Pile-up Mitigation

Although there are a number of observing strategies that will raise the threshold flux level at which pile-up becomes a significant problem [ASC1997], some piled-up ACIS observations will nevertheless be performed. We hope that an algorithm for correcting a piled-up spectrum will eventually be developed. In the meantime, however, we are investigating simple analysis strategies that will separate reliable events from corrupted events, allowing the observer to discard the corrupted ones.

Here we describe an obvious technique suitable for isolated point sources: Since pile-up is related to the surface brightness of the incident photon distribution, an event's position in the source PSF is related to the probability that the event is corrupted. So, one can reduce pile-up effects to any desired level by discarding a sufficient number of events from the PSF core.

We choose to discard events by applying a circular mask centered on the PSF, as depicted in Figure 6.41. Our investigation explores the relationships between the photon flux level, the size of the mask, and the magnitude of the spectral distortion remaining in the unmasked events. Since both the CCD behavior and the AXAF PSF are energy dependent, we simulated on-axis (aimpoint of the I-array) monochromatic point sources at three different energies - Oxygen (525 eV), Aluminum (1.49 keV), and Copper (8.03 keV).
 
 

Figure 6.41: Mask applied to aspect-corrected events.

 

• Simulation Results
• A Recipe for Pile-up Mitigation?
• Comments on the Use of Masking