My Observation is Piled Up! Now What?

There are two ways to remove the effects of pile up from an observation:

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Using the region selection and event file filtering procedures demonstrated in earlier sections, you can excise the inner-most regions of a source (as they are the most heavily piled up), re-extract the spectrum, and continue your analysis on the excised event file. For this procedure, it is recommended that you take an iterative approach: remove an inner region, extract a spectrum, check with epatplot, and repeat, each time removing a slightly larger region, until the model and observed distribution functions agree.

and/or

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Using the event file filtering procedures, consider only the pattern 0 events (PATTERN==0). Pattern 0 events are less sensitive to pile up than other patterns.