The ACIS background consists of a relatively soft Cosmic X-ray Background contribution and cosmic ray-induced events with a hard spectrum. These components are changing very slowly with time and can be predicted well (although they cannot be separated at present). There is an occasional strong third component producing background flares.

There are many resources on the CXC website about the ACIS background and how it should be included in the analysis:

• ACIS Background Memos from the Calibration group
• Maxim Markevitch's ACIS background pages
• CIAO thread: Using the ACIS "Blank-Sky" Background Files