Unitarity constraint for threshold coherent pion photoproduction on the deuteron

The contribution of the two-step process γd-->pn-->π⁰d to the imaginary part of the amplitude for coherent pion production on the deuteron is calculated at threshold exploiting unitarity constraints. The result shows that this absorptive process is not negligible and has to be considered in an extraction of the elementary neutron production amplitude from the γd-->π⁰d cross section at threshold.