIS contamination model. This has produced a discrepancy of about 0.4 optical depths at 0.7 keV and about 0.05 optical depths at Al-K (1.5 keV) between the data and the CALDB version of the contamination model. The calibration team is currently working on updating the CALDB version of the ACIS contamination model to account for these recent changes.

In addition, the calibration team continues to monitor the gain and quantum efficiency (QE) of the two HRC detectors through gratings observations of the white dwarf HZ 43 (for the soft response) and imaging observations of the supernova remnant G21.5-09 (for the hard response). The gain and QE in both detectors continue to decline with time, with an average QE loss of about 2% per year. For the HRC-S, the QE decline in the central plate (S2) is nearly wavelength-independent. For the outer two plates of the HRC-S (S1 and S3), a wavelength-dependent adjustment to the QE is necessary and is contained in the QE uniformity map (QEU). To counter the loss in gain and QE of the two HRC detectors, the High Voltage (HV) setting of the two detectors has been periodically increased over the course of the mission. The HV of the HRC-S has been increased three times and the HV of the HRC-I has been increased two times. The latest HV increase for the two HRC detectors occurred in September 2024. This recent increase in the HV restored the response of both detectors to their state about two years prior. Over the past year, the calibration team released the final CALDB products for the two HRC detectors with the old HV setting, and updated calibration products for the two HRC detectors with the new HV setting. In CALDB 4.11.2 and CALDB 4.11.3 (from June to August 2024), the final updates to the HRC-I QE and HRC-S QEU were released for the old HV setting. Updates to the HRC-S QE, HRC-S QEU, and HRC-I QE were released in CALDB 4.11.5, 4.11.6, and 4.12.0 (from December 2024 to January 2025) for the new HV setting. Applying the latest HRC CALDB products produces an rms scatter in the fluxes of HZ43 and G21.5-09 of about 1.5% with both HV settings.

As has been the case for many years, the Chandra calibration team continues to support the International Astronomical Consortium for High Energy Calibration (IACHEC). The 17th annual meeting of the IACHEC was held May 12–15, and many Chandra calibration scientists participated, giving presentations at the meeting and chairing several of the calibration Working Groups.