Search for Magnetically Broadened Cascade Emission from Blazars with VERITAS

Abstract

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