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ACIS Background Memos

Last update 27 Dec 2005

ACIS Background Subsections:

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. The memo discusses these components and is a required reading before attempting to model the background by using the quiescent datasets (a link to the datasets is given inside).

To calibrate the ACIS instrumental background in the normal imaging mode, Chandra performed a series of dark Moon exposures in 2001 and observations with ACIS stowed outside the focal area in 2002-2005. This memo describes the two datasets, compares their spectra, and has links to the relevant event files.

ACIS particle background can be reduced significantly compared to the standard grade selection by screening out events with significant flux in border pixels of the 5×5 event islands. The particle background above 6 keV is reduced by a factor of 1.4 in the front-illuminated chips and ~ 1.25 in the back-illuminated chips. The background rejection is much better at soft energies - by a factor of 2 near 0.5 keV in FI chips and by a factor of 3 near 0.3 in BI chips. In the intermediate energies, 1-5 keV, the background is reduced by a factor of 1.1-1.15. Real X-ray photons are practically not affected by such cleaning - only about 2% of them are rejected, independently of the energy band, provided there is no pileup.

We have analyzed ~30 ACIS data sets collected in "Event Histogram" mode in order to characterize the background of the S3 and I3 CCDs produced by high-energy charged particles. We have computed count rates in four energy bands (0.5-2.0 keV, 0.5-7.0 keV, 5.0-10.0 keV, and 0.3-10.0 keV) at the bottom and top of the CCDs. We have determined that the S3 background is flatter, albeit higher, than the I3 background which exhibits a 20% increase from bottom to top. We have created composite background spectra for both CCDs. These spectra have dramatically different shapes and features for the BI and FI CCDs. We have also investigated the time variability of this background and have found that this component varied by ~20% over the 1 1/4 year interval considered. We have also compared the composite background spectra to the background data from sky observations and from the Dark Moon pointings. We find an excellent agreement between Event Histogram spectra and the Dark Moon spectra.

Analysis of the ACIS particle background soon after launch, prior to opening the aft HRMA cover for an FI (S2) and BI (S3) chips. Rates in different spectral bands and event grades, spectral line identification.

Comments/Questions/Changes to Maxim Markevitch (maxim % head.cfa.harvard.edu)

Last modified: 11/15/10

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