CalDB 4.9.5 Public Release Notes

I. INTRODUCTION

CalDB 4.9.5 is an upgrade to the Chandra CalDB, which includes the following items:

• ACIS T_GAIN Update Epochs 83+84
• ACIS CTI Correction Update Effective 2020-01-01
• LETG/HRC-I LSFPARMs Added

For the CIAO 4.13 / CalDB 4.9.5 release notes see How CalDB 4.9.5 Affects Your Analysis.

II. SUMMARY OF CHANGES

A. ACIS T_GAIN Update Epochs 83+84

Location:	$CALDB/data/chandra/acis/t_gain/
Filenames:
	acisD2020-05-02t_gainN0008.fits	Epochs 81+82 finalized, non-GRADED
	acisD2020-05-02t_gainN0008B.fits	Epochs 81+82 finalized, GRADED and CC33_GRADED
	acisD2020-11-01t_gain_biN0002.fits	Epochs 83+84 provisional, non-GRADED
	acisD2020-11-01t_gainN0002.fits	Epochs 83+84 provisional, GRADED and CC33_GRADED

The ACIS Calibration Team has generated time-dependent gain (T_GAIN) corrections from onboard External Calibration Source (ECS) data taken during the period of 01 August 2020 through 31 January 2021 (Epochs 83+84). With this release it is now possible to finalize the T_GAIN for the previous bi-epochal period (Epochs 81+82, February through July 2020). These updates are included in the files above with effecive dates 2020-11-01 (provisional) and 2020-05-02 (finalized) respectively.

A correction is included in this upgrade, specifically to change the FP_TEMP boundary condition in the previous provisional T_GAIN files for Epochs 81+82. These files were released in CalDB 4.9.3 in October 2020, and specified the range as FP_TEMP(148.16-164.16)K (-124.99C to -108.99C) rather than the expanded prescription approved and released in CalDB 4.9.2 (July, 2020), which set the required range to (148.15-168.16)K, or -125.0C to -104.99C, for all temperature- dependent ACIS calibrations. While no OBS_IDs taken since July 01, 2020 have violated the -108.99C upper limit, future observations will admit warmer FP_TEMP values, and hence upgrading to CalDB 4.9.5 is prudent for all users of any future ACIS observations.

This release affects all ACIS observations taken since 2020-05-02T00:00:00, and users of any OBS_IDs with DATE-OBS later than that date are encouraged to reprocess their datasets. The degree of the corrections is similar to those of previous T_GAIN releases, and those analyzing narrow line-source data are the most likely to be affected signficantly. See the Section III.a technical details section below for further information regarding these specific file upgrades.

See the ACIS Time-dependent Gain why page for a more basic explanation of the time-dependent gain correction.

PIPELINES/TOOLS AFFECTED:

ACIS Level 1 pipeline tool acis_process_events.

CIAO script chandra_repro uses acis_process_events to reprocess ACIS observation data.

THREADS AFFECTED:

Reprocessing Data to Create a New Level=2 Event File

B. ACIS CTI Correction Update Effective 2020-01-01

Location:	$CALDB/data/chandra/acis/cti/
Filename:	acisD2020-01-01ctiN0009.fits

A new 5-year addition has been made to the time- and temperature-dependent CTI correction of the ACIS-S3 (ACIS-7) chip. The new file above, which modifies only the ACIS-S3 CTI correction, contains a single modification, specifically to change the TCTIY value from 0.0234, as exists in the current 2015-01-01 file, to 0.0418. A further note is that this new file will be used only for OBS_IDs in non-GRADED DATAMODEs; For GRADED and CC33_GRADED DATAMODE configurations, the 2002-08-01 N0009B file will continue to be selected. This latter N0009B file includes no CTI corrections for the back-illuminated (BI) chips; There is no CTI correction calibration for the BI chips with GRADED modes.

See the ACIS CTI correction why page for a more basic explanation of the time- and temperature-dependent CTI correction. A further illustration of the specific change here is given in the Section IIIb technical details below.

PIPELINES/TOOLS AFFECTED:

ACIS Level 1 pipeline tool acis_process_events.

CIAO script chandra_repro uses acis_process_events to reprocess ACIS observation data.

THREADS AFFECTED:

Reprocessing Data to Create a New Level=2 Event File

C. LETG/HRC-I LSFPARMs Added

Location:	$CALDB/data/chandra/hrc/lsfparm/
Filenames:	hrcileg1D1999-07-22lsfparmN0004.fits hrcileg-1D1999-07-22lsfparmN0004.fits

Because of the recently increased use of the HRC-I/LETG configuration, the LETGS Calibration Team has suggested that LSFPARM files be added to the CalDB for this configuration, although no specific LSF parameters have been derived for this configuration. In the absence of direct calibrations, the HRC-S/LETG LSFPARM N0004 files currently in CalDB have been copied directly for the HRC-I/LETG case, with the only change besides the filenames above, is to reset the DETNAM values from "HRC-S" to "HRC-I" in the headers of the copied files.

CIAO TOOLS AFFECTED:

See the CIAO scripts mktgresp and chandra_repro These scripts were recently modified to include the HRC-I/LETG LSFPARM files from CalDB for this configuration.

THREADS AFFECTED:

HRC-I Grating RMFs

III. TECHNICAL DETAILS

A. ACIS T_GAIN Update Epochs 83+84

The general description and method of derivation for the ACIS T_GAIN are given in the memorandum posted on ACIS Calibrationweb page http://cxc.harvard.edu/cal/Acis/Cal_prods/tgain/index.html. However, additional steps have been added to the derivation procedure, due to the ever poorer statistics that are attainable from the ECS due to the source's diminishing flux levels. In particular, the ACIS calibration team has used trends from previous epochs (when more ECS flux was available) to "nudge" their more recent results to provide more predictable time-dependent gain corrections. The method is to combine earlier epochal data with current (i.e. new) epochal data to generate a 12-month average for T_GAIN corrections, and use the trends form the 12-month averages to modify the 6-month averages, resulting in a smoother correction with better statistical support.

The data are increasingly noisy as the ECS flux decreases with the age of the spacecraft. As a result the ACIS team is has selectively increased the range of acceptable operating focal plane temperatures (FP_TEMP values) for three of the 10 chips (ACIS-4, 8, and 9) in order to include more events for better statistics in the T_GAIN correction values. This expanded range is 151.96K to 154.96K. The previous more restricted range of 151.96-152.96K has been used for the other seven ACIS chips. Subsequent T_GAIN releases will include the expanded FP_TEMP range of ECS data for all 10 ACIS chips.

Long term trends for chips I3 and S3 are plotted below in Figures 1a and b, through Epoch 84, which includes the latest release. The chip locations for these plots are given in the respective labels above the plots. The chip locations selected are near the aimpoints. The full CTI corrections have been applied to the data in both figures. While some fluctuations have occurred recently in the results for the Ti and Mn lines of the ECS for I3 in particular, there is nothing out-of-expectation for the newest datasets. The fluctuations in the traces are due to counting statistics.

Fig. 1a: Long-term T_GAIN correction tgain_trends at the ACIS-I3 aimpoint, for specific ECS line energies, from Epoch 1 to Epoch 84.

Fig. 1b: Long term T_GAIN correction tgain_trends at the aimpoint of ACIS-S3, for specific ECS line energies, from Epoch 1 to Epoch 84.

B. ACIS CTI Correction Update Effective 2020-01-01

The ACIS Calibration team has provided an update for the time- and temperature-dependent CTI corrections for ACIS-S3, effective 2020-01-01T00:00:00. Specifically, a single value in an existing CTI file in CalDB is all that needs to be changed in order to generate the new CTI CalDB file. The new file shall be acisD2020-01-01ctiN0009.fits. All of the values are identical to 2015-01-01, except for TCTIY for S3 in BLOCK 2 of the file, or the first BINTABLE extension. The value in 2015-01-01 is 0.0234 and the new value shall be 0.0418.

The new 2020-01-01 CTI file has been tested by reprocessing all of the ECS data for T_GAIN EPOCH 81 (Feb-Apr 2020) in CIAO 4.13 applying the new CTI file, and compared the results with corresponding events from a previous run which used the 2015-01-01 CTI file. The result is the plot below, Figure 2. It was also found that no events on the other ACIS chips were affected, as is expected from this change.

Fig. 2: Delta-PHA values derived from ACIS T_GAIN Epoch 81 (Feb-Apr 2020) on ACIS-S3, using the 2015 and 2020 CTI files respectively. The T_GAIN correction has not been applied in either case. The PHA values thus derived are thus consistent with one another. Any additional recovered events with the new CTI file applied are not included.