Generate a spectral energy distribution for a source

What you will learn

You will learn how to use :ref:`ctlike` to derive the spectral energy density of a source from Fermi-LAT data.

So far the :ref:`csspec` script does not work for Fermi-LAT data, but you will learn here a method that you can use to derive the spectrum of any source using Fermi-LAT data. For this you first need a little Python code to transform the spectral component of the Vela source in the :ref:`model definition file <glossary_moddef>` from a super exponentially cut-off power law into a node function. You do this as follows:

>>> import gammalib
>>> models   = gammalib.GModels('vela_results.xml')
>>> energies = gammalib.GEnergies(10, gammalib.GEnergy(100.0, 'MeV'), gammalib.GEnergy(100.0, 'GeV'))
>>> nodes    = gammalib.GModelSpectralNodes()
>>> for energy in energies:
>>>     nodes.append(energy, models['Vela'].spectral().eval(energy))
>>> for par in nodes:
>>>     if 'Energy' in par.name():
>>>         par.fix()
>>>     if 'Intensity' in par.name():
>>>         par.min(1.0e-30)
>>> models['Vela'].spectral(nodes)
>>> models.save('model_nodes.xml')

This will create a :ref:`model definition file <glossary_moddef>` with the following structure:

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<source_library title="source library">
  <source name="Vela" type="PointSource">
    <spectrum type="NodeFunction">
      <node>
        <parameter name="Energy" value="100" scale="1" free="0" />
        <parameter name="Intensity" value="7.40280399439422e-08" error="0" scale="1" min="1e-30" free="1" />
      </node>
      <node>
        <parameter name="Energy" value="215.443469003188" scale="1" free="0" />
       <parameter name="Intensity" value="2.27587615163047e-08" error="0" scale="1" min="1e-30" free="1" />
      </node>
      ...
</source_library>

Instead of the super exponentially cut-off power law the model now contains a node function with ten intensity nodes between 100 MeV and 100 GeV, and the :ref:`ctlike` tool will now adjust the intensities of each of the nodes which will result in a spectrum. To do this type now:

$ ctlike
Input event list, counts cube or observation definition XML file [obs.xml]
Input model definition XML file [models.xml] model_nodes.xml
Output model definition XML file [vela_results.xml] vela_nodes_results.xml

This will produce the :ref:`model definition file <glossary_moddef>` vela_nodes_results.xml with the intensities of the fitted source at each energy. The results are displayed graphically below.

Note

The NodeFunction spectral models implements a piece-wise power law that is defined by the intensity values at each node.