Chandra Announcement: LETG/HRC-S observations: PI filtering and gain maps + observations after 2018

[Tara Gokas]

LETG/HRC-S observations: PI filtering and gain maps + observations after 2018

The HRC-S gain map was updated to version 3 in CALDB 4.9.2 (released 9 July 2020). However, the congruous LETG/HRC-S PI region file, version 2, was not updated at that time.

The PI Region filter (version 2) is included in the latest CALDB 4.9.4 (released 16 December 2020).

Observations reprocessed between July and December 2020 with chandra_repro and CALDB 4.9.2, CALDB 4.9.2.1 and CALDB 4.9.3 would have the incorrect PI filter applied. 

The CXC advises using the latest chandra_repro in CIAO 4.13 and CALDB 4.9.4 to reprocess any LETG/HRC-S observation originally processed  between July and December 2020. 

The thread A Note on Processing Versions explains how to check which CALDB version is installed in the system and which version of the CALDB was used in the reprocessing of the file.

For observations from 2018 onwards

Recent calibration studies have shown that, for data obtained from 2018 onwards, application of PI Region background filtering removes an unpredictable fraction of valid source X-ray events that can significantly exceed the target maximum of 1.25% loss. This behavior results from the gradual loss of instrument gain.  

It is expected that a new calibration to handle the 2018 onwards data will be made available in the near future.  In the meantime, the CXC advises not to apply any PI Region background filtering to observations made from 2018 onwards unless the user verifies through comparison with manual processing of unfiltered data that the X-ray loss due to filtering is acceptable.  The script chandra_repro is currently written to automatically apply PI filtering, therefore the CXC advises users to instead follow the LETG/HRC-S data reprocessing thread for the affected data to experiment with and without PI filtering.