  The injection of ultra-high energy cosmic rays in the intergalactic medium
leads to the production of a GeV-TeV gamma-ray halo centered on the source
location, through the production of a high electromagnetic component in the
interactions of the primary particles with the radiation backgrounds. This
paper examines the prospects for the detectability of such gamma ray halos. We
explore a broad range of astrophysical parameters, including the inhomogeneous
distribution of magnetic fields in the large scale structure as well as various
possible chemical compositions and injection spectra; and we consider the case
of a source located outside clusters of galaxies. With respect to the gamma-ray
flux associated to synchrotron radiation of ultra-high energy secondary pairs,
we demonstrate that it does not depend strongly on these parameters and
conclude that its magnitude ultimately depends on the energy injected in the
primary cosmic rays. Bounding the cosmic ray luminosity with the contribution
to the measured cosmic ray spectrum, we then find that the gamma-ray halo
produced by equal luminosity sources is well below current or planned
instrument sensitivities. Only rare and powerful steady sources, located at
distances larger than several hundreds of Mpc and contributing to a fraction >
10% of the flux at 10^19 eV might be detectable. We also discuss the gamma-ray
halos that are produced by inverse Compton/pair production cascades seeded by
ultra-high energy cosmic rays. This latter signal strongly depends on the
configuration of the extragalactic magnetic fields; it is dominated by the
synchrotron signal on a degree scale if the filling factor of magnetic fields
with B > 10^(-14) G is smaller than a few percents. Finally, we discuss briefly
the case of nearby potential sources such as Centaurus A.
