Neutron-proton bremsstrahlung: One-pion-exchange current

The order ${\mathrm{eg}}^{ 2}$ and the ${\Delta }_{33}$ excitation parts of the one-pion-exchange neutron-proton electromagnetic current have been included in a calculation of the $\mathrm{np}$ bremsstrahlung cross section. The order ${\mathrm{eg}}^{ 2}$ exchange current was found to have a large effect. We have shown that the one-pion-exchange current operator can be very well approximated by its zero-photon-energy limit for the existing experiments. By comparison with other work which uses this same approximation but which includes all nonlocal and exchange terms of the potential, we have concluded that contributions to the two-body current other than one-pion exchange are also important.