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

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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
54 CHAPTER 5. BAT ANALYSIS PROCEDURES Using the Input Catalog We can use the input catalog in a call to batcelldetect. Issue the following modified command, batcelldetect preslew.img preslew.cat 5 \ pcodefile=preslew.pcodeimg regionfile=preslew.reg \ incatalog=incat.cat \ clobber=YES The command is the same as the previous one, with the addition of the input catalog that we created in the previous step. The Output The output list of sources has now changed. # RAcent DECcent POSerr Theta Phi Peak Cts SNR Name 19.5113 +65.2918 0.0000 23.7 73.5 -0.0397 -0.3 4U 0114+65 320.5664 +77.0719 0.0069 33.0 114.3 5.0688 28.2 UNKNOWN There are two sources listed instead of one. The first source is 4U 0114+65, the source we entered in the input catalog. It has a position error of 0 since it was assumed to be fixed at its known position. The second source is the GRB we already detected with the previous command. It should be obvious that with a significance of -0.3 sigma, 4U 0114+65 is not detected, and therefore it is not interfering with the measurement of the GRB. Since 4U 0114+65 is quite weak, it will appear in DS9 as a blue circle instead of a green one. 5.9.5 Advanced Usage batcelldetect is a complex tool with many options. Many users will be satisfied with the options described here. Users with more advanced needs can consult the batcelldetect documentation for more detailed information.
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vi CONTENTS D.1.6 SEE ALSO . . . .
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viii CONTENTS D.10.2 USAGE . . . .
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x CONTENTS D.18.5 EXAMPLES . . . .
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