If a source or region is bright, two or more photons can interact with the same or adjacent pixels during a single frame time (nominally 3.2 s for full frame). This appears as a single detected event but with energy the sum of all interacting-photon energies. Often the recorded energy is high enough to cause the event to be rejected. Pileup leads to:
- distortion of energy spectrum
- distortion of image (A bright point source can appear as a ring with dark central region.)
- underestimation of observed count rate
[ Chandra Proposer's Guide pile up section ]
During frame readout, the ACIS CCDs take 40 microsec to (parallel) transfer the charge by one pixel. As a result of this, each frame readout, each pixel is exposed for 40 microsec to a source (plus cosmic X-ray background) at each location along its column. This is most noticeable if a row contains a bright point source, but on average, all sources along the column contribute to every pixel along the column. Note also that the image does not just trail, it extends along the whole column. For a given source, the part of the column closer to the frame store is exposed during frame readout, and the part further away from the frame store is exposed as the new frame is read in.
[ Chandra Proposer's Guide trailed image section ]