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Carpet-3 detection of a photon-like air shower with estimated primary energy above 100 TeV in a spatial and temporal coincidence with GRB 221009A

T0 review · 3 major / 3 minor · reviewed 2026-08-09 · deepseek-v4-flash

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Pith's one-line read A ~300 TeV photon-like air shower coincident with GRB 221009A is reported, with only a 3 × 10^-4 chance of being a hadronic cosmic ray.

desk verdict A careful single-event analysis that adds real new data, but the headline hadron probability of ~3e-4 is softer than it looks and needs a systematic robustness check. read the letter →

arxiv 2502.02425 v1 pith:FERHE4H2 submitted 2025-02-04 astro-ph.HE

D. D. Dzhappuev , I. M. Dzaparova , T. A. Dzhatdoev , E. A. Gorbacheva , I. S. Karpikov , M. M. Khadzhiev , N. F. Klimenko , A. U. Kudzhaev
show 13 more authors
A. N. Kurenya A. S. Lidvansky O. I. Mikhailova V. B. Petkov E. I. Podlesnyi N. A. Pozdnukhov V. S. Romanenko G. I. Rubtsov S. V. Troitsky I. B. Unatlokov N. A. Vasiliev A. F. Yanin K. V. Zhuravleva
This is my paper · ORCID
classification astro-ph.HE
keywords gamma-rayburstsveryhighenergygammaraysairshowersmuondetectorsneuralnetworkclassificationGRB221009AextragalacticbackgroundlightCarpet-3
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

This paper reports a single air-shower event recorded by the Carpet-3 array on 9 October 2022, arriving from a direction 1.8 degrees away from GRB 221009A about 75 minutes after the burst. The shower's reconstructed size implies a primary energy of about 300 TeV if the particle was a photon, and its low muon content together with a neural-network classifier trained on simulated showers puts the probability that it was a hadronic cosmic ray at roughly 3 × $10^{-4}$. A chance coincidence with the burst is estimated at about 9 × $10^{-3}$. If the particle really is a gamma ray from this GRB, it would be the highest-energy photon ever associated with any gamma-ray burst, and its mere arrival would be surprising because such photons should be absorbed by extragalactic background light. The paper also derives a fluence above 100 TeV that is in order-of-magnitude agreement with the extrapolation of LHAASO's lower-energy measurements.

What carries the argument

The analysis is carried by the upgraded 410 $m^{2}$ underground muon detector of Carpet-3, which recorded only 3 muons for this event, and by a neural-network classifier that combines the 20x20 pattern of energy releases in the central Carpet array with reconstructed shower parameters (arrival angles, axis position, shower size, muon counts in both detector parts, and the azimuthal-asymmetry variable C_k). The classifier was trained on 6750 simulated photon showers and 6750 simulated proton showers generated with CORSIKA using the QGSJET-II-04 and FLUKA hadronic interaction models over an energy range of 100-1000 TeV. The quoted hadron probability is the fraction of simulated proton events that the network scores at or above the observed event's score, and the energy estimate comes from a power-law fit relating simulated gamma-ray primary energy to shower size N_e.

What would settle it

One decisive check would be to measure the distribution of muon counts for hadronic cosmic-ray showers of the same reconstructed size with the same detector and compare it with the simulation: if the observed fraction of showers with three or fewer muons is much larger than the simulated 0.127, the hadron probability rises accordingly. A second check would be an archival search in other air-shower datasets for photon-like events within the same time window and direction, which would either corroborate the GRB association or show it to be a chance fluctuation.

Watch

Extended reading notes

Core claim

The central claim is that Carpet-3 detected a photon-like air shower with estimated primary energy 300+43-38 TeV, arriving 4536 seconds after the Fermi-GBM trigger from a direction 1.8 degrees from GRB 221009A, with a chance-coincidence probability of about 9 × $10^{-3}$ and an estimated probability of ~3 × $10^{-4}$ that the primary was hadronic. The paper states that this is the highest-energy event ever associated with any gamma-ray burst. Under the photon interpretation, the event also implies a fluence above 100 TeV of roughly (1.1 ± 0.9) × $10^{-3}$ erg/$cm^{2}$, consistent with the extension of the LHAASO spectrum to lower energies.

Load-bearing premise

The load-bearing premise is that the CORSIKA/QGSJET-II-04 and FLUKA simulations accurately reproduce both the Carpet-3 detector response and the muon content of real hadronic cosmic-ray showers; if actual proton or nuclei showers fluctuate to low muon counts more often than the simulations predict, the quoted 3 × $10^{-4}$ hadron probability and the 300 TeV gamma-ray energy estimate would be biased and the gamma-ray interpretation would lose support.

Editorial extensions

If this is right

  • If the event is a genuine ~300 TeV photon from GRB 221009A, it would be the most energetic gamma ray ever linked to a GRB, exceeding the ~18 TeV LHAASO events by more than an order of magnitude.
  • Its detection would imply that the standard absorption of such photons by extragalactic background light is somehow overcome, providing a target for axion-like-particle, Lorentz-invariance-violation, and other new-physics models.
  • The event would set a lower bound on the GRB's very-high-energy emission that any afterglow model must reproduce, and the derived fluence above 100 TeV would connect to the LHAASO spectrum.
  • The observation demonstrates the value of continuous, wide-field air-shower monitoring: at 4536 s after trigger the burst was at high elevation for Carpet-3 while LHAASO's field of view was closing and HAWC's was below the horizon.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • The authors do not attempt a search for additional events in the same direction; a dedicated archival search over other air-shower arrays' data could either strengthen the GRB association or show that the 9 × 10^-3 chance probability is the more relevant number.
  • The neural-network classification rests on the simulated muon deficit for photons; a direct measurement of the muon-count distribution for hadronic showers of this size at the same altitude would test the 3 × 10^-4 figure independently of the next GRB.
  • If the event is real but not from the GRB, the high Galactic latitude and proximity to known ultra-high-energy gamma-ray sources mean the Galactic-plane diffuse emission could produce occasional look-alikes, so future statistics rather than this single event will settle the origin.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

3 major / 3 minor

Summary. The paper reports a single air-shower event detected by the Carpet-3 array on 2022-10-09, with reconstructed Ne = 36400, zenith angle 26.5 deg, direction 1.8 deg from GRB 221009A, and at 4536 s after the GRB trigger. The event shows only 3 muons in the 410 m^2 underground muon detector. Using CORSIKA/QGSJET-II-04/FLUKA simulations, the authors estimate the primary energy as 300+43-38 TeV under a photon assumption, and using a neural-network classifier trained on simulated photon and proton showers they estimate the probability that the primary was a hadron as ~3e-4. They also estimate a data-driven chance coincidence probability of ~9e-3 for a background event satisfying the same selection on the day of the GRB. They conclude that this is the highest-energy event ever associated with any GRB and discuss implications for new physics in view of extragalactic background light absorption.

Significance. If correct, this would be a remarkable observation: a ~300 TeV photon candidate from a GRB, far above the previously highest-energy GRB photons (~18 TeV from LHAASO), with strong implications for GRB emission mechanisms, EBL opacity, and new-physics scenarios. The analysis is careful in several respects: the background estimate is data-driven rather than based on assumed source models; the neural network is evaluated on a held-out Monte Carlo test set; and the detector response is simulated with standard, publicly available codes. However, the headline probabilities rest on model-dependent simulations and post-hoc selection threshold choices, so the strength of the claim is sensitive to systematic effects that are not quantified in the manuscript.

major comments (3)
  1. [Sec. III C] The quoted hadron probability of ~3e-4 is the fraction of simulated proton events with neural-network prediction at or above the observed value 0.927. The Monte Carlo sample contains only proton and photon primaries generated with a single hadronic interaction model (QGSJET-II-04/FLUKA), while the real hadronic background includes nuclei and the high-energy hadronic model is itself uncertain. The paper does not assign any systematic uncertainty to this probability from model choice or composition. Given that Sec. III B shows that 12.7% of real hadronic events at matching Ne have n_mu <= 3, the additional suppression to 3e-4 comes primarily from spatial-pattern correlations that are not validated against data. Without a sensitivity study using alternative hadronic models or heavier primary species, the quoted probability could vary by an order of magnitude; a value of ~1e-2 would no longer strongly support the photon interpretation. This is a load-bearing number for the central claim.
  2. [Sec. II B and Sec. III C] The background selection criteria (Ne >= Nev_e and n_mu <= n_mu_ev, and later neural-network prediction >= 0.927) are defined using the observed event's own values. The reported chance probabilities (3.0e-3 and 9.0e-3) are therefore conditional on post-hoc thresholds, with no correction for the trials associated with choosing these cuts. For a valid frequentist p-value, one would need either a pre-specified selection or an explicit penalty for the cut optimization. As presented, the association probability is likely understated, and this directly affects the claim that the event is temporally and directionally coincident with GRB 221009A.
  3. [Sec. III A and Sec. III C] The Monte Carlo sample size is described inconsistently: the text states that 6750 photon and 6750 proton showers were generated, but the full Monte Carlo set is later quoted as 80609 events, split into 62007 training and 18602 test events. This discrepancy matters because the statistical precision of the 3e-4 hadron fraction depends on the total number of simulated hadrons; if only about 6750 protons were used, the fraction corresponds to only about 2 simulated events, giving a large Poisson uncertainty. The authors should reconcile these numbers and state explicitly how many simulated photon and proton events enter the neural-network classification and the energy estimate.
minor comments (3)
  1. [Sec. III B] The text contains a typographical error: 'Fig. 2)' should be 'Fig. 2.'.
  2. [Abstract and Sec. IV D] The phrase 'highest-energy event ever associated with any GRB' should be qualified as 'photon-like event' given that the hadronic probability is model-dependent; the conclusion should restate the systematic caveat from Sec. III C.
  3. [Sec. III D] The fluence uncertainty is estimated from Poisson statistics alone; the systematic uncertainty from the assumed E^-2 source spectrum and from the Monte Carlo efficiency is not propagated into the final fluence range.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the event classification, energy estimate, and coincidence probability are derived from independent Monte Carlo simulations and data-driven background rates, not from the conclusion.

full rationale

The paper's derivation chain is self-contained. The primary identification as photon-like rests on a neural network trained on independent CORSIKA/QGSJET-II-04/FLUKA Monte Carlo showers (6750 photons and 6750 protons), with the quoted hadron probability equal to the measured false-positive fraction of simulated protons at or above the observed network output (Sec. III C). The energy estimate of 300 TeV is conditional on the photon assumption and is obtained by inverting an MC-calibrated Ne-energy relation (Sec. III A), which is a standard estimator rather than a conclusion built into the input. The chance coincidence probability is data-driven: it is the observed rate of events satisfying the same photon-like selection criteria from the GRB direction over 667 live days (Sec. II B and III C), not a quantity derived from the GRB hypothesis. The fluence estimate in Sec. III D normalizes an assumed E^-2 spectrum to the single detected event and is explicitly labeled as an estimate with large Poisson uncertainties, so it is not presented as an independent prediction. Self-citations, including the preliminary telegram [48] and the Carpet-2 Monte Carlo description [86], are methodological references and are not load-bearing in the sense of substituting for evidence or forbidding alternatives. The main caveats—single-model hadronic simulation and proton-only training—are correctness/systematic-uncertainty concerns, not circularity. No equation or fitted parameter reduces to the paper's own conclusion by construction.

Assumptions & free parameters 3 free parameters · 5 assumptions · 0 invented entities

The paper introduces no new particles, forces, or dimensions; possible new physics (axion-like particles, Lorentz invariance violation) is mentioned only as external explanations from the literature. The free parameters listed are choices made in the analysis rather than fundamental constants.

free parameters (3)
  • GRB spectrum power-law index for fluence = 2 (assumed E^-2)
    The fluence estimate in Sec. III D assumes an E^-2 power-law spectrum for the GRB; the result depends on this choice but it is not fitted to data.
  • Fluence normalization = 1.1e-3 erg/cm2 above 100 TeV
    Equated to the observed single event in Sec. III D; this is fitted to the same event being interpreted.
  • Neural network classification threshold = 0.71
    Chosen on the MC test set to give a 1e-3 proton background rejection (Sec. III C, Fig. 3). The event's score is 0.927, so the conclusion is not extremely sensitive to this threshold, but the quoted hadron probability is threshold-dependent.
assumptions (5)
  • domain assumption The Monte Carlo code CORSIKA with QGSJET-II-04 and FLUKA accurately describes air shower development and muon production for photon and proton primaries in the 100-1000 TeV range.
    Used throughout Secs. III A-III C for energy estimation and for training the neural network; no data/MC closure test is shown.
  • domain assumption The reconstructed direction of the Carpet-3 event has a 90% CL uncertainty of 4.7 degrees, so the 1.8 degree offset from the GRB direction is consistent with association.
    Stated in Sec. II B and used for the on-source background region; this angular resolution comes from prior Carpet work.
  • domain assumption The extragalactic background light model of Ref. [109] correctly predicts the attenuation of very high energy photons, so a 300 TeV photon from z=0.151 would be unexpected in the Standard Model.
    Used in Sec. IV D to argue for possible new physics; if the EBL model is wrong, the new-physics interpretation weakens.
  • standard math Poisson statistics govern the occurrence of background events in the 667-day data set.
    Used in Sec. II B and III C to convert event counts into chance probabilities.
  • domain assumption The observed event is from an astrophysical primary rather than detector noise.
    The event passed the standard Carpet trigger and reconstruction criteria; the paper does not discuss detector malfunction for this event.

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Cite this review

Pith. "Pith review of Carpet-3 detection of a photon-like air shower with estimated primary energy above 100 TeV in a spatial and temporal coincidence with GRB 221009A." pith.science (2026). https://pith.science/paper/FERHE4H2

@misc{pith2026250202425,
  author       = {Pith},
  title        = {Pith review of: Carpet-3 detection of a photon-like air shower with estimated primary energy above 100 TeV in a spatial and temporal coincidence with GRB 221009A},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/FERHE4H2}},
  note         = {Machine review of arXiv:2502.02425}
}
abstract

The brightest cosmic gamma-ray burst (GRB) ever detected, GRB 221009A, was accompanied by photons of very high energies. These gamma rays may be used to test both the astrophysical models of the burst and our understanding of long-distance propagation of energetic photons, including potential new-physics effects. Here we present the observation of a photon-like air shower with the estimated primary energy of $300^{+43}_{-38}$ TeV, coincident (with the chance probability of $\sim 9\cdot 10^{-3}$) with the GRB in its arrival direction and time. Making use of the upgraded Carpet-3 muon detector and new machine learning analysis, we estimate the probability that the primary was hadronic as $\sim 3 \cdot 10^{-4}$. This is the highest-energy event ever associated with any GRB.

Figures

Figures reproduced from arXiv: 2502.02425 by the authors.

Figure 1
Figure 1. Layout (plot to scale) of the Carpet-3 EAS array [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Distribution (PDF) of the number of muons in the [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. ROC curve of the gamma-ray – proton classifying [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗
Figures from the paper (5 more)
Figure 4
Figure 4. Figure 4: Predictions distribution of the gamma-ray – proton [PITH_FULL_IMAGE:figures/full_fig_p005_4.png]
Figure 6
Figure 6. Figure 6: Temporal dependence of the zenith angle of the [PITH_FULL_IMAGE:figures/full_fig_p006_6.png]
Figure 7
Figure 7. Figure 7: Comparison of the photon fluence of [PITH_FULL_IMAGE:figures/full_fig_p007_7.png]
Figure 8
Figure 8. Figure 8: Same as Fig [PITH_FULL_IMAGE:figures/full_fig_p007_8.png]
Figure 9
Figure 9. Figure 9: The architecture of the neural network classifier. [PITH_FULL_IMAGE:figures/full_fig_p008_9.png]

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Forward citations

Cited by 1 Pith paper

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score. Full citation record

  1. Gamma Ray Burst GRB 221009A: two distinct hints at once at new physics

    astro-ph.HE 2025-02 conditional novelty 4.0 of 10

    For the 251 TeV photon from GRB 221009A, Lorentz invariance violation can raise the survival probability to near one, but the predicted count of 0.1 makes this only a weak hint.

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