Add link to radiation model

recipes/fitting-with-gamera/fitting_with_gamera.ipynb

@@ -81,7 +81,7 @@
     "\n",
     "Below is the implementation of the `GameraSpectralModel` class. It is a subclass of `SpectralModel`. With every evaluation, it evolves a population of electrons in a time-independent environment (i.e. fixed magnetic field and radiation field), takes the output electron spectrum and calculates the gamma-ray spectrum from this.\n",
     "\n",
-    "As the radiation field, we use the model from [Popescu et al. 2017](https://doi.org/10.1093/mnras/stx1282) together with the CMB spectrum, in this case evaluated at the position of the crab nebula. Any other model or radiation spectrum is also possible. It is read here from a txt file with the energies and energy densities of the radiation.\n",
+    "As the radiation field, we use the model from [Popescu et al. 2017](https://doi.org/10.1093/mnras/stx1282) together with the CMB spectrum, in this case evaluated at the position of the crab nebula. The data of the model can also be found [here](http://cdsarc.u-strasbg.fr/viz-bin/Cat?J/MNRAS/470/2539#/browse). Any other model or radiation spectrum is also possible. It is read here from a txt file with the energies and energy densities of the radiation.\n",
     "\n",
     "The model has a number of parameters related to the modelled source. Many of these should not be fit, but just be set at the model instantiation and frozen. \n",
     "\n",