Evidence for Gamma-Ray Halos Around Active Galactic Nuclei and the First Measurement of Intergalactic Magnetic Fields

TL;DR

This study provides evidence for gamma-ray halos around active galactic nuclei, indicating the presence of intergalactic magnetic fields, and offers the first measurement of their strength, which has implications for understanding cosmic magnetic field origins.

Contribution

First detection of gamma-ray halos around AGN and the first measurement of intergalactic magnetic field strength using Fermi data.

Findings

Gamma-ray halos detected at 3.5σ significance.

Halo properties consistent with IGMF of ~10^{-15} G.

Results support the existence of primordial magnetic fields.

Abstract

Intergalactic magnetic fields (IGMF) can cause the appearance of halos around the gamma-ray images of distant objects because an electromagnetic cascade initiated by a high-energy gamma-ray interaction with the photon background is broadened by magnetic deflections. We report evidence of such gamma-ray halos in the stacked images of the 170 brightest active galactic nuclei (AGN) in the 11-month source catalog of the Fermi Gamma-Ray Space Telescope. Excess over point spread function in the surface brightness profile is statistically significant at 3.5\sigma (99.95% confidence level), for the nearby, hard population of AGN. The halo size and brightness are consistent with IGMF, B_{IGMF} ~ 10^{-15} G. The knowledge of IGMF will facilitate the future gamma-ray and charged-particle astronomy. Furthermore, since IGMF are likely to originate from the primordial seed fields created shortly after the Big Bang, this potentially opens a new window on the origin of cosmological magnetic fields, inflation, and the phase transitions in the early Universe.