We calculate the vacuum energy of a spinor field in the background of a
Nielsen-Olesen vortex. We use the method of representing the vacuum energy in
terms of the Jost function on the imaginary momentum axis. Renormalization is
carried out using the heat kernel expansion and zeta functional regularization.
With this method well convergent sums and integrals emerge which allow for an
efficient numerical calculation of the vacuum energy in the given case where
the background is not known analytically but only numerically. The vacuum
energy is calculated for several choices of the parameters and it turns out to
give small corrections to the classical energy.Comment: 22 pages, 6 figure