Testing new-physics scenarios with the combined LHAASO and Carpet-3 fluence spectrum of GRB 221009A: axion-like particles and Lorentz-invariance violation

From gamma-ray burst (GRB) 221009A, very high-energy photons were detected: >10 TeV with LHAASO and >100 TeV with Carpet-3. Such energetic photons are expected to be absorbed via electron-positron pair production on their way to the Earth. Their observation might be explained by new physics, including Lorentz invariance violation (LIV) or photon mixing with axion-like particles (ALPs). Here, we construct a joint fluence spectrum by combining flux measurements from both experiments, and fit it under these hypotheses. While LIV can account for the Carpet-3 observation, it provides only a modest improvement over standard physics in the overall fit and requires parameters excluded by other constraints. ALP mixing improves the description of both LHAASO and Carpet-3 data, yielding a substantial enhancement in fit quality for a specific region of the ALP parameter space.

💡 Research Summary

The paper addresses the striking detection of ultra‑high‑energy photons from the exceptionally bright gamma‑ray burst GRB 221009A. The Large High‑Altitude Air Shower Observatory (LHAASO) reported photons up to ~18 TeV, while the Carpet‑3 array observed a single event with an estimated energy of ~300 TeV. In standard astrophysics such photons should be almost completely absorbed by electron‑positron pair production on the extragalactic background light (EBL) and the cosmic microwave background (CMB) during their propagation to Earth. The authors therefore construct a combined fluence spectrum by merging the time‑integrated fluxes from LHAASO (0–2000 s after trigger) and Carpet‑3 (a single event at 4536 s) and perform a statistical fit under three hypotheses: (i) conventional physics with the Saldana‑Lopez 2021 EBL model, (ii) photon–axion‑like particle (ALP) mixing, and (iii) Lorentz‑invariance violation (LIV) with a quadratic sub‑luminal modification of the photon dispersion relation.

For the standard case they compute the optical depth τ(E,z) using the chosen EBL model plus CMB photons. The resulting attenuation predicts essentially zero probability for a 250 TeV photon, contradicting the Carpet‑3 observation.

In the ALP scenario, photons can oscillate into ALPs in the magnetic fields of the GRB host galaxy and the Milky Way. The authors adopt realistic magnetic‑field models, treat the propagation with a density‑matrix formalism, and marginalise over two astrophysical nuisance parameters (the line‑of‑sight position y₀ and the spiral‑arm orientation φ) by averaging χ² over 20 random realisations. Scanning the (mass m, photon‑coupling g_{aγγ}) plane they find a best‑fit point at m ≈ 5.2 × 10⁻⁷ eV and g_{aγγ} ≈ 6 × 10⁻¹¹ GeV⁻¹. This improves the total χ² by Δχ² = 30.48 for 24 degrees of freedom, a highly significant enhancement. The ALP model reproduces the modest dip seen in the KM2A data around 5 TeV and yields a partial suppression (≈ 1/3 of the standard flux) in the strong‑mixing regime at ≳ 10 TeV, thereby accommodating both the LHAASO and Carpet‑3 points.

For LIV, the authors consider a quadratic (n = 2) sub‑luminal modification, E² − k² = −E⁴/(E_{LIV,2}²). Using the exact modified γγ→e⁺e⁻ cross‑section from the literature, they recompute τ(E). The χ² curve shows a shallow minimum at E_{LIV,2} ≈ 4 × 10¹² GeV, giving Δχ² = 12.99 for 25 degrees of freedom. This modest improvement is insufficient to outweigh the strong constraints from air‑shower observations, which require E_{LIV,2} > 10¹³–10¹⁴ GeV. Moreover, LIV only affects the CMB‑absorbed part of the spectrum and cannot reproduce the 5 TeV dip.

The authors test the robustness of their conclusions by swapping the EBL model (Franceschini & Rodighiero 2017) and varying the magnetic‑field strengths by ±30 %. The ALP best‑fit region remains within the 68 % confidence contour, while the LIV result is unchanged. Introducing a slight curvature in the intrinsic source spectrum (e.g., a log‑parabola) worsens the fit for both new‑physics scenarios, but does not alter the relative preference for ALPs.

In summary, the combined fluence spectrum of GRB 221009A cannot be explained by standard photon‑background attenuation. Photon–ALP mixing provides a statistically robust and physically plausible explanation, with best‑fit parameters that are compatible with existing laboratory (CAST) and astrophysical (white‑dwarf polarisation) bounds. The quadratic LIV hypothesis yields only a marginal improvement and is in tension with independent limits from ultra‑high‑energy air‑shower measurements. The study therefore highlights ALPs as the most promising candidate for new physics that can account for the extraordinary very‑high‑energy photons from GRB 221009A.