  We propose a novel a posteriori error estimator for conforming finite element
discretizations of two- and three-dimensional Helmholtz problems. The estimator
is based on an equilibrated flux that is computed by solving patchwise mixed
finite element problems. We show that the estimator is reliable up to a
prefactor that tends to one with mesh refinement or with polynomial degree
increase. We also derive a fully computable upper bound on the prefactor for
several common settings of domains and boundary conditions. This leads to a
guaranteed estimate without any assumption on the mesh size or the polynomial
degree, though the obtained guaranteed bound may lead to large error
overestimation. We next demonstrate that the estimator is locally efficient,
robust in all regimes with respect to the polynomial degree, and asymptotically
robust with respect to the wavenumber. Finally we present numerical experiments
that illustrate our analysis and indicate that our theoretical results are
sharp.
