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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.