Pion photoproduction in an isobar consistent model

The pion photoproduction amplitude is calculated within an isobar model already used to determine the Δ++(1232 MeV) parameters in pion-nucleon scattering. This amplitude is expressed in terms of physical on-shell quantities, and the contribution involving the off-shell behavior of the final state interaction is isolated as principal value integrals on the nonpolar pion-nucleon K-matrix. Different form factors to regularize these integrals are proved, we getting a model independent determination of the couplings GM and GE, which is consistent with the modern quark models for the resonance.