Morphology of blazar-induced gamma ray halos due to a helical intergalactic magnetic field

Andrew J. Long, Tanmay Vachaspati

Research output: Contribution to journalArticlepeer-review

17 Scopus citations


We study the characteristic size and shape of idealized blazar-induced cascade halos in the 1-100,GeV energy range assuming various non-helical and helical configurations for the intergalactic magnetic field (IGMF). While the magnetic field creates an extended halo, the helicity provides the halo with a twist. Under simplifying assumptions, we assess the parameter regimes for which it is possible to measure the size and shape of the halo from a single source and then to deduce properties of the IGMF. We find that blazar halo measurements with an experiment similar to Fermi-LAT are best suited to probe a helical magnetic field with strength and coherence length today in the ranges 10-17 ≲ B0 / Gauss ≲ 10-13 and 10 Mpc ≲ λ ≲ 10 Gpc where ∼ B02 / λ is the magnetic helicity density. Stronger magnetic fields or smaller coherence scales can still potentially be investigated, but the connection between the halo morphology and the magnetic field properties is more involved. Weaker magnetic fields or longer coherence scales require high photon statistics or superior angular resolution.

Original languageEnglish (US)
Article number065
JournalJournal of Cosmology and Astroparticle Physics
Issue number9
StatePublished - Sep 29 2015


  • cosmic magnetic fields theory
  • extragalactic magnetic fields
  • gamma ray theory
  • primordial magnetic fields

ASJC Scopus subject areas

  • Astronomy and Astrophysics


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