HRC-S Time-dependent Gain Corrections and PI Filter
A new HRC-S gain map and pulse-height filter was introduced in CALDB 4.9.6 (21 September 2021) fixing issues found in the prior gain map version introduced in CALDB 4.9.2 (6 July 2020). The original intent was to remove over half of Level=2 background events at wavelenghts >20 Å with a loss of ~1% of X-ray events; however, it was found that the HRC-S/LETG background filter sometimes removed an excessive amount of valid X-ray events, primarily at negative orders, with observations beginning ~2017 and increasing over time, with losses up to 6% at some wavelengths by the end of 2020. The new gain maps and pulse-height filter has been now corrected, as described in the "HRC-S Adjustment for Very Low Gain" memo.
The primary differences from the old gain map and PI filter are:
Gain corrections are time dependent; the HRC-S gain is now roughly half what it was at the beginning of the mission.
PHA is replaced with scaled SUMAMPS, which creates a much more smoothly-varying gain map.
PI is replaced with SAMP-based PI.
The old PI filter became less effective over time because it did not account for decreasing gain. Less background is removed at short wavelengths because the filtering threshold must be higher to avoid excluding X-ray events.
Relevant Plots
- HRC-S gain curves of 18 CalDB epochs normalized to epoch 2001.5. The green curve in the top panel is for epoch 2001.22, when gains are close to those during pre-flight laboratory calibration, and the blue curve in the bottom panel is the gain around the time of this time-dependence calibration (Jan 2020). The gain calibration is valid to at least 2022.
- PI pulse height distributions for flight background between 2000 and 2019: the difference between X-ray and background distributions is the basis of the pulse-height filter.
- Comparison of old and new PI filters
Users working with HRC-S data should reprocess to improve the TGAIN calibration in their data by running the chandra_repro script or by following the Create a New Level=2 Event File thread. The PI background filter is applied to HRC-S/LETG data, as explained in the "Apply background filter" section of the LETG/HRC-S Grating Spectra thread.