The 300 TeV photon from GRB 221009A: a Hint at Non-linear Lorentz Invariance Violation?

TL;DR

The study examines whether Lorentz invariance violation (LIV) can reconcile a 300 TeV photon from GRB 221009A with standard physics, by analyzing LIV-induced changes to gamma-gamma absorption thresholds and photon time-of-flight. By combining TOF constraints from the LHAASO afterglow with EBL-absorption considerations and the Carpet-3 detection, the authors map the LIV parameter space $(E_{ m LIV}, n)$ and find first-order LIV incompatible while higher-order, particularly quadratic, subluminal LIV remains viable, with $E_{ m LIV2} \,=\ 1.30_{-0.35}^{+0.56}\times 10^{-7} E_{ m Pl}$ (95.4% CL). They evaluate the likelihood of a genuine GRB association versus a coincidence and discuss how LIV could simultaneously address both the lack of EBL attenuation and the late arrival, though the interpretation hinges on a single event. The results highlight GRB 221009A as a unique probe of non-linear LIV and quantum-gravity effects, motivating future multi-messenger observations to test these non-standard physics scenarios.

Abstract

The air shower array Carpet-3 detected a 300 TeV photon from the direction of GRB 221009A at 4536 s after the Fermi-GBM trigger for this event. If the association with this gamma-ray burst is real, it poses two puzzles. First, why was this photon not absorbed by the extragalactic background light? ``New physics'' beyond the Standard Model is required to explain how it managed to reach Earth from a cosmological distance. Second, why was this photon detected when the VHE afterglow observed by LHAASO already faded? A novel astrophysical mechanism is required to explain this delay. In this work we show that Lorentz invariance violation (LIV), which arises as a low-energy limit of certain quantum gravity theories, can solve both puzzles. It shifts thresholds of particle interaction and changes the opacity of the extragalactic background, and cause energy-dependent variations of the photon velocity, which changes the photon time of flight. We investigate the LIV parameter space assuming that the 300 TeV photon is a part of the VHE afterglow detected by LHAASO in the TeV range. We identify viable solutions and place stringent two-sided constraints on the LIV energy scale required to resolve the observational puzzles. First-order LIV appears to be incompatible with the constraints set by analyzing the TeV afterglow of this GRB. Viable solutions emerge for higher orders. In particular, the commonly studied second-order subluminal LIV with $E_{\rm LIV2} = 1.30_{-0.35}^{+0.56} \times 10^{-7} E_{\rm Pl}$ (95.4% credibility level; $E_{\rm Pl}$ is the Planck energy) is consistent with all the observed data.

Figures (4)

• Figure 1: (a) The best-fit light curve of the GRB 221009A afterglow measured by LHAASO in the $0.3...5\,$TeV range LHAASOSci2023_aftrglw. The dashed part depicts the times when no reliable signal were detected by LHAASO. (b) The probability distribution Eq. \ref{['eq:prob_LIVTOF']} for the LIV TOF delay of the Carpet-3 photon, estimated from the LHAASO afterglow light curve.
• Figure 2: Constraints on the LIV parameters $E_{\text{LIV}}$ and $n$ arising from analysis of GRB 221009A's afterglow assuming that the Carpet-3 photon is associated with this event. Panel (b) is the same as (a) but the main trend of the lines $n\propto 1/(\text{const} + \log E_{\text{LIV}})$ is subtracted. The thick black line is the constraint that the 300 TeV photon is not absorbed by EBL (Eq. \ref{['eq:EBL_Carpet']}). The black dashed line is the EBL absorption constraint from the LHAASO afterglow observation (Eq. \ref{['eq:EBL_LHAASO']}). The blue line is the constraint from the TOF delay analysis of the LHAASO afterglow (Eq. \ref{['eq:dt_LHAASO']}). The green, orange, and red bands stand for the conditions that the LIV TOF delay of the 300 TeV photon is 213 s (Eq. \ref{['eq:dt_Carpet_bottom']}), 1437 s (Eq. \ref{['eq:dt_Carpet_99']}), and 4310 s (Eq. \ref{['eq:dt_Carpet_999']}), correspondingly (bands thickness show the 68% credible range for $E_\gamma^\text{(Cpt)}$). The percents near the arrows on the panel (b) show the probabilities that the LIV TOF delay is less or greater than the given value according to the association with the GRB afterglow, Eq. \ref{['eq:prob_LIVTOF']}. The single-hatched region on the right is the most conservative inference on the LIV parameters. Within this range LIV TOF is consistent with the LHAASO data PiranOfengeimPRD2024_LIVLHAASOSci2024_LIV and it satisfies the condition $\delta t^\text{(LIV)} < t_\text{(Cpt)}-t_0$, and LIV threshold shifts enable the Carpet-3 photon to traverse the EBL. The assumption that the 300 TeV light curve is the same as the lower energy LHAASO light curve yields much stricter limits, as depicted by the red and green stripes.
• Figure 3: A schematic diagram of the possible solutions to the Carpet-3 photon puzzles. If the photon was not produced by the regular afterglow, this requires both "new physics" to explain the suppressed EBL absorption and "new astrophysics" to explain the late arrival time. Alternatively if the photon was emitted by the afterglow early on and was delayed on flight, LIV can explain both puzzles.
• Figure 4: The coincidence probability conditioned by that LIV has (a) given $n$ or (b) given $E_{\text{LIV}}$, Eq. \ref{['eq:coince']}, within the assumption that the light curve is the same for the TeV and sub-PeV ranges.