[Paper Review] Large-Scale Diffuse Intergalactic Magnetic Fields Constraints with the Cherenkov Telescope Array

Paramita Barai, E. M. de Gouveia Dal Pino|arXiv (Cornell University)|Nov 14, 2018

Astrophysics and Cosmic Phenomena1 references3 citations

TL;DR

This paper proposes using the Cherenkov Telescope Array (CTA) to detect diffuse intergalactic magnetic fields (IGMF) via gamma-ray observations of distant blazars. High-energy photons from blazars produce electron-positron pairs in the intergalactic medium, whose deflection by IGMF creates extended GeV emission (pair halos) that CTA’s enhanced sensitivity can detect, setting a lower limit of $ B_{\text{IGM}} \gtrsim 10^{-16} \, \text{G} $ with $ 60\% $ filling factor.

ABSTRACT

Magnetic fields of the order of $μ$-Gauss are observationally detected in galaxies and galaxy clusters, which can be (at least) in part originated by the amplification of much weaker primordial seed fields. These fields should be carried out by strong galactic outflows, magnetically enriching the InterGalactic Medium (IGM). However direct observation of magnetic fields in the IGM is scarce. This talk will give a review of how Intergalactic Magnetic Field (IGMF) can be constrained using gamma-ray observations. High-energy TeV photons emitted by distant blazars can interact with the cosmic extragalactic optical/infrared/microwave background light, producing electron-positron pairs, and initiating electromagnetic cascades in the IGM. The charged component of these cascades is deflected by IGMFs, thereby reducing the observed point-like TeV flux, and creating an extended image in the GeV energy range, which can potentially be detected with $γ$-ray telescopes (Fermi-LAT, HESS, CTA). Studies (e.g., Neronov & Vovk 2010, Dolag et al. 2011) have put lower limits on the IGMF strength of the order of $10^{-16} - 10^{-15} G$, and filling factors of $60\%$. This talk will describe the constraints which the Cherenkov Telescope Array sensitivity is expected to give (CTA Consortium 2018).

Motivation & Objective

• To constrain the strength and spatial filling factor of large-scale diffuse intergalactic magnetic fields (IGMF) using high-energy gamma-ray observations.
• To assess the sensitivity of the Cherenkov Telescope Array (CTA) to detect secondary GeV gamma-ray emission (pair halos) from electromagnetic cascades initiated by VHE photons in the IGM.
• To improve upon current lower limits on IGMF by leveraging CTA’s superior angular resolution, energy resolution, and sensitivity in the 100 GeV–10 TeV energy range.
• To evaluate the detectability of pair halos and pair echoes as signatures of IGMF deflection of electron-positron pairs in cascades initiated by TeV photons from distant blazars.

Proposed method

• Simulate 3D electromagnetic cascades using Monte Carlo methods, modeling pair production from VHE photons interacting with extragalactic background light (EBL).
• Track deflection of secondary electron-positron pairs by IGMF, which alters the angular distribution of secondary GeV photons, forming spatially extended pair halos.
• Estimate the expected flux of pair halo emission for a blazar at 120 Mpc with $ B_{\text{IGM}} = 10^{-14} \, \text{G} $, using theoretical differential angular distributions.
• Compare simulated pair halo fluxes with CTA sensitivity curves (for both northern and southern sites) to assess detectability over the 0.1–5 TeV energy range.
• Use time-resolved spectral analysis to search for pair echoes—delayed GeV emission—when IGMF is below $ 10^{-16} \, \text{G} $.
• Apply Bayesian statistics to stacking analyses of multiple blazars (e.g., 24 at $ z < 0.5 $) to infer IGMF constraints from non-detections of secondary components.

Experimental results

Research questions

• RQ1 Can the Cherenkov Telescope Array detect spatially extended GeV emission (pair halos) from electromagnetic cascades initiated by VHE photons in the presence of intergalactic magnetic fields?
• RQ2 What lower limits on IGMF strength and filling factor can be derived from non-detections of secondary GeV emission in Fermi-LAT and HESS data?
• RQ3 How does CTA’s improved sensitivity and angular resolution enhance the detectability of pair halos compared to current instruments like HESS and MAGIC?
• RQ4 What is the expected flux of pair halo emission for a blazar at 120 Mpc with $ B_{\text{IGM}} = 10^{-14} \, \text{G} $, and can it be resolved by CTA?
• RQ5 Can pair echoes—time-delayed GeV emission—be observed as a signature of weak IGMF, and what sensitivity is required for detection?

Key findings

• CTA is expected to detect pair halo emission from electromagnetic cascades in the 0.1–5 TeV energy range, with sensitivity sufficient to observe halos from blazars at 120 Mpc with $ B_{\text{IGM}} = 10^{-14} \, \text{G} $.
• The CTA southern (I) and northern (NB) site sensitivity curves exceed the simulated pair halo flux, indicating high detectability potential.
• Current non-detections of secondary GeV emission in Fermi-LAT and HESS data imply a lower limit of $ B_{\text{IGM}} \gtrsim 3 \times 10^{-16} \, \text{G} $, consistent with earlier studies.
• A stacking analysis of 24 low-redshift blazars ($ z < 0.5 $) using Fermi-LAT data suggests $ B_{\text{IGM}} \sim 10^{-17} - 10^{-15} \, \text{G} $, with Bayesian statistics supporting non-zero IGMF.
• The IGMF filling factor is constrained to at least 60% for fields stronger than $ 10^{-16} - 10^{-15} \, \text{G} $, based on Dolag et al. (2011).
• CTA’s improved sensitivity (5–10× better than HESS/MAGIC) and wider field of view ($ >1.5^\circ $) make it uniquely suited to resolve extended pair halo morphology.