CalDB 4.7.8 Public Release Notes

I. INTRODUCTION

CalDB 4.7.8 is an upgrade to the Chandra CalDB, which includes the following items:

• ACIS T_GAIN Epochs 71+72 Updates
• HRC-S GAPLOOKUP Version N0004
• 2016 T_GAIN Provisional Files RETIRED

For the CIAO 4.9 / CalDB 4.7.8 release notes see How CalDB 4.7.8 Affects Your Analysis.

II. SUMMARY OF CHANGES

A. ACIS T_GAIN Epochs 71+72 Updates

Location:	$CALDB/data/chandra/acis/t_gain/
Filenames:	acisD2017-05-02t_gainN0005.fits acisD2017-05-02t_gainN0006.fits acisD2017-11-01t_gainN0002.fits acisD2017-11-01t_gain_biN0002.fits

The ACIS Calibration team has released new time-dependent gain corrections derived from the External Calibration Source (ECS) during quarterly Epochs 71 (Aug-Oct 2017) and 72 (Nov 2017 - Jan 2018), which have been combined to obtain sufficient counting statistics. With this release, the combined epoch 69+70 CalDB files may be finalized. Users with observations taken between (DATE-OBS=) 2017-05-02T12:00:00 and 2017-11-01T00:00:00 are advised to reprocess with their data with finalized versions of the applicable T_GAIN files.

Users with observations taken since 2017-11-01T00:00:00 will have access to an updated provisional T_GAIN file (one of the version N0002 filenames above); They will not however have a finalized file until after Epoch 74 (May-July 2018) is completed.

For basic information concerning the ACIS Time-dependent Gain Correction, See ACIS Time-dependent Gain why page. For a more detailed explanation, see the Technical Details section 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

B. HRC-S GAPLOOKUP Version N0004

Location:	$CALDB/data/chandra/hrc/gaplookup/
Filename:	hrcsD1999-07-22gaplookupN0004.fits

The CXC HRC-S calibration team has released a correction to the existing GAPLOOKUP table version 3 in the CalDB (released in 27 June 2012 with CalDB 4.5.0), which had a small offset in positions of photons near the aimpoint. This new version N0004 degap file listed above rectifies the issue, by including a shift of +2.5 pixels (approximately 0.018 Ang) to all degap corrections (in CRSV) near the aimpoint position. The new degap solution ameliorates a wavelength shift as seen from tests run on Capella LETGS+HRC-S observations. A full explanation of the new degap correction is given one the HRC Calibration Web page at DEGAP Update for HRC-S2

See the why page for more information on the degap solution in general. For details on this newest GAPLOOKUP table release, see the HRC-S DEGAP Technical Details section below.

PIPELINES/TOOLS AFFECTED:

HRC Level 1 pipeline tool hrc_process_events

CIAO script chandra_repro, which uses hrc_process_events to reprocess HRC observation data.

THREADS AFFECTED:

Reprocessing Data to Create a New Level=2 Event File

C. 2016 T_GAIN Provisional Files RETIRED

Location:	$CALDB/data/chandra/acis/t_gain/
Filenames:	acisD2016-02-01t_gain_biN0002.fits acisD2016-02-01t_gainN0002.fits acisD2016-11-01t_gain_biN0002.fits acisD2016-11-01t_gainN0002.fits

The above provisional files (flat, non-interpolating) are being deleted from the CalDB distribution with version 4.7.8, and removed from the CalDB ACIS branch index file. These have been in CalDB distribution for more than one year. If users' data file headers show this file as the TGAINFIL value, we suggest they use an upgraded CalDB such as the current release to reprocess their data with the appropriate permanent (finalized) T_GAIN CalDB file for their particular datasets.

PIPELINES/TOOLS AFFECTED:

None.

THREADS AFFECTED:

None.

III. TECHNICAL DETAILS

A. ACIS T_GAIN Epochs 71+72 Updates

The ACIS T_GAIN epoch 71+72, combining two quarterly epochs, and covering the period of Aug 2017 through Jan 2018 has recently been completed, and the ACIS External Calibration Source (ECS) radiation zone data has been reduced as per some modified procedures, to generate the time-dependent gain correction tables for this epoch. The procedural modifications were made to address the decreasing statistics available from the ECS due to its natural decay in count rates. The two epochs are being combined to produce a single new T_GAIN correction update.

The original method of derivation for the ACIS T_GAINs is given in the memorandum "Corrections for time-dependence of ACIS gain", posted on ACIS Calibration web page ACIS TGAIN.

The magnitudes (in eV) of the new gain corrections, versus photon energy, are given in Figs. 1-3 below. The corrections are of the usual order in magnitude, specifically less than 1.0% of the photon energy value. Fig. 1 below gives the corrections for the ACIS-I aimpoint on chip ACIS-3, for the CTI-corrected case, which is the only one applicable to FI chips. Figure 2 gives the corrections for the ACIS-S aimpoint on ACIS-7, for the case where the BI chips are CTI-corrected. Finally Fig. 3 gives the corrections for ACIS-7 for NON-CTI-corrected BI chips. These would be relevant to GRADED DATAMODE observations with ACIS-S, for example.

Fig. 1: Time dependent gain corrections versus photon energy for the ACIS-I aimpoint on ACIS-I3 (CHIPX=970, CHIPY=975) for the new and recent comparable T_GAIN files in CalDB. The parallel CTI correction for the FI chips has been applied to all data.

Fig. 2: Time-dependent gain corrections versus energy for the ACIS-S aimpoint on ACIS-7 (CHIPX=210, CHIPY=520) for the new and recent comparable CalDB T_GAIN files. For these data, the series and parallel CTI correction for the BI chips has been applied.

Fig. 3: T_GAIN correctons versus energy for the ACIS-S aimpoint on ACIS-7, this time with no CTI correction applied to the data, as would be the case for GRADED DATAMODE observations.

Long term trends for chips I3 and S3 are plotted below in Figures 4a and b, through Epoch 71+72, 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, as with Figs 1 and 2 above. The full CTI correction has been applied to the data in both figures.

Fig. 4a: Long-term T_GAIN correction trends at the ACIS-I3 aimpoint, for specific ECS line energies, from Epoch 1 to Epochs 71+72.

Fig. 4b: Long term T_GAIN correction trends at the aimpoint of ACIS-S3, for specific ECS line energies, from Epoch 1 to Epochs 71+72.

B. HRC-S GAPLOOKUP Version N0004

The HRC-S calibration team has been aware of a small residual error in photon positions near the aimpoint of the central plate of the detector (HRC-S2) since it was released in 2012. Measurements of proper-motion corrected positions of cosmic sources AR Lac and HZ43 show a ~0.5 arc second offset along the dispersion direction (along the length of the plate), which is much larger than the 0.1-0.2 arcsecond shifts seen on HRC-I for the same sources.

Recently, a correction that employs a +2.5 pixel (approximately 0.018 Ang) correciton in the degap solution for CRSV between 95 and 102. The updated degapping solution ameliorates the wavelength shift as seen from tests run on Capella LETGS+HRC-S observations.

The CXC Calibration web page Degap Update for HRC-S2 includes a thorough explanation of the changes made in this new HRC-S DEGAP solution.