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

36 CHAPTER 5. BAT ANALYSIS PROCEDURES
Figure 5.1: Schematic diagram of mask weighting operation.
The BAT has a coded-aperture imaging system, which means that does not contain any focussing
optics, and point sources on the sky are not imaged to a single point on the BAT detector
array. BAT imaging and spectroscopy involves cross-correlating the detected shadow pattern with
the expected shadow pattern. The first step in this process for GRB analysis is to determine the
degree of shadowing for each event recorded during the GRB via ray tracing, and convert shadowing
fraction into a weight. At a later step, these weights can be summed to produce a light curve
or spectrum.
BAT mask weights are only valid for a given position on the sky! If you are interested in a
different (or revised) position, then you must re-run the mask weighting.
5.5.2 Reasons to Re-run the Mask Weighting
The Swift Data Center runs the mask weighting tool in the automated pipeline. In principle, you
will not need to re-run it yourself. However, in practice, there are several reasons you may want to.
• You have a refined position for the GRB. Mask weighting is only valid for one position on
the sky, so if you have a new position, you need to re-calculate the weights.
• You are interested in another source than the GRB.
• To capture improvements in the newer software (see below).
• To make an auxiliary raytracing file.
Checking the event file for the position
You can check the event file to see if it was mask-weighted with the proper position of the GRB.
Run the command:
fkeyprint sw00145675000bevshsp_uf.evt.gz BAT_
You should see the following results (among other entries):
BAT_RA = 320.5402 / [deg] Right ascension of source
BAT_DEC = 77.0751 / [deg] Declination of source
If these lines do not exist, or they contain the incorrect position, then you will need to re-run
the mask-weighting task.