The Flight Spectral Response of the ACIS Instrument

Kavli Affiliate: Catherine E. Grant

| First 5 Authors: Paul P. Plucinsky, Norbert S. Schulz, Herman L. Marshall, Catherine E. Grant, George Chartas

| Summary:

We discuss the flight calibration of the spectral response of the Advanced
CCD Imaging Spectrometer (ACIS) on-board the Chandra X-ray Observatory (CXO).
The spectral resolution and sensitivity of the ACIS instrument have both been
evolving over the course of the mission. The spectral resolution of the
frontside-illuminated (FI) CCDs changed dramatically in the first month of the
mission due to radiation damage. Since that time, the spectral resolution of
the FI CCDs and the backside-illuminated (BI) CCDs have evolved gradually with
time. We demonstrate the efficacy of charge-transfer inefficiency (CTI)
correction algorithms which recover some of the lost performance. The detection
efficiency of the ACIS instrument has been declining throughout the mission,
presumably due to a layer of contamination building up on the filter and/or
CCDs. We present a characterization of the energy dependence of the excess
absorption and demonstrate software which models the time dependence of the
absorption from energies of 0.4 keV and up. The spectral redistribution
function and the detection efficiency are well-characterized at energies from
1.5 to 8.0 keV. The calibration at energies below 1.5 keV is challenging
because of the lack of strong lines in the calibration source and also because
of the inherent non-linear dependence with energy of the CTI and the absorption
by the contamination layer. We have been using data from celestial sources with
relatively simple spectra to determine the quality of the calibration below 1.5
keV. The analysis of these observations demonstrate that the CTI correction
recovers a significant fraction of the spectral resolution of the FI CCDs and
the models of the time-dependent absorption result in consistent measurements
of the flux at low energies for data from a BI (S3) CCD.

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