Semi-Analytic Model for Intergalactic Gamma-Ray Cascades in Extragalactic Magnetic Fields

TL;DR

This paper presents a semi-analytic model for gamma-ray cascades in intergalactic space, accounting for magnetic field effects, and applies it to observational data to constrain the strength of extragalactic magnetic fields.

Contribution

It introduces a novel semi-analytic approach to model gamma-ray cascades considering magnetic deflections and applies it to real observational data for EGMF constraints.

Findings

Lower limits on EGMF strength derived from gamma-ray observations.

Model successfully explains gamma-ray halo features around blazars.

Constraints depend on source livetime assumptions.

Abstract

Primary gamma rays emitted by extragalactic sources, such as blazars, will generate electromagnetic cascades in intergalactic space. These cascades proceed via electron-positron pair production and inverse Compton scattering on cosmic background radiation, mainly the cosmic microwave background (CMB) and extragalactic background light (EBL) fields. The existence of an extragalactic magnetic field (EGMF) could deflect electron-positron pair trajectories and scatter the cascade photons, possibly creating a halo around the source while suppressing the cascade flux collected by a detector. We develop a semi-analytic model for the cascade process and apply it to combine GeV-TeV data on high-frequency-peaked Bl Lacertae objects (HBLs) from the Fermi Large Area Telescope (LAT) and ground-based Cherenkov telescopes, comparing observation results with model predictions using a robust statistical framework. Lower limits with different confidence levels on the field strength of the EGMF derived from this procedure are discussed under various assumptions about the source livetime.