Gamma-ray induced cascades and magnetic fields in intergalactic medium

TL;DR

This paper uses Monte-Carlo simulations to study gamma-ray induced cascades in the intergalactic medium, revealing how magnetic fields influence extended gamma-ray emission around extragalactic sources and enabling measurement of weak intergalactic magnetic fields.

Contribution

It introduces a simulation-based method to infer intergalactic magnetic field strengths from gamma-ray cascade morphology and energy dependence.

Findings

Extended gamma-ray emission depends on magnetic field properties.

Size of the emission decreases with increasing energy, allowing magnetic field measurement.

Weak magnetic fields in voids can be constrained to < 10^{-16} G.

Abstract

We present the results of Monte-Carlo simulations of three-dimensional electromagnetic cascade initiated by interactions of the multi-TeV gamma-rays with the cosmological infrared/optical photon background in the intergalactic medium. Secondary electrons in the cascade are deflected by the intergalactic magnetic fields before they scatter on CMB photons. This leads to extended 0.1-10 degree scale emission at multi-GeV and TeV energies around extragalactic sources of very-high-energy gamma-rays. The morphology of the extended emission depends, in general, on the properties of magnetic fields in the intergalactic medium. Using Monte-Carlo simulated data sets, we demonstrate that the decrease of the size of extended source with the increase of energy allows to measure weak magnetic fields with magnitudes in the range from < 1e-16 G to 1e-12 G if they exist in the voids of the Large Scale Structure.