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

52 CHAPTER 5. BAT ANALYSIS PROCEDURES
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# batcelldetect v1.60
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...
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Found 1 Images (and selected 1 images)
Analyzing Image: 1
Detection Iteration 1
Found 21 cumulative pixels
Detection Iteration 2
Found 21 cumulative pixels
Detected 1 sources
# RAcent DECcent POSerr Theta Phi Peak Cts SNR Name
320.5664 +77.0719 0.0069 33.0 114.3 5.0688 28.2 UNKNOWN
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batcelldetect works in two passes. In the first pass, it scans for groups of pixels above the noise
level. The “Found NN cumulative pixels” messages shows that batcelldetect found some strong
excess pixels. In the second pass, batcelldetect fits a point spread function to each excess in order
to refine the position and intensity. Finally, it prints a report of the detected sources.
In this particular case, batcelldetect found one source. The “RAcent,DECcent” columns give
the best-fit position in celestial coordinates. Theta and Phi are the position in BAT instrumental
coordinates. “Peak Cts” gives the best-fit peak intensity. The name is “UNKNOWN” because we
performed a blind search with no input catalog.
SNR gives the local signal to noise ratio for that source. In this case the signal to noise ratio is
28.2, indicating a highly significant detection. Please note that the position centroid uncertainty
depends systematically on the signal to noise ratio, as documented in the BAT calibration reports.
In some cases, the formal statistical centroid error will be too small. Scientists should use the
guidance in the calibration reports in order to determine a more conservative position error.
The region file, called preslew.reg in this example, can be loaded into an image viewer like DS9
to highly the detected sources. Highly significant sources are highlighted in green. Weak sources
appear in blue.
The Output Catalog
While some simple diagnostic data is printed to the console, a much larger amount of information is
saved in the output catalog. This file contains a detailed accounting of every image excess analyzed
by batcelldetect. More information about the contents of this file can be found in the documentation
for batcelldetect.
Lowering the Threshold?
The significance threshold in the example above was “5” meaning that an excess must be at least
5 sigma above the local noise level to be detected. Knowing that the single-trial probability of
detecting a >5 sigma excess by chance is less than 10 -6 , users might be tempted to lower the
threshold. However, users should bear in mind that there are of order 300000 independent pixels in