The contributions from meson exchange and isobar excitation currents to proton-proton bremsstrahlung are discussed within the framework of a relativistic $\mathrm{NN}$ interaction. Below the pion-production threshold the Δ isobar is shown to give the dominant contribution to the two-body currents. The nonrelativistic and static limits to these current operators are discussed, and are shown to give a good approximation to the full relativistic meson-exchange current in the considered kinematic region. For the Δ isobar the static limit is a poor approximation. Including, however, also the energy dependence of the Δ propagation yields a reasonable representation of the isobar excitation current.

• Received 13 February 1998

DOI:https://doi.org/10.1103/PhysRevC.58.686