As described in Section 2, the displacement of the PHA distribution of an observation from prelaunch conditions can be compensated by shifting the (ground-based) spectral response matrix accordingly. Calibration sources have been analyzed to quantify the relationship between the response matrix shift and the bright earth gain state. The calibration sources are listed in Table 3. The PHA distribution of each calibration observation was fitted (in XSPEC) with a spectral model derived from PSPC observations, initially using the ground-based (unshifted) response matrix. The normalization was the only free parameter, and a minimum derived. This procedure was repeated using response matrices shifted in channel space (generated using the FTOOL ``gcorrmf''). Multiple response matrices were used with incremental shifts of one-tenth of a channel. These matrices covered the whole range of possible shifts between -1 and +4. This best fit incremental shift was recorded. The corresponding gain state was determined from the bright earth data for this detection position and observation time. From 72 calibration observations, a direct correlation has been shown to exist between the channel shift of the response matrix and bright earth mean PHA channel (Figure 2 and 3).
Figure 2: The weighted bright earth gain level is plotted against the response matrix shift which produces the minimum reduced value. The PHA channels 2-10 were included in the spectral fits.
Figure 3: The weighted bright earth gain level is plotted against the response matrix shift which produces the minimum reduced value. The PHA channels 4-10 were included in the spectral fits.
Table 3: Calibration Sources