Search for Magnetically Broadened Cascade Emission From Blazars with VERITAS

TL;DR

This study searches for magnetically broadened gamma-ray emission from blazars using VERITAS data, aiming to detect effects of intergalactic magnetic fields on cascade emission, and sets limits on magnetic field strengths and extended flux.

Contribution

It provides the first constraints on intergalactic magnetic fields around 10^{-14} G by analyzing angular broadening in gamma-ray cascades from multiple blazars.

Findings

Excluded IGMF strengths around 10^{-14} G at 95% confidence level.

No detection of angularly broadened cascade emission from the studied blazars.

Set limits on the flux of extended gamma-ray emission around the sources.

Abstract

We present a search for magnetically broadened gamma-ray emission around active galactic nuclei (AGN), using VERITAS observations of seven hard-spectrum blazars. A cascade process occurs when multi-TeV gamma rays from AGN interact with extragalactic background light (EBL) photons to produce electron-positron pairs, which then interact with cosmic microwave background (CMB) photons via inverse-Compton scattering to produce gamma rays. Due to the deflection of the electron-positron pairs, a non-zero intergalactic magnetic field (IGMF) would potentially produce detectable effects on the angular distribution of the cascade emission. In particular, an angular broadening compared to the unscattered emission could occur. Through non-detection of angularly broadened emission from 1ES 1218+304, the source with the largest predicted cascade fraction, we exclude a range of IGMF strengths around $10^{-14}$G at the 95% confidence level. The extent of the exclusion range varies with the assumptions made about the intrinsic spectrum of 1ES 1218+304 and the EBL model used in the simulation of the cascade process. All of the sources are used to set limits on the flux due to extended emission.