Generate a butterfly diagram for a model component

What you will learn

You will learn how to use :ref:`ctbutterfly` to create a butterfly diagram for a model component from COMPTEL data.

A butterfly diagram shows the envelope of all spectral models that are statistically compatible with the data. You can create such a butterfly diagram for the Crab using the :ref:`ctbutterfly` tool by providing the :ref:`model definition file <glossary_moddef>` output file from :ref:`ctlike` as input:

$ ctbutterfly fit=yes
Input event list, counts cube or observation definition XML file [events.fits] obs.xml
Source of interest [Crab]
Input model definition XML file [$CTOOLS/share/models/crab.xml] models.xml
Start value for first energy bin in TeV [0.1] 0.00000075
Stop value for last energy bin in TeV [100.0] 0.00003
Output ASCII file [butterfly.txt]

:ref:`ctbutterfly` produces the ASCII file butterfly.txt that contains the envelope of the butterfly diagram. The figure below gives a graphical representation of the envelope. The red data points show the combined Crab nebular and pulsar spectrum from Kuiper et al. 2001, A&A, 378, 918 for comparison.