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

Abstract

We study the characteristic size and shape of idealized blazar-induced cascade halos in the $1-100\mathrm{GeV}$ 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 $10^{-17}\lesssim B_0/\mathrm{Gauss}\lesssim 10^{-13}$ and $10\mathrm{Mpc}\lesssim \lambda \lesssim 10\mathrm{Gpc}$ 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.

v3 - 33 pages, 14 figures; reduced file size; matches JCAP version

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• Constraints on primordial magnetic fields from magnetically-induced perturbations: current status and future perspectives with LiteBIRD and future ground based experiments
• Probing Intergalactic Magnetic Fields with Simulations of Electromagnetic Cascades
• Physics of cosmological cascades and observable properties
• Magnetic heating across the cosmological recombination era: Results from 3D MHD simulations
• Bow Ties in the Sky I: The Angular Structure of Inverse Compton Gamma-ray Halos in the Fermi Sky
• Fundamental Implications of Intergalactic Magnetic Field Observations
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• Constraints on the Intergalactic Magnetic Field from Bow Ties in the Gamma-ray Sky
• LiteBIRD Science Goals and Forecasts: Primordial Magnetic Fields
• On the Measurement of the Helicity of Intergalactic Magnetic Fields Using Ultra-High-Energy Cosmic Rays
• Probing stochastic inter-galactic magnetic fields using blazar-induced gamma ray halo morphology
• Searching for primordial helical magnetic fields
• Imprints of the post recombination dissipation of helical magnetic field on the Cosmic Microwave Background Radiation