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

TL;DR

This paper investigates how the size and shape of gamma-ray halos around blazars can reveal properties of the intergalactic magnetic field, especially its helicity, using observational data in the 1-100 GeV range.

Contribution

It introduces a method to infer the helicity and other properties of the intergalactic magnetic field from the morphology of blazar-induced gamma-ray halos.

Findings

Halo shape is influenced by magnetic helicity, creating a twist.

Optimal detection of magnetic helicity occurs for specific field strengths and coherence lengths.

Measurement feasibility depends on photon statistics and angular resolution.

Abstract

We study the characteristic size and shape of idealized blazar-induced cascade halos in the $1-100 \, {\rm GeV}$ energy range assuming various non-helical and helical configurations for the intergalactic magnetic field (IGMF). While the magnetic field creates an extended halo, the helicity provides the halo with a twist. Under simplifying assumptions, we assess the parameter regimes for which it is possible to measure the size and shape of the halo from a single source and then to deduce properties of the IGMF. We find that blazar halo measurements with an experiment similar to Fermi-LAT are best suited to probe a helical magnetic field with strength and coherence length today 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. Stronger magnetic fields or smaller coherence scales can still potentially be investigated, but the connection between the halo morphology and the magnetic field properties is more involved. Weaker magnetic fields or longer coherence scales require high photon statistics or superior angular resolution.