  This paper is focused on the study of an inverse problem for a
non-self-adjoint hyperbolic equation. More precisely, we attempt to stably
recover a first order coefficient appearing in a wave equation from the
knowledge of Neumann boundary data. We show in dimension n greater than two, a
stability estimate of H{\"o}lder type for the inverse problem under
consideration. The proof involves the reduction to an auxiliary inverse problem
for an electromagnetic wave equation and the use of an appropriate Carleman
estimate.
