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We study the characteristic size and shape of idealized blazar-inducedcascade halos in the $1-100 \, {\rm GeV}$ energy range assuming variousnon-helical and helical configurations for the intergalactic magnetic field(IGMF). While the magnetic field creates an extended halo, the helicityprovides the halo with a twist. Under simplifying assumptions, we assess theparameter regimes for which it is possible to measure the size and shape of thehalo from a single source and then to deduce properties of the IGMF. We findthat blazar halo measurements with an experiment similar to Fermi-LAT are bestsuited to probe a helical magnetic field with strength and coherence lengthtoday in the ranges $10^{-17} \lesssim B_{0} / {\rm Gauss} \lesssim 10^{-13}$and $10 \, {\rm Mpc} \lesssim \lambda \lesssim 10 \, {\rm Gpc}$ where$\mathcal{H} \sim B_0^2 / \lambda$ is the magnetic helicity density. Strongermagnetic fields or smaller coherence scales can still potentially beinvestigated, but the connection between the halo morphology and the magneticfield properties is more involved. Weaker magnetic fields or longer coherencescales require high photon statistics or superior angular resolution.

Morphology of blazar-induced gamma ray halos due to a helical intergalactic magnetic field