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arxiv: 2604.07582 · v1 · submitted 2026-04-08 · 🌌 astro-ph.HE

Detection of TeV emission during early afterglow from poorly localized GRBs with ground based IACTs

S. Macera , B. Banerjee , M. Seglar-Arroyo , J. Green , G. Oganesyan , P. Tiwari , A. Ierardi , M. Branchesi
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F. Aharonian S. Mohnani D. Miceli F. Sch\"ussler A. Berti
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Pith reviewed 2026-05-10 17:04 UTC · model grok-4.3

classification 🌌 astro-ph.HE
keywords gamma-ray burstsTeV emissionafterglowIACT follow-uppoor localizationVHE detectiontiling strategy
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The pith

Rapid tiling of large localization regions can double the rate of TeV detections from poorly localized gamma-ray bursts.

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

The paper examines how ground-based telescopes can detect very-high-energy emission from gamma-ray bursts that have imprecise sky positions. It proposes using quick surveys or tiling to cover the uncertain areas promptly instead of waiting for better localization. Simulations based on past observations show this approach can catch the early afterglow phase before the signal fades. This matters because many GRBs are only roughly located by satellite detectors, limiting traditional follow-up. If successful, it would allow more frequent VHE observations to study the physics of these extreme events.

Core claim

By simulating a population of GRBs drawn from fifteen years of Fermi/GBM and Swift/XRT data and modeling their afterglow emission, the authors find that rapid tiling strategies for follow-up observations with Imaging Atmospheric Cherenkov Telescopes can significantly increase the detection of TeV emission during the early afterglow. Specifically, this method boosts detection rates by up to a factor of two for instruments like ASTRI and LACT compared to focusing only on well-localized events, and enables up to four VHE detections per year with CTAO.

What carries the argument

The rapid tiling strategy for covering large localization error regions, which enables observation of the rapidly decaying early afterglow despite imprecise initial positions from MeV detectors.

If this is right

  • For ASTRI and LACT the detection rate increases by up to a factor of two over well-localized-only strategies.
  • CTAO could achieve up to four VHE detections per year with the tiling approach.
  • Detection rates vary with observational latency, exposure time per tile, and overall strategy.
  • The evaluation uses next-generation telescopes with larger fields of view.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Observatories might adopt automated tiling protocols as standard response to MeV-detector alerts.
  • More early VHE detections could tighten constraints on particle acceleration in GRB outflows.
  • The same tiling logic could apply to other poorly localized transients such as certain fast radio bursts.
  • Direct comparison of real alert follow-ups against these simulations would test the input population model.

Load-bearing premise

The simulations rely on a GRB population and afterglow emission models that match fifteen years of Fermi and Swift observations.

What would settle it

A multi-year campaign applying tiled follow-ups to poorly localized GRBs and recording substantially fewer TeV detections than the simulated rates would falsify the predicted gains.

Figures

Figures reproduced from arXiv: 2604.07582 by A. Berti, A. Ierardi, B. Banerjee, D. Miceli, F. Aharonian, F. Sch\"ussler, G. Oganesyan, J. Green, M. Branchesi, M. Seglar-Arroyo, P. Tiwari, S. Macera, S. Mohnani.

Figure 1
Figure 1. Figure 1: Information retrieved from https://heasarc.gsfc. nasa.gov/w3browse/fermi/fermigtrig.html. The instru￾ments which localized GRBs that are detected by GBM are mentioned in the legend. In total 2967 GRBs are localized only by GBM. Sources with LAT err-radius more than 5◦ excluded (17/3807; counted until 14 July, 2024; 16 years since the launch of Fermi). GRBs localized with MAXI, MASTER, AGILE, IN￾TEGRAL, and… view at source ↗
Figure 2
Figure 2. Figure 2: Flowchart for preparation of the GRB catalog. See Sect. [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: The simulated 220 long GRBs (red points) and the sensi [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: The sensitivity curves for ASTRI (Scuderi 2024), LACT (Zhang et al. 2025), and LHAASO. In addition, we also show the sensitivity of CTAO-N (obtained from CTAO performance documentation). All the sensitivities are obtained under standard assumptions, such as, dark sky condition, low zenith angle (20◦ ) and high atmospheric transmission. and CTAO-N) remains limited (diameter of 4◦ − 10◦ ), the well￾localized… view at source ↗
Figure 5
Figure 5. Figure 5: The simulated light curve of the synthetic population of GRBs in 0.02-5 TeV (energy integrated light curve; top left), 0.02 [PITH_FULL_IMAGE:figures/full_fig_p007_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: The intrinsic VHE light curves of simulated 220 long [PITH_FULL_IMAGE:figures/full_fig_p008_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: Heatmap showing the detectability of the GRBs in the [PITH_FULL_IMAGE:figures/full_fig_p008_7.png] view at source ↗
Figure 9
Figure 9. Figure 9: Number of detectable GRBs by CTAO-N, ASTRI and [PITH_FULL_IMAGE:figures/full_fig_p009_9.png] view at source ↗
Figure 8
Figure 8. Figure 8: Same as Fig [PITH_FULL_IMAGE:figures/full_fig_p009_8.png] view at source ↗
Figure 10
Figure 10. Figure 10: Detectability heatmaps for CTAO-N as a function of la [PITH_FULL_IMAGE:figures/full_fig_p010_10.png] view at source ↗
Figure 11
Figure 11. Figure 11: Simulated follow-up observation strategy for the Ground [PITH_FULL_IMAGE:figures/full_fig_p011_11.png] view at source ↗
read the original abstract

Gamma-ray bursts (GRBs) are among the most luminous and rapidly evolving transients in the Universe. While space-based instruments have extended GRB observations up to energies of $\sim$100 GeV, the detection of very-high-energy (VHE; $E>100$ GeV) emission from ground-based telescopes, especially during prompt or/and the early afterglow phase, remains challenging. These difficulties arise from the rapid temporal decay of GRB afterglows, strong attenuation by the extragalactic background light (EBL), observational latency, and the typical poor sky localization provided by MeV-detectors such as Fermi/GBM. In this work, we investigate the prospects for detecting TeV ($\sim$100 GeV--1 TeV) emission from poorly localized GRBs by adopting optimized follow-up strategies based on rapid tiling of large localization regions. We simulate a realistic population of GRBs informed by more than fifteen years of Fermi/GBM and Swift/XRT detections and recent progresses in the afterglow emission modeling. Using these simulations, we evaluate the detectability of GRB early afterglows by the next-generation Imaging Atmospheric Cherenkov Telescopes, equipped with larger field-of-view (FoV), as a function of latency, exposure time, and observational strategy. Our strategy can significantly enhance the detection rate; for instruments such as ASTRI and LACT, it increases by up to a factor of two compared to strategies limited to well-localized (Swift-like) events. For CTAO, our proposed approach provides up to four VHE detections per year.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript simulates a realistic population of GRBs drawn from >15 years of Fermi/GBM and Swift/XRT catalogs, folds in current afterglow emission models to predict TeV early-afterglow emission, and evaluates detection prospects for next-generation IACTs (ASTRI, LACT, CTAO) when using rapid tiling of large localization regions from poorly localized events. It reports that optimized follow-up increases detection rates by up to a factor of two relative to Swift-like well-localized strategies, with CTAO yielding up to four VHE detections per year.

Significance. If the underlying GRB population, redshift distribution, and early-time TeV afterglow properties are accurately represented, the work supplies concrete quantitative guidance for transient follow-up strategies that could meaningfully raise the annual yield of VHE GRB detections with wide-FoV IACT arrays. The simulation framework itself, informed by real catalogs and published models, is a useful forward-looking tool for observatory planning.

major comments (2)
  1. [GRB population and afterglow modeling section] The headline detection-rate gains (factor-of-two for ASTRI/LACT; up to four CTAO events yr^{-1}) rest on the fidelity of the simulated early-afterglow VHE population for GBM-only events. The manuscript does not report a direct comparison of the adopted luminosity function, redshift distribution, or t < 100 s TeV light-curve shapes against the existing sample of VHE-detected GRBs (e.g., GRB 190114C, GRB 180720B). Without this cross-check, it is impossible to quantify how much the quoted enhancement factors would shift if the true fraction of bright early TeV afterglows among poorly localized bursts differs from the model inputs.
  2. [Results and detection-rate calculations] The Monte Carlo results are presented as a function of latency and exposure time, yet no systematic variation is shown for key model ingredients such as the EBL attenuation prescription or the assumed fraction of bright early afterglows. A 30–50 % change in either parameter would proportionally rescale the reported detection rates and the claimed factor-of-two improvement; the absence of such a sensitivity study leaves the robustness of the central numbers untested.
minor comments (2)
  1. [Figures] Figure captions and axis labels should explicitly state the assumed EBL model and the precise definition of 'detection' (e.g., significance threshold and energy range) so that readers can reproduce the quoted rates.
  2. [Abstract and Results] The abstract states 'up to four VHE detections per year' for CTAO; the corresponding section should clarify whether this is a median, mean, or optimistic value over the simulated population and whether it includes only prompt/early-afterglow phases.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the constructive and detailed comments. We address each major point below, explaining our approach and the revisions we will implement to strengthen the manuscript.

read point-by-point responses

  1. Referee: [GRB population and afterglow modeling section] The headline detection-rate gains (factor-of-two for ASTRI/LACT; up to four CTAO events yr^{-1}) rest on the fidelity of the simulated early-afterglow VHE population for GBM-only events. The manuscript does not report a direct comparison of the adopted luminosity function, redshift distribution, or t < 100 s TeV light-curve shapes against the existing sample of VHE-detected GRBs (e.g., GRB 190114C, GRB 180720B). Without this cross-check, it is impossible to quantify how much the quoted enhancement factors would shift if the true fraction of bright early TeV afterglows among poorly localized bursts differs from the model inputs.

    Authors: We agree that explicit validation against the observed VHE GRB sample is valuable. The afterglow models adopted in our work are taken from recent literature that reproduces the TeV properties of events such as GRB 190114C and GRB 180720B. However, we did not include a side-by-side comparison of the simulated distributions specifically for the GBM-only subpopulation. In the revised manuscript we will add a dedicated subsection that directly compares the adopted luminosity function, redshift distribution, and early-time (t < 100 s) TeV light-curve shapes to the properties inferred from the known VHE detections. This addition will allow readers to evaluate any potential offset in the bright-afterglow fraction for poorly localized bursts. We note that the reported factor-of-two improvement is driven by the difference in sky-coverage strategy between rapid tiling and Swift-like pointing; because this factor is a ratio, it is expected to remain stable even if the overall normalization of the VHE population is rescaled. revision: yes

  2. Referee: [Results and detection-rate calculations] The Monte Carlo results are presented as a function of latency and exposure time, yet no systematic variation is shown for key model ingredients such as the EBL attenuation prescription or the assumed fraction of bright early afterglows. A 30–50 % change in either parameter would proportionally rescale the reported detection rates and the claimed factor-of-two improvement; the absence of such a sensitivity study leaves the robustness of the central numbers untested.

    Authors: We acknowledge that a sensitivity analysis would better demonstrate robustness. In the revised manuscript we will add a new figure and accompanying text that recomputes the annual detection rates after (i) switching between standard EBL models (Franceschini et al. versus Gilmore et al.) and (ii) varying the fraction of bright early afterglows by ±30 % and ±50 %. The results will show that while absolute rates scale with these parameters, the relative improvement factor of up to two (for ASTRI/LACT) and the CTAO yield of up to four events per year remain essentially unchanged, because the gain originates from the observational tiling strategy rather than from the absolute VHE normalization. revision: yes

Circularity Check

0 steps flagged

No significant circularity: forward Monte-Carlo simulation from external catalogs and models

full rationale

The paper simulates a GRB population drawn from 15+ years of Fermi/GBM and Swift/XRT catalogs, folds it through published afterglow emission models, EBL attenuation, and instrument response, then computes detection rates under different tiling strategies. No equation or central claim reduces by construction to a quantity defined from the authors' own fitted parameters or prior self-citations. The quoted enhancement factors (factor-of-two for ASTRI/LACT, up to four CTAO detections/year) are direct outputs of the simulation pipeline rather than re-statements of its inputs. This is a standard forward-looking forecast study whose validity rests on the fidelity of the external inputs, not on any self-referential loop.

Axiom & Free-Parameter Ledger

2 free parameters · 1 axioms · 0 invented entities

The central claim rests on (1) a GRB population model fitted to historical Fermi/GBM and Swift/XRT catalogs and (2) afterglow emission models taken from recent literature; no new physical entities are postulated.

free parameters (2)
  • GRB population parameters
    Rate density, luminosity function, and redshift distribution are informed by fifteen years of Fermi/GBM and Swift/XRT detections and therefore fitted to data.
  • Afterglow model parameters
    Parameters controlling early afterglow light curves and TeV emission are taken from recent modeling progress and implicitly fitted to existing observations.
axioms (1)
  • domain assumption The simulated GRB population and afterglow physics accurately represent the true distribution and emission processes for poorly localized events.
    Invoked when the authors state the simulations are 'informed by' catalog data and 'recent progresses in the afterglow emission modeling'.

pith-pipeline@v0.9.0 · 5655 in / 1462 out tokens · 68064 ms · 2026-05-10T17:04:15.066582+00:00 · methodology

discussion (0)

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Reference graph

Works this paper leans on

3 extracted references · 3 canonical work pages

  1. [1]

    Abdalla, H. et al. 2019, Nature, 575, 464 Abdo, A. A., Ackermann, M., Ajello, M., et al. 2009a, ApJ, 706, L138 Abdo, A. A., Ackermann, M., Arimoto, M., et al. 2009b, Science, 323, 1688 Abe, H. et al. 2023, Mon. Not. Roy. Astron. Soc., 527, 5856 Abe, K. et al. 2025a, Astrophys. J. Lett., 988, L42 Abe, S. et al. 2025b, Astron. Astrophys., 700, A96 Acciari, ...

    work page arXiv 2019

  2. [2]

    This can be explained by the fact that we are sampling few GRBs compared to the all GRBs detected by GBM, and very bright GRBs are typically less frequent

    Both distributions peak atS γ ∼10 −6–10−5,erg,cm −2 and show similar behavior over several orders of magnitude in fluence, with an exception for high fluences values. This can be explained by the fact that we are sampling few GRBs compared to the all GRBs detected by GBM, and very bright GRBs are typically less frequent. Appendix D: Observational Strategy...

    work page 2026

  3. [3]

    Right panel: normalized fluence distributions ofP 2 andP 5, illustrating that the mock sample repro- duces the prompt-emission properties of GBM-detected GRBs

    The vertical dashed lines indicate the mean redshift of each sample, showing close agreement between the two distributions. Right panel: normalized fluence distributions ofP 2 andP 5, illustrating that the mock sample repro- duces the prompt-emission properties of GBM-detected GRBs. Article number, page 17 of 20 A&A proofs:manuscript no. arxiv_03042026 0 ...

    work page 2026