Proton-proton bremsstrahlung below and above pion-threshold: the   influence of the $\Delta$-isobar
  The proton-proton bremsstrahlung is investigated within a coupled-channel model with the $\Delta$ degree of freedom. The model is consistent with the $NN$ scattering up to 1 GeV and the $\gamma N\Delta$ vertex determined in the study of pion photoproduction reactions. It is found that the $\Delta$ excitation can significantly improve the agreements with the $pp \rightarrow pp\gamma$ at $E_{lab}=280$ MeV. Predictions at $E_{lab}=550$ and $800$ MeV are presented for future experimental tests. 
F. de Jong, K. Nakayama and T.-S. H. Lee