Letter to the Editor

Intergalactic magnetism in a γ-ray beam as a model of Porphyrion

¹ Université Paris Cité, CNRS, Astroparticule et Cosmologie, 75006 Paris, France
² Laboratory of Astrophysics, École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland
³ Dipartimento di Fisica e Astronomia, Universita di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
⁴ INAF Istituto di Radioastronomia, Via P. Gobetti 101, 40129 Bologna, Italy
⁵ Nordita, KTH Royal Institute of Technology and Stockholm University, Hannes Alfvéns väg 12, 10691 Stockholm, Sweden
⁶ The Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova, 10691 Stockholm, Sweden
⁷ McWilliams Center for Cosmology & Department of Physics, Carnegie Mellon University, Pittsburgh, PA 15213, USA
⁸ School of Natural Sciences and Medicine, Ilia State University, 3-5 Cholokashvili Avenue, 0194 Tbilisi, Georgia
⁹ Département de Physique Théorique and Center for Astroparticle Physics, Université de Genève, 1211 Geneve 4, Switzerland
¹⁰ Theoretical Physics Department, CERN, 1211 Geneva 23, Switzerland

^⋆ Corresponding author.

Received: 20 November 2024
Accepted: 24 March 2025

Abstract

We estimate the magnetic field in the jets of the recently discovered 7 Mpc long Porphyrion system. We used nondetection of the system in gamma-rays to derive a lower bound on the co-moving magnetic field strength at the level of ∼10 nG (comoving). This value is consistent with recent estimates of magnetic fields in the filaments of the large-scale structure. We discuss the possibility that, instead of being the extreme case of a radio jet formation scenario, Porphyrion actually traces a very high-energy γ-ray beam emitted by an active galactic nucleus. In such a model, jets do not need to spread into the voids of the large-scale structure to appear straight on a very large distance range, and several anomalies of the standard radio jet scenarios can be solved at once.