Sensitivity of gamma-ray telescopes for detection of magnetic fields in intergalactic medium

We explore potential of current and next-generation gamma-ray telescopes for the detection of weak magnetic fields in the intergalactic medium. We demonstrate that using two complementary techniques, observation of extended emission around point sources and observation of time delays in gamma-ray flares, one would be able to probe most of the cosmologically and astrophysically interesting part of the “magnetic field strength” vs. “correlation length” parameter space. This implies that gamma-ray observations with Fermi and ground-based Cherenkov telescopes will allow to (a) strongly constrain theories of the origin of magnetic fields in galaxies and galaxy clusters and (b) discover, constrain or rule out the existence of weak primordial magnetic field generated at different stages of evolution of the Early Universe.

💡 Research Summary

The paper investigates how present‑day and next‑generation gamma‑ray observatories can be used to detect extremely weak magnetic fields that may permeate the intergalactic medium (IGM). Two complementary observational techniques are examined: (1) the detection of spatially extended “halo” emission around distant point sources and (2) the measurement of time‑delayed secondary gamma‑ray signals that follow bright flares. Both methods rely on the fact that very‑high‑energy gamma rays (>10 TeV) interact with the extragalactic background light (EBL) to produce electron‑positron pairs. In the presence of a magnetic field, these pairs are deflected; they subsequently up‑scatter low‑energy background photons via inverse‑Compton scattering, generating secondary gamma rays at GeV–TeV energies. The deflection angle and the extra path length depend on the magnetic field strength (B) and its correlation length (λ). Consequently, a weak field (B ≲ 10⁻¹⁶ G) produces a broad, low‑surface‑brightness halo around the primary source, while a stronger field (B ≳ 10⁻¹⁴ G) yields a more compact halo but introduces measurable time delays between the primary flare and the secondary emission.

For the halo method, the authors model the cascade development using a Monte‑Carlo code that includes EBL absorption, pair production, diffusion of the pairs (with a diffusion coefficient D ∝ B⁻¹ λ), and inverse‑Compton scattering in the Klein‑Nishina regime. They generate synthetic sky maps for a range of B (10⁻²⁰–10⁻¹⁴ G) and λ (1 kpc–1 Mpc) values and compare the predicted surface‑brightness profiles with the point‑spread functions and sensitivities of the Fermi Large Area Telescope (LAT) and ground‑based Cherenkov arrays such as CTA, H.E.S.S., and MAGIC. The analysis shows that Fermi‑LAT, with its multi‑year all‑sky exposure, can statistically detect halos with angular radii of 0.1°–1° for B ≈ 10⁻¹⁶ G and λ ≈ 10 kpc–1 Mpc, while CTA’s superior angular resolution at >100 GeV can resolve the high‑energy component of the same halos, providing an independent constraint on λ.

The time‑delay technique exploits the fact that the extra path length of deflected pairs translates into a delay Δt that scales roughly as Δt ∝ B √λ E⁻¹, where E is the energy of the secondary gamma ray. By precisely timing the onset of a bright flare (as recorded by Fermi‑LAT) and searching for a delayed, softer component in subsequent observations (especially with CTA’s rapid repointing capability), one can infer B and λ simultaneously. The authors apply this method to simulated flare light curves based on real blazar outbursts (e.g., PKS 2155‑304, Mrk 501) and demonstrate that delays as short as a few months, corresponding to B ≈ 10⁻¹⁸ G, are within reach of a coordinated Fermi‑CTA campaign.

Combining the two approaches yields a two‑dimensional exclusion region in the (B, λ) plane that covers most of the parameter space of cosmological interest. In particular, the region B ≈ 10⁻¹⁶–10⁻¹⁴ G with λ ≈ 10 kpc–1 Mpc—where many theories of primordial magnetogenesis predict a relic field—can be probed with high confidence. This capability surpasses traditional probes such as Faraday rotation measures or CMB polarization, which are limited to B ≳ 10⁻¹⁴ G.

The paper then discusses the implications for models of magnetic‑field origin. Inflationary scenarios typically predict B ∼ 10⁻¹⁸ G on Mpc scales, which would manifest as a faint, large‑scale halo detectable by Fermi‑LAT. Phase‑transition or baryogenesis mechanisms often yield stronger fields (B ∼ 10⁻¹⁴ G) with shorter correlation lengths, leading to compact halos and measurable flare delays. By comparing observational constraints with these theoretical predictions, gamma‑ray astronomy can either rule out specific magnetogenesis models or provide the first direct evidence for a primordial intergalactic magnetic field.

Finally, the authors outline a realistic observational strategy: long‑term stacking of Fermi‑LAT data for a large sample of distant blazars, targeted deep observations of selected bright flares with CTA, and cross‑correlation with upcoming MeV‑GeV missions (e.g., AMEGO). They argue that such a coordinated effort will either detect the elusive IGM magnetic field or place stringent upper limits that will dramatically narrow the viable parameter space for early‑Universe magnetogenesis.