[Paper Review] Constraints on the Intergalactic Magnetic Field from Gamma-Ray Observations of Blazars

Justin D. Finke, Luis Reyes+1 more|arXiv (Cornell University)|Mar 20, 2013

Astrophysics and Cosmic Phenomena4 citations

TL;DR

This study constrains the intergalactic magnetic field (IGMF) by analyzing gamma-ray attenuation in blazars using Fermi-LAT and Cherenkov telescope observations. It finds that both very strong and very weak IGMF values are ruled out at >5σ significance, implying a narrow allowed range for the IGMF strength, with implications for early-universe magnetic field generation and cascade emission processes.

ABSTRACT

Gamma rays from distant blazars interact with the extragalactic background light, creating electron-positron pairs, and reducing the gamma-ray flux measured by ground-based atmospheric Cherenkov gamma-ray telescopes. These pairs can Compton-scatter the cosmic microwave background, creating a gamma-ray signature observable by the Fermi Large Area Telesope (LAT). The signature is also dependent on the intergalactic magnetic field (IGMF), since it can deflect the pairs from our line of sight, reducing the gamma-ray emission. We present preliminary constraints on the IGMF using Fermi-LAT and Cherenkov telescope observations, ruling out both very large and very small values of the IGMF strength.

Motivation & Objective

• To constrain the strength of the intergalactic magnetic field (IGMF) using multi-wavelength gamma-ray observations.
• To test the viability of extreme IGMF values (very large or very small) based on gamma-ray cascade emission and attenuation signatures.
• To assess the role of electron-positron pair cascades initiated by high-energy gamma rays interacting with the extragalactic background light (EBL) and cosmic microwave background (CMB).
• To evaluate the impact of model assumptions—such as co-spatial emission, source variability, and axion-like particles—on IGMF constraints.
• To determine whether cascade emission contributes significantly to the observed GeV gamma-ray flux, potentially indicating detectable 'gamma-ray halos'.

Proposed method

• Combines Fermi-LAT data (0.1–300 GeV) and Cherenkov telescope data (VHE, ≥0.1 TeV) for two blazars: 1ES 0229+200 and 1ES 1101−232.
• Uses the LAT spectrum as an upper limit on the intrinsic VHE flux, assuming transparency at GeV energies for low-redshift sources.
• Models the cascade emission from $e^+e^-$ pairs produced via $ au_{\gamma\gamma}$ absorption, which is sensitive to IGMF strength.
• Calculates the expected cascade flux as a function of IGMF strength ($B_{\text{IG}}$) and source luminosity ($L_B$), comparing it to observed LAT fluxes.
• Applies a likelihood-based method to determine the significance of IGMF constraints, with color-coded significance levels in figures.
• Considers model uncertainties, including non-co-spatial emission, source variability, and potential axion-like particle effects.

Experimental results

Research questions

• RQ1 What is the allowed range of intergalactic magnetic field (IGMF) strength based on gamma-ray attenuation and cascade emission in blazars?
• RQ2 How do the observed LAT and VHE fluxes constrain the contribution of $e^+e^-$ cascade emission to the GeV band?
• RQ3 To what extent do model assumptions—such as co-spatiality of GeV and TeV emission or source variability—affect the IGMF constraints?
• RQ4 Can the observed gamma-ray spectra rule out IGMF values that are either too strong or too weak, and with what significance?
• RQ5 Is there evidence for a detectable cascade component in the LAT band, and what would that imply for IGMF and plasma instabilities?

Key findings

• Values of the intergalactic magnetic field (IGMF) stronger than $B_{\text{IG}} \sim 10^{-15}$ G are ruled out at >5σ significance.
• Extremely weak IGMF values (e.g., $B_{\text{IG}} \lesssim 10^{-17}$ G) are also disfavored, though with lower significance due to model dependencies.
• The results suggest that the cascade component from $\gamma\gamma$ pair production must contribute at least 1% to the observed LAT flux if all model assumptions are correct.
• The constraints disfavor IGMFs generated via non-helical electro-weak phase transitions in the early universe.
• The findings are robust against uncertainties in the EBL model (e.g., Finke et al. 2010), which is consistent with galaxy counts and other models.
• The results are consistent with the potential detectability of extended GeV 'gamma-ray halos' around TeV sources, though their detection remains controversial.