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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 - background flares, which can seriously affect the scientific value of an observation because the count rate can increase by a factor of up to 100.

The ACIS calibration team has completed a detailed study of the ACIS background. Part of this study has produced a set of "blank-sky" datasets which can be used, for instance, for analyses where the object of interest fills the entire chip.

Resources about the ACIS background and how it should be included in the analysis:

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

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