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From: Bram Boroson <bboroson@head.cfa.harvard.edu>
Subject: ONTIME and LIVETIME headers
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Date: Tue, 19 Aug 2008 18:56:51 -0400 (EDT)
From: Bram Boroson <bboroson@head.cfa.harvard.edu>
To: xmmhelp@sciops.esa.int
Subject: ONTIME and LIVETIME headers
Hello,

I am analyzing a PN spectrum. I use "merge" on two XMM EPIC observations,
then apply a GTI filter, and then produce the spectrum.

The ONTIME keywords in the PN event file header give times shorter than
the LIVETIME keywords. Is this ok? If the LIVETIME takes into account
detector deadtime, should LIVETIME be shorter?

The final spectrum file has an EXPOSURE time that is closer to the
LIVETIME. The spectrum does not match the MOS spectra, but it would if the
ONTIME were used instead.

Can you tell me whether the time keywords may be set improperly in my
analysis?

Thank you very much,

Bram Boroson


Reply 1

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From: Matthias Ehle <xmmhelp@sciops.esa.int>
To: bboroson@head.cfa.harvard.edu
Subject: Re: ONTIME and LIVETIME headers (PR#37879)
Date: Wed Aug 20 08:36:51 2008
Hello,

Checking the merge task online-documentation (section Comments), see
http://xmm2.esac.esa.int/sas/8.0.0/doc/merge/node9.html
you will note that special care must be taken using this task together 
with any GTI filtering. 

Otherwiese, I think you have two options:
1) apply the GTI filter first on the individual observations and use merge
afterwards - again a careful checking of the result is needed
2) create spectra from the two observations individually and do your
spectral analysis without merging them (just loading both simultaneously 
in Xspec).

I would recommend using option 2) as keeping the two spectra separately
will not reduce the significance of your spectral fitting and will avoid
any possible problem that 'merge' might introduce wrt to exposure times.

It might also be good to try both options to have a possibility of
cross-checking
your results.

Kind regards,
  Matthias 


> Hello,
> 
> I am analyzing a PN spectrum. I use "merge" on two XMM EPIC observations,
> then apply a GTI filter, and then produce the spectrum.
> 
> The ONTIME keywords in the PN event file header give times shorter than
> the LIVETIME keywords. Is this ok? If the LIVETIME takes into account
> detector deadtime, should LIVETIME be shorter?
> 
> The final spectrum file has an EXPOSURE time that is closer to the
> LIVETIME. The spectrum does not match the MOS spectra, but it would if the
> ONTIME were used instead.
> 
> Can you tell me whether the time keywords may be set improperly in my
> analysis?
> 
> Thank you very much,
> 
> Bram Boroson
> 

--
   Dr Matthias Ehle 
   XMM-Newton User Support Group
   European Space Astronomy Centre

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