Final state rescattering as a contribution to B->rho gamma

We provide a new estimate of the long-distance component to the radiative transition B-->rho gamma. Our mechanism involves the soft scattering of on-shell hadronic products of nonleptonic B decay, as in the chain B-->rho rho-->rho gamma. We employ a phenomenological fit to scattering data to estimate the effect. The specific intermediate states considered here modify the B-->rho gamma decay rate at roughly the 5-8% level, although the underlying effect has the potential to be larger. Contrary to other mechanisms of long distance physics which have been discussed in the literature, this yields a non-negligible modification of the B-0-->rho(0) gamma channel and hence will provide an uncertainty in the extraction of V-td. This mechanism also affects the isospin relation between the rates for B--->rho(-)gamma and B-0-->rho(0) gamma and may generate CP asymmetries at experimentally observable levels.