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

Abstract

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.

Figures (5)

• Figure 1: Figure \ref{['fig:spec-fit']}. Combined fluence spectrum for the entire duration of GRB 221009A. Black symbols -- LHAASO (circles: derived from 230$-$300 s, boxes: derived from 300$-$900 s; empty symbols for WCDA, filled symbols for KM2A). Green diamond -- Carpet-3 (dark error bars are 68% CL, light error bars are 95% CL). Dark blue line -- WCDA best-fit power-law intrinsic spectrum, extrapolated as the dotted line. Red line -- this best-fit spectrum after absorption. Thin blue line -- example of the spectrum with ALPs for the best-fit ALP parameters; dashed black line -- best-fit spectrum with LIV. See the text for details.
• Figure 2: Figure \ref{['fig:ALPparam']}. ALP parameter space $(m, g_{a\gamma\gamma})$. Blue shading shows the $\Delta\chi^2$ distribution (arbitrary scale; lighter colors indicate better fits). The red cross marks the best-fit point, and the thin red contour outlines the 68% confidence region. Gray lines indicate existing upper limits on $g_{a\gamma\gamma}$: the solid line is the experimental bound from CAST CAST:2024, while the dash-dotted line represents a model-dependent constraint derived from polarization observations of magnetized white dwarfs MWD-polarization. The white line with its hatched 68% CL band shows the range favored by stellar-evolution arguments ST-globularCl.
• Figure 3: Figure \ref{['fig:LIVlikely']}. Comparison of $\Delta\chi^2$ for the quadratic subluminal LIV scenario (solid blue line: exact cross section, Eq. (\ref{['eq:exactCS']}); dashed blue line: approximation Fairbairn:2014LIV) as a function of $E_{\rm LIV, 2}$. For reference, the horizontal green dash-dotted and red dashed lines indicate the $\Delta\chi^2$ values for the best-fit ALP model, 0, and the standard-physics case, 30.48, respectively. Gray shading indicates the ranges of $E_{\rm LIV, 2}$ excluded from the development of air showers induced by primary gamma rays Satunin:2021constraints (dark shade) and cosmic rays EAS-LIV (light shade).
• Figure 4: Figure \ref{['fig:residuals']}. Residuals of fluence spectrum fits for the best-fit ALP (green circles) and LIV (blue pluses) models, and for the standard absorption (red crosses). To combine Gaussian and Poisson residuals in the same plot, we expressed both in terms of "standard deviations" $\sigma$, determined in each case from the p-values of individual points. Shading indicates $1\sigma$ and $2\sigma$ bands for reference. For WCDA data points below 2 TeV, new-physics effects in the best-fit models are negligible. The best-fit LIV model differs from the standard physics only for the 300-TeV point.
• Figure 5: Figure \ref{['fig:shifts']}. ALP parameter space $(m, g_{a\gamma\gamma})$: zoom in Fig. \ref{['fig:ALPparam']}. Best-fit parameters are shown for the baseline model (red cross; red contour encircles the 68% CL region), variations in the host-galaxy (HG) and Milky-Way (MW) magnetic field normalization by $\pm30\%$ ($+, \ -$), and for the Franceschini & Rodighiero (2017) Franceschini:2017iwq EBL model (F17). See Fig. \ref{['fig:ALPparam']} for other notations.