  The nucleon-nucleon bremsstrahlung reaction is investigated based on a fully
gauge-invariant relativistic meson-exchange model approach. In order to account
consistently for the complicated part of the interaction current (which at
present is too demanding to be calculated explicitly), a generalized contact
current is introduced following the approach of Haberzettl, Nakayama, and
Krewald [Phys. Rev. C74, 045202 (2006)]. The contact interaction current is
constructed phenomenologically such that the resulting full bremsstrahlung
amplitude satisfies the generalized Ward-Takahashi identity. The formalism is
applied to describe the high-precision proton-proton bremsstrahlung data at 190
MeV obtained at KVI [H. Huisman et al., Phys. Rev. C65, 031001(R) (2002)]. The
present results show good agreement with the data, thus removing the
longstanding discrepancy between the theoretical predictions and experimental
data. The present investigation, therefore, points to the importance of
properly taking into account the interaction current for this reaction.
