The SWIFT BAT Software Guide Version 6.3 30 ... - HEASARC - Nasa

5.5. MASK WEIGHTING (RAY TRACING) 35
5.4.9 What to do if no detector enable/disable map is present
Detector enable/disable maps are periodically produced by the BAT flight software to indicate
which detectors have been disabled in flight. When noisy detectors appear and are detected by
the flight software, they are automatically disabled to prevent them from interfering with the BAT
triggering process.
It is important to understand that the detector enable/disable map production is not synchronized
with the observing plan. The map may have been produced during a previous observation.
Normally the SDC retrieves the proper map for GRB data and includes it with the data set.
If there are processing errors, or for non-GRB observations, the detector enable/disable maps may
be missing. If that is the case, then the map must be found manually from the trend data.
Search in the BAT trend data by month. See /swift/data/trend/YYYY MM/bat/bdetflag on
the HEASARC FTP site.
You want the next earliest detector enable/disable map produced before the event data in
question. The files are listed by time (expressed as mission elapsed time, or MET), so one needs
to scan the list until the right time is found.
First, determine the time of the data you are interested in. Usually you can look at the TSTART
keyword,
ftlist sw00145675000bevshsp_uf.evt.gz K | grep TSTART
which produces the following
TSTART = 142921443. / Start time
A quick scan of the trend directory in /swift/data/trend/2005 07/bat/bdetflag shows that
the next earliest detector enable/disable map is swt0142921337bdecb.fits.gz. In this case the
time listed in the file name is about 106 seconds before the start of the event data.
It is not vitally important to get exactly the right detector map. Having a few detectors
incorrectly labeled as enabled or disabled will not substantially alter the analysis.
5.5 Mask Weighting (Ray Tracing)
Mask weighting (also known as ray-tracing) is the fundamental operation required to produce
background-subtracted light curves and spectrum from the BAT. This section shows you how to
check the event file for how it was ray-traced; how to make a new set of mask weights; how to make
an auxiliary ray tracing file. It is important to stress that the mask weighting approach works for
one source at a time. If you are interested in several sources in the same observation, then you
should make one copy of the event file for each source, and perform the mask weighting calculation
for each separately.
5.5.1 Introduction
Mask weighting is required for all BAT light curve and spectral analysis. This is the first operation
needed before background-subtracted light curves or spectra can be analyzed.