In response to Mallory Robert's email on this topic May 25th, Maxim Markevitch
at CXC has provided the following comments:
"
Dr. Roberts is entirely correct in that the background should be subtracted
from the images before dividing by the exposure maps, and from the spectra
before fitting the models. Just like for ASCA there were night Earth and
blank sky event files, we have accumulated blank-sky datasets (that include
particle and sky background components) for several ACIS temperatures. They
are available as event files. Please see the following web pages:
General discussion of the background:
http://asc.harvard.edu/cal/, sections "ACIS", "ACIS Background"
Blank-field datasets and tools for their use:
http://hea-www.harvard.edu/~maxim/axaf/acisbg/
Examples of their use can be found, e.g., in ApJ 541,542 and
astro-ph/0105093. In particular, the latter paper tries to deal with a
problem with modeling the low-energy part of the background. Nonuniformity
of the soft sky background turns out to be a more serious problem than the
well-predictable hard particle component.
In August-October 2001, we also plan to observe dark Moon to help separate
the soft sky and particle components.
_________________________________________________________________
Maxim Markevitch, CXC maxim@head-cfa.harvard.edu 617-495-7138
"
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Dr. Belinda Wilkes Tel : (617) - 495 - 7268
Harvard/Smithsonian email: belinda@cfa.harvard.edu
Center for Astrophysics fax : (617) - 495 - 7356
60 Garden Street Assistant Director
Cambridge MA 02138 Chandra X-ray Center (CXC)
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