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Figure 3.4: Rate-Gain and after-pulse effects in the Crab data.
The top panel shows the power-law fit to the crab data,
with nominal gain. The high count rate of the crab saturates
the PSPC gain.
The lower panel shows the fit after the gain assumed in the
response matrix has been manually lowered by 5%.
The very soft residual in the lower panel is after-pulse
contamination
[Hasinger1993].
At very high count rates, such as from the Crab
(count rates 
800 counts s 
),
the detector gain saturates due to the reduction in the electric field
caused by the pseudo-continuous presence of a substantial electron cloud.
However, it is currently thought that this effect was also important
for the observation of Mkn 421 ( 
150countss 
).
Figure 3.4
shows the effect in the Crab data (top panel).
Here the Crab data have been fit with a power-law of photon index
and 
H Icm 
using a detector response matrix with the nominal gain
and a gain lowered by 5%.
Measurements in the PANTER test facility showed a gain
drop of 1% for a 1.5 keV X-ray count rate of 500 counts s 
, and a
drop of 2% for a 3.0 keV X-ray count rate of 500 counts s 
.
This effect will be further investigated using the PANTER measurements.