ACIS Calibration Issues in Progress

                Richard J. Edgar
                edgar@head-cfa.harvard.edu

The low energy response of the back-illuminated chips, (especially S3) is under investigation on several fronts:

• Energy scale. Off-axis pointings with the LETG/ACIS-S placed the low-energy part of the spectrum of PKS 2155 at various places on the S3 chip. An analysis of these is here: http:Energyscale_120.html This shows a ~20eV offset between mean pulse heights and true energies for energies between about 400 and 1000 eV, and larger differences due to the event threshold at lower energies.
• Response shape. The entire BI response matrix set is being reworked based on the MIT physical CCD model (by Gregory Prigozhin). We are fitting the simulated resulting event list pulse height distributions, including the main peak, tails, Si K escape peak, fluorescence peak, etc. (Previous efforts included just 2 Gaussians: the main peak and the escape peak.) This will produce a definitive response for the BI chips in the spring of 2001.
• Quantum Efficiency. There is some evidence (ref: Herman Marshall's page http://space.mit.edu/ASC/calib/letg_acis/ck_cal.html) that the S3/S2 sensitivity ratio is larger than we have advertised by 10-20% in the energy regime below 1 or 1.2 keV. We are investigating this by several methods, including flight grating spectra of multiple AGN and QSO targets, re-analysis of low-energy ground calibration data, and fitting of spectra from other flight targets (notably hot clusters of galaxies). It's difficult to disentangle this effect from the (to date poorly modeled) asymmetries in the line shape function at low energies. Re-analysis of ground calibration data will be required to decide which (if either) of the QE curves is the correct one. This is an ongoing topic of debate within the calibration group and the PI team, and may not be resolved until summer of 2001 (or later).

Also in work:

• Fix Bad Pixel memo on the web to describe what's in the CALDB 2.0 release for -120 C
• Make a Bad Pixel list for -110 C
• Rework web page to make it easier to find descriptions of CALDB products and files.

Last revised: Fri Jan 12 13:57:03 EST 2001