Revisiting Very High Energy Gamma-Ray Absorption in Cosmic Propagation under the Combined Effects of Axion-Like Particles and Lorentz Violation

TL;DR

This work tackles the anomalous transparency of the universe to very high energy gamma rays by proposing a unified framework that combines axion-like particle (ALP)–photon mixing and second-order Lorentz invariance violation (LIV). Using a density-matrix formalism for photon–ALP propagation in magnetic fields and LIV-modified pair-production thresholds, the authors fit GRB 221009A’s TeV spectrum with a Bayesian approach, adopting a power-law intrinsic spectrum and an LIV-corrected optical depth. They show that ALPs alone cannot explain the 300 TeV photon, while quadratic subluminal LIV is required; a hybrid ALP–LIV model with best-fit parameters ($g_{a\gamma}=1.685\times10^{-10}\ \mathrm{GeV}^{-1}$, $m_a=9.545\times10^{-8}\ \mathrm{eV}$, $E_{\rm LIV,2}=(1.30_{-0.35}^{+0.56})\times10^{-7} E_{\rm Pl}$) provides an excellent fit to both the 18 TeV and 300 TeV data. This indicates a potential pathway to physics beyond the Standard Model and motivates further tens-to-hundreds TeV observations to refine ALP and LIV parameters.

Abstract

Very high energy (VHE) gamma rays with energies above 100 GeV are expected to experience strong attenuation during cosmic propagation due to pair production with the extragalactic background light (EBL). However, recent observations-particularly the detection of GRB 221009A with photons up to 18 TeV by LHAASO and a 300 TeV photon by Carpet-3,pose a significant challenge to conventional EBL absorption models. These extraordinarily high-energy photons suggest the presence of new physics influencing photon propagation over cosmological distances. The gamma-ray spectrum in the tens-to-hundreds of TeV range provides a powerful probe of possible Lorentz invariance violation (LIV) or photon-axion-like particle (ALP) mixing in cosmic magnetic fields. Yet, when treated independently, neither LIV nor ALPs can fully explain the observed transparency of the Universe to VHE gamma rays. In this work, we propose a unified framework that combines both effects-ALPs and LIV-to account for the unexpectedly high survival probability of VHE photons. We analyze the multi-wavelength spectrum of GRB 221009A under this hybrid model and demonstrate that the combined influence of ALPs, characterized by a coupling constant $g_{aγ} = 1.685 \times 10^{-10},\mathrm{GeV}^{-1}$ and mass ($m_a = 9.545 \times 10^{-8},\mathrm{eV}$) alongside second-order LIV characterized by an energy scale $E_{\rm LIV,2} = (1.30_{-0.35}^{+0.56}) \times 10^{-7} E_{\rm Pl}$ adopted from the literature, significantly enhances the photon survival probability across the 10-300 TeV range. This synergistic ALP-LIV model provides a coherent explanation for the observed extreme-energy transparency of the Universe and the delayed arrival of ultra-high-energy photons, offering valuable insights into physics beyond the Standard Model.

Figures (4)

• Figure 1: Corresponding best-fit SEDs of GRB 221009A under only ALPs scenario with a coupling constant $g_{a\gamma} = 1.489 \times 10^{-10} \, \mathrm{GeV}^{-1}$ and $m_a = 4.191 \times 10^{-7} \, \mathrm{eV}$.
• Figure 2: Photon survival probability within the host galaxy of GRB 221009A as a function of ALPs mass ($m_a$) and the ALP–photon coupling constant ($g_{a\gamma}$), evaluated for photon energies of 18 TeV and 300 TeV. The results indicate that a substantial fraction of photons can survive across the explored $m_a$–$g_{a\gamma}$ parameter space.
• Figure 3: Corresponding best-fit SEDs of GRB 221009A under only LIV scenario.
• Figure 4: Corresponding best-fit SEDs of GRB 221009A under ALP-LIV hybrid scenario with a coupling constant $g_{a\gamma} = 1.685 \times 10^{-10} \, \mathrm{GeV}^{-1}$ and $m_a = 9.545 \times 10^{-8} \, \mathrm{eV}$.