To compute the significance of a given source you need to add the attribute tscalc="1" to the <source> tag in the model definition file and execute ctlike using this file. The structure of the XML file with significance computation requested for Src001 is shown below.
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<source_library title="source library">
<source name="Src001" type="PointSource" tscalc="1">
<spectrum type="PowerLaw">
<parameter name="Prefactor" value="1" error="0" scale="5.7e-18" min="0" free="1" />
<parameter name="Index" value="1" error="-0" scale="-2.48" min="-4.03225806451613" max="4.03225806451613" free="1" />
<parameter name="PivotEnergy" value="1" scale="300000" free="0" />
</spectrum>
<spatialModel type="PointSource">
<parameter name="RA" value="266.424004498437" error="0" scale="1" free="1" />
<parameter name="DEC" value="-29.0049010253548" error="0" scale="1" free="1" />
</spatialModel>
</source>
...
</source_library>
ctlike will compute the so-called Test Statistic, which is twice the difference between the log-likelihood value obtained for a fit that includes Src001 and a fit that excludes Src001. The Test Statistic value will follow a \(\chi^2_p\) distribution with \(p\) degrees of freedom, where \(p\) is the number of free parameters of Src001. For \(p=1\), the source significance is the square root of the Test Statistic.
An excerpt of the ctlike log file is shown below where signifance computation was requested for Src001 and Src002 and the diffuse Galactic emission component IEM. The computation has been done using a stacked analysis.
2017-03-03T23:36:07: === GModelSky ===
2017-03-03T23:36:07: Name ......................: Src001
2017-03-03T23:36:07: Instruments ...............: all
2017-03-03T23:36:07: Test Statistic ............: 12650.600597267
...
2017-03-03T23:36:07: === GModelSky ===
2017-03-03T23:36:07: Name ......................: Src002
2017-03-03T23:36:07: Instruments ...............: all
2017-03-03T23:36:07: Test Statistic ............: 2154.31897658715
...
2017-03-03T23:36:07: === GModelSky ===
2017-03-03T23:36:07: Name ......................: IEM
2017-03-03T23:36:07: Instruments ...............: all
2017-03-03T23:36:07: Test Statistic ............: 43070.3063685261
The Test Statistic values are also written into the model definition file created by ctlike:
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<source_library title="source library">
<source name="Src001" type="PointSource" ts="12650.601" tscalc="1">
...
</source>
<source name="Src002" type="PointSource" ts="2154.319" tscalc="1">
...
</source>
<source name="IEM" type="DiffuseSource" ts="43070.306" tscalc="1">
...
</source>
...
</source_library>