Blazar halos as probe for extragalactic magnetic fields and maximal acceleration energy

Abstract

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.