There is a flaw in the serial readout of the ACIS chips, causing a significant amount of charge to be randomly deposited along pixel rows as they are read out. Chip S4 (ccd_id=8) is most affected by this problem.

On account of the serial readout problem, the event file shows a variable pattern of linear streaks. These events may be flagged in or removed from the file with the destreak tool, as shown in the Destreak the ACIS-S4 Chip thread.

Technical details on the correction are available in the destreak memo.