A. Vikhlinin, R. Edgar, N. Schulz
July 30, 2004
There is a secular drift of the average PHA values for photons of a fixed energy E.
This drift is caused primarily by gradual changes of the CTI in ACIS CCDs and also
due to electronic drift in I2. These percent-level changes in gain are very
important for the scientific analysis. However, the gain changes are sufficiently
small so that possible modifications of the shape of the ACIS spectral
response can be neglected. This opens a possibility for a simple but sufficiently
accurate correction: a pre-computed time- and energy-dependent shift can be added
to the PHA values and the existing gain table is applied to the modified PHA's to
recompute the photon energies and PI channels. Existing RMF calibration can be used
with the corrected data.
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