From b58ef319098ed02711ba53f29e0c61592a291a2c Mon Sep 17 00:00:00 2001 From: Jürgen Knödlseder Date: Wed, 1 Jul 2015 14:06:09 +0200 Subject: [PATCH] Add note about the effect of fitting pivot energies. --- doc/source/issues.rst | 16 ++++++++++++++++ 1 file changed, 16 insertions(+), 0 deletions(-) diff --git a/doc/source/issues.rst b/doc/source/issues.rst index 6685683..389bacf 100644 --- a/doc/source/issues.rst +++ b/doc/source/issues.rst @@ -6,6 +6,7 @@ Known issues Below you will find a list of known issues. - :ref:`Broken power law has unreliable errors ` +- :ref:`Errors become unreliable when fitting the pivot energy ` .. _issues_bplaw: @@ -17,3 +18,18 @@ Below you will find a list of known issues. and this is related to the fact that the law's gradient is discontinous in energy. There is not very much we can do about it, it's inherent in the law. + +.. _issues_pivot: + +.. topic:: Errors become unreliable when fitting the pivot energy + + The spectral ``PowerLaw``, ``ExpCutoff`` and ``LogParabola`` models + have a pivot energy, specified by the ``Scale`` parameter, and this + pivot energy can not be determined in a fit together with the other + model parameters. The reason is that the pivot energy is not an + independent parameter of these models, and hence when all other + spectral parameters are free, the pivot energy is unconstrained. + So please make sure that the pivot energy is fixed, or fix other + parameters of the model to assure non-degeneracy of the free + parameters. + -- libgit2 0.21.4