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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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A. Vikhlinin

Last modified: 11/15/10

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