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

5.9. DETECTING SOURCES IN BAT IMAGES 53
a typical BAT sky image, which results in about 300000 trials. Thus, the true chance probability
of getting a >5 sigma fluctuation somewhere in the image is 10-20%. This result also means that if
you look at enough images (say 5-10 images), you will get a >5 sigma excess just by chance. Users
should always carefully account for the number of trials appropriately in assessing the significance
of a particular excess, and generally should never lower the threshold below 5 sigma.
5.9.4 Finding the Intensities of Known Sources
In addition to performing a blind search, batcelldetect can also measure the intensities of sources
at known positions. No matter what the intensities are at those known positions, batcelldetect will
report them (they can even be negative, since BAT images have gaussian statistics).
Let us suppose we are worried about whether the X-ray binary 4U 0114+65 is interfering
with the measurement of the GRB. Generally speaking bright sources increase the overall noise
level everywhere in the image by about 1% of the bright source’s intensity. Thus, a very bright
source can distort the intensity of another source. Since 4U 0114+65 is about 21 degrees from the
GRB position, it is in the field of view. We can check the the intensities of both sources using
batcelldetect.
Targeted searches are done by specifying an input catalog (or “analysis catalog”). batcelldetect
expects the input catalog to be in a certain format which is easy to create from ASCII files using
a tool like ‘ftcreate’. The catalog must be a FITS file with at least these three columns: NAME,
RA OBJ, DEC OBJ. As one might expect, these are the name and position of the source.
Creating a Catalog from Scratch
Here are some simple steps to create a catalog from scratch. Remember that we are interested in
the flux at the position of 4U 0114+65, which has a known position. Let us first create a text file
with the name and position of this source source and call it ‘incat.dat’:
# Name RA_OBJ DEC_OBJ
’4U 0114+65’ 19.5113 65.2918
Of course, it is straightforward to add more sources to the catalog by adding more lines to this
file. In fact, you can use a large catalog of your favorite sources which might not even be in the field
of view. batcelldetect automatically filters the input catalog to include only those sources which
are in the field of view of the image.
We will use the task ‘ftcreate’ to convert this to a FITS file. ftcreate needs a ’column description’
file. This is an ASCII file which we will call ‘incat.col’:
NAME 30A
RA_OBJ E deg
DEC_OBJ E deg
The three lines of the file correspond to the three columns of the FITS file.
Finally, we create the FITS catalog, ‘incat.cat’, with the following command,
ftcreate incat.col incat.dat incat.cat