Prospect on intergalactic magnetic field measurements with gamma-ray instruments

Abstract

Abstract. Observing high-energy gamma-rays from Active Galactic Nuclei (AGN) offers a unique potential to probe extremely tiny values of the intergalactic magnetic field (IGMF), a long standing question of astrophysics, astroparticle physics and cosmology. Very high energy (VHE) photons from blazars propagating along the line of sight interact with the extragalactic background light (EBL) and produce e + e − pairs. Through inverse-Compton interaction, mainly on the cosmic microwave background (CMB), these pairs generate secondary GeV-TeV components accompanying the primary VHE signal. Such secondary components would be detected in the gamma-ray range as delayed "pair echos" for very weak IGMF (B < 10 −16 G), while they should result in a spatially extended gamma-ray emission around the source for higher IGMF values (B > 10 −16 G). Coordinated observations with space (i.e. Fermi) and groundbased gamma-ray instruments, such as the present Cherenkov experiments H.E.S.S., MAGIC and VERITAS, the future Cherenkov Telescope Array (CTA) Observatory, and the wide-field detectors such as HAWC and LHAASO, should allow to analyze and finally detect such echos, extended emission or pair halos, and to further characterize the IGMF.

Simulations of extragalactic magnetic fields and of their observables

“…of detected IC cascade component at ∼ GeV energies is probing the magnetic fields in the intervening cosmic volume (mostly dominated by voids) and/or the filling factor of magnetic fields along the line of sight. From this effect, lower limits of ≥ 10 −16 G on ∼ Mpc scales have been derived Neronov and Vovk, 2010;Dolag et al, 2011;Arlen et al, 2014;Caprini and Gabici, 2015;Chen et al, 2015) The Cherenkov Telescope Array (CTA) will have the potential to further limit large-scale magnetic fields through this effect (Sol et al, 2013;Meyer et al, 2016). Finally, the presence of significant large-scale magnetic fields has been suggested as a possible explanation for the puzzling lack of infrared absorption in the observed spectra of distant blazars (e.g.…”

Capabilities beyond Gamma Rays

“…-Probing Extreme Environments : Data from the AGN KSP, together with multi-wavelength (MWL) and possibly multi-messenger (MM) data, will bring us closer to a comprehensive understanding of the different types of blazars and their supposed parent population of radio galaxies, through the exploitation of a reference sample of high-quality spectra and light curves from different AGN classes. The signals detected from AGN over a large range of redshifts will also be used as beacons for a precise measurement of the extragalactic background light (EBL) [31] and to constrain the strength of the intergalactic magnetic field (IGMF) [444].…”