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Due to contamination of the gas supply, a number of observational data sets are effected by after-pulses (APs). These are low pulse height events (PHA channels less than ) associated with both X-ray photons or particle background events, and occur within approximately 0.35 ms of the primary event (the nominal dead time for a PSPC event is ms). The most likely cause of APs is negative ion formation in the vicinity of the anodes. They have a soft, steep spectrum effecting only PHA channels 15 and below, in which they can contribute up to a few % of the total count rate. The spectrum changes slightly as a function of time, but is similar to a single electron spectrum. The number of AP events also changes as a function of time and this along with its correlation to the PSPC gas tank swaps and other variables is currently under investigation.
Figure 3.4 shows (bottom panel) the effect of the after-pulses on spectra. In this case the crab data have been corrected manually for the rate-gain effect before fitting with a power-law spectrum.
While AP events associated with photons can be rejected (eliminate all events occurring within 0.35 ms of the preceeding event; note that this produces an additional dead time component which must be included), those associated with particle background events cannot, since the precursor event is almost always vetoed by the on-board S/W and hence not included in the telemetry stream. The number of APs appears to be: - worse for high gain datasets, and - worse for harder spectra.
AP events are not always adequately taken into account using standard background subtraction techniques, and the effect can be quite significant for extended sources - see [Snowden et al.1994].