  We apply linear-response analysis to the Gross-Pitaevskii equation to obtain
the excitation frequencies of a Bose-Einstein condensate in a vortex state, and
apply it to a system of Rubidium atoms confined in a time-averaged orbiting
potential trap. The excitation frequencies of a vortex differ significantly
from those of the ground state, and may therefore be used to obtain a
spectroscopic signature of the presence of a vortex state.
