[Paper Review] Pair-production opacity at high and very-high gamma-ray energies

D. Horns, M. Meyer|arXiv (Cornell University)|Jan 1, 2013

Astrophysics and Cosmic Phenomena13 references5 citations

TL;DR

This study analyzes Fermi-LAT data to search for anomalous transparency in the universe to high-energy gamma rays, finding three photons from high-redshift AGN with optical depth τ > 2. The combined post-trial significance of 3.68σ suggests reduced pair-production opacity, indicating potential deviations from standard extragalactic background light models or new physics such as axion-like particles.

ABSTRACT

The propagation of high energy (HE, $E_γ>100$ MeV) and very high-energy gamma-rays (VHE, $E_γ>100$ GeV) in the extra-galactic photon field leads to pair-production and consequently energy- and distance-dependent attenuation of the primary intensity. The spectroscopy of an increasing number of extra-galactic objects at HE and VHE energies has demonstrated indeed the presence of such an attenuation which in turn has been used to constrain the photon density in the medium. At large optical depth ($τ\gtrsim 2$) potential modifications of pair-production due to competing but rare processes (as, e.g., the presence of sub-neV axion-like particle) may be found. Indications for a pair-production anomaly have previously been found with VHE-spectra. Here, we present further indications (at the level of $3.68 σ$) for a reduced optical depth at high energies from an analysis of Fermi- extit{LAT} data.

Motivation & Objective

• To investigate potential deviations from standard gamma-ray attenuation in the extragalactic background light (EBL) using high-energy Fermi-LAT data.
• To test whether the observed high-energy gamma-ray spectra from distant AGN show signs of reduced pair-production opacity at τ > 2.
• To explore the implications of such anomalies for new physics, including axion-like particles or modified photon propagation.
• To combine results from Fermi-LAT with prior VHE observations to strengthen constraints on EBL normalization and transparency.

Proposed method

• Selected photons with Eγ > 10 GeV from high Galactic latitude (|b| > 10°) and zenith angle Z < 100° using the ULTRACLEAN event class.
• Matched photons to known gamma-ray emitting AGN from the 2FGL and 3FGL catalogs with known redshifts.
• Excluded photons with energy > 500 GeV and association probability < 90% to reduce background contamination.
• Calculated expected source photon counts via power-law extrapolation of the spectrum from 1 GeV down to 1% flux loss due to absorption.
• Combined probabilities from three high-τ photons using Fisher’s method to assess global significance.
• Applied pre- and post-trial corrections to account for multiple testing and systematic uncertainties in energy and spectral index.

Experimental results

Research questions

• RQ1 Is there evidence for reduced gamma-ray opacity at high energies (Eγ > 100 GeV) in the Fermi-LAT energy range, beyond what is predicted by standard EBL models?
• RQ2 Do the observed high-energy photons from distant AGN with τ > 2 indicate a deviation from expected pair-production attenuation?
• RQ3 Could the observed anomaly be explained by new physics such as axion-like particles or modified photon propagation?
• RQ4 How does the significance of the anomaly compare between Fermi-LAT and prior VHE observations?
• RQ5 What is the robustness of the result under systematic variations in energy calibration and intrinsic source spectral index?

Key findings

• Three photons with optical depth τ > 2 were identified from three distinct AGN: GB6J1001+2911 (τ = 2.18, z = 0.558), S4 0218+35 (τ = 2.46, z = 0.944), and Ton 599 (τ = 3.1, z = 0.725).
• The combined pre-trial probability of detecting these three photons under standard absorption was 6.57 × 10⁻⁶, corresponding to a significance of 4.36σ.
• After correcting for trials, the post-trial significance was 1.17 × 10⁻⁴, equivalent to 3.68σ, indicating a statistically significant anomaly.
• Systematic variations in energy uncertainty and a harder intrinsic spectrum increased the pre-trial probability but did not reduce the significance below 3σ.
• The result is consistent with earlier indications from VHE data, suggesting a potential universal deviation from standard EBL-based attenuation.
• The findings point toward possible modifications in photon propagation, such as those induced by sub-neV axion-like particles, though no definitive explanation is provided.