  High energy photons from blazars interact within tens of kpc with the
extragalactic photon background, initiating electromagnetic pair cascades. The
charged component of such cascades is deflected by extragalactic magnetic
fields (EGMF), leading to halos even around initially point-like sources. We
calculate the intensity profile of the resulting secondary high-energy photons
for different assumptions on the initial source spectrum and the strength of
the EGMF, employing also fields found earlier in a constrained simulation of
structure formation including MHD processes. We find that the observation of
halos around blazars like Mrk~180 probes an interesting range of EGMF strengths
and acceleration models: In particular, blazar halos test if the photon energy
spectrum at the source extends beyond \sim 100 TeV and how anisotropic this
high energy component is emitted.
