Chandra X-Ray Observatory (CXC)

ACIS time-dependent QEU

ACIS calibration team

December 2012


We present a set of time-dependent QEU files for ACIS. These files were released in CALDB 4.5.5 (December 2012) and are included in the PIMMS Cycle 15 QE files for ACIS. The files are produced at two year intervals. There is a set appropriate for FAINT/VFAINT mode data on all chips and a set for GRADED mode data on the back-illuminated chips (S1 and S3). This page documents the creation and testing of the new QEU files.

QE decline and modeling

The time-dependent QEU files were created using External Calibration Source (ECS) data at two-year epochs. Details on the QEU modeling methods can be found in this memo.

The following plot shows the percentage QE loss over time at the aimpoint regions for S3 (with and without CTI correction) and I3:

The ratio images below show the changes in QE from 2000 to 2012 for I3 at 2.15 keV (top) and S3 at 3.16 keV (bottom) for CTI-corrected data.

The following ratio image shows the changes in QE from 2000 to 2012 for S3 with no CTI correction at 3.16 keV:

Testing the new files

To test the new QEU files for CTI-corrected data, we generate ARFs for ACIS-S3 and I3 subarray observations of SNR G21.5-0.9 using the previous QEU file (acisD2000-01-29qeuN0006.fits) and the new QEU files (acisDyyyy-mm-ddqeuN0007.fits), and fit with an absorbed powerlaw model. This PDF shows a comparison of best-fit parameters with the previous and new QEU. (The results with the new QEU are referred to as "test QEU" in the plot legends.) As expected, the fluxes (in all but one case) increase slightly when the new time-dependent QEU files are used.

Last modified: 12/12/12

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