CalDB 4.10.2 Public Release Notes

I. INTRODUCTION

CalDB 4.10.2 is an upgrade to the Chandra CalDB, which includes the following items:

• ACIS Contamination Model (CONTAM) version N0015

For the CIAO 4.14 / CalDB 4.10.2 release notes see How CalDB 4.10.2 Affects Your Analysis.

II. SUMMARY OF CHANGES

A. ACIS Contamination Model (CONTAM) version N0015

Location:	$CALDB/data/chandra/acis/contam/
Filename:	acisD1999-08-13contamN0015.fits

As the ACIS OBF contamination continues to accumulate, periodic adjustments to the model are necessary to keep up with its variation with advancing mission elapsed time. Fig. 1 below gives the optical depth Tau (0.66 keV) at the center of the ACIS-S array from Abell 1795 observations throughout the mission, along with Mkn 421 "big dither" observations since year 2013, compared with the CONTAM model N0014 prediction, which is now coming in at 65% of the July 2022 measured value. Fig. 2 gives the Tau at 1.49 keV as derived from ECS and Mkn 421 "big dither" observations, compared with CONTAM N0014 as well, where the model comes in at about 7% below measured. The ACIS Calibration Team has taken note of these growing discrepancies, and generated a new CONTAM (N0015) model for CalDB. This new model does not change the center-to-edge ratio, affects the model significantly only for 2020 and beyond.

Fig. 1: Contamination layer derived from Abell 1795 and Mkn 421 at the center of ACIS-S versus time in years, showing the previous model (CONTAM N0014) and the recent data from observations, at 0.66 keV. The model is fine up to 2020, but has begun to underestimate since.

Fig. 2: Same as Figure 1, but derived from ECS 1.49 keV data, as well as Mkn 421 observations for the same energy.

See the ACIS QE CONTAM why page for basic information concerning ACIS contamination. For further details on the latest CONTAM N0014 effects on the ACIS aimpoint EAs, see the technical details below.

CIAO SCRIPTS/TOOLS AFFECTED:

See the CIAO scripts as follows:

• fluximage, which uses mkinstmap
• specextract, which uses mkwarf
• fullgarf, which uses mkgarf

THREADS AFFECTED:

For exposure-corrected imaging analysis with ACIS, see the threads:

• Single Chip ACIS Exposure Map and Exposure-corrected Image
• Multiple Chip ACIS Exposure Map and Exposure-corrected Image

For imaging spectroscopy with ACIS, see the threads:

• Extract Spectrum and Response Files for a Pointlike Source
• Extract Spectrum and Response Files for an Extended Source
• Extract Spectrum and Response Files for Multiple Sources

For grating spectroscopy with ACIS, see the threads:

• HETG/ACIS-S Grating ARFs
• LETG/ACIS-S Grating ARFs

III. TECHNICAL DETAILS

A. ACIS Contamination Model (CONTAM) version N0015

The ACIS Calibration Team has generated, tested, and released an upgrade to the CalDB ACIS OBF contamination (CONTAM) model. The new version is N0015, an upgrade to the previous version (N0014) that was released on 16 December 2020 with CalDB 4.9.4. That previous model has been shown to undercorrect for the contamination effect in the most recent calibration source data for both ACIS-I and ACIS-S. The new model adjusts the level and rate of OBF contamination for the years 2020-2022, for the central (aimpoint) positions (tau0 in the FITS file), without changing the center-to-edge (tau1) models.

With no changes required in the tau1 values in the CONTAM file, the process of developing the upgrade was simplified. The "time" column in each of the extensions is the same, as always. However it has been appended to extend until nearly the end of year 2029, and the vector extended by 1666 time values, the last of which is 1009780085.0 (Real8), which corresponds to UTC 2029-12-31T06:26:56. This latter date will be used for the Calibration Validity End Date and Time in the file headers and the resulting ACIS branch index file in CalDB.