Date: Wed, 15 Oct 2003 14:36:23 +0100 (BST)
From: Phil Evans <email@example.com>
Subject: Background scalings
I am trying to examine phase-resolved EPIC spectroscopy, where the source
spectra are extracted using a (PHASE in [x:y]) type selection expression,
a PHASE column having been created in the eventlist using PHASECALC.
I am not slecting the background spectrum based on phase, so as to
maintain a decent S/N ratio for the background.
However, while the EXPOSURE key-value for the background file appears to
be more-or-less the exposure time, the phase-resolved source regions have
exactly the same value as this, and as each other, despite covering
different phase ranges.
As far as I can tell therefore, XSPEC will assume that the source and
background spectra have the same time exposures, and will thus
oversubtract the background. If I manually calculate the exposure time for
each source as (totaltime * phasecoverage), and set the EXPOSURE value to
these, will this result in an accurately subtracted background?
Astrophysics Research Student.
Keele University Astrophysics Group
Tel: (01782) 583530