background

From: Maurice Leutenegger <Maurice.A.Leutenegger_at_email.domain.hidden>
Date: Tue, 24 Feb 2009 15:32:43 -0500
Hi,

I have a few questions about the treatment of background in isis that 
are not totally clear to me from the manual:

1. Suppose I have a spectrum with corresponding response and background 
files, and I use "define_back" to load the background file and associate 
it with the spectrum.

What does this do? Does it subtract the background from the data before 
a model is fit to it, like xspec? Or does it add onto the model in lieu 
of an explicitly modeled background? If the latter is true, how does the 
fit algorithm account for the statistical error on the background?

Also, is "define_back" sufficient to include a treatment of the 
background in spectral fitting, or must further steps be taken during 
the fitting process?

2. I can load a type I pha spectrum with corresponding type I pha 
background (e.g. XMM RGS data). What about HETGS data? The background 
information is contained in the columns BACKGROUND_UP and _DOWN of the 
main type II pha spectrum file, or it can be split off into a separate 
type II file. But "define_back" only takes type I pha files. Does this 
mean I need to split off the background into type I pha files? Or is 
there a more elegant way to do this?

3. How can I plot a background subtracted spectrum?


I also have one other unrelated question. When I load XMM RGS data into 
isis, I can only plot it in keV. _isis_plot.x_unit does not seem to 
affect this. How can I change back to Angstrom?


Thanks,

Maurice
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Received on Tue Feb 24 2009 - 15:59:02 EST

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