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The (missing) gamma-ray counterparts to EP events

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Maria Edvige Ravasio, Ansh Chopra, Peter G. Jonker, Qinyu Wu, Eric Burns, Biswajit Banerjee, Gor Oganesyan, Stefano Ascenzi
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Jesse T. Palmerio, Frederic Daigne, Joshua Wood, Rachel Hamburg, Andrew J. Levan, Jonathan A. Quirola-Vasquez, Javier Sanchez-Sierras, Joyce N. D. van Dalen, Agnes P. C. van Hoof, Hui Sun, Weimin Yuan, Nikhil Sarin, Antonio Martin-Carrillo, Dylan Adrien, Ce Cai, YanQiu Zhang, Samuele Ronchini, Marica Branchesi

High Energy Astrophysical Phenomena · Submitted 20 Oct 2026 · 300 references

Pith's verdict: A careful and transparent census of gamma-ray counterparts to EP's first-year sample; the central conclusion that most are low-energy GRB extensions is plausible but rests on untested extrapolation of empirical correlations, which the authors openly acknowledge.

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arXiv ID2610.28734 (version 1)
Pith IDCPWYTJ23, Pith's permanent record of this paper and its review
CitationAgnes P. C. van Hoof, Andrew J. Levan, Ansh Chopra, Antonio Martin-Carrillo, Biswajit Banerjee, Ce Cai, Dylan Adrien, Eric Burns, Frederic Daigne, Gor Oganesyan, Hui Sun, Javier Sanchez-Sierras, Jesse T. Palmerio, Jonathan A. Quirola-Vasquez, Joshua Wood, Joyce N. D. van Dalen, Maria Edvige Ravasio, Marica Branchesi, Nikhil Sarin, Peter G. Jonker, Qinyu Wu, Rachel Hamburg, Samuele Ronchini, Stefano Ascenzi, Weimin Yuan, Yanqiu Zhang. "The (missing) gamma-ray counterparts to EP events." arXiv:2610.28734 (2026). https://pith.science/paper/CPWYTJ23
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The paper

The paper claims

A systematic search shows that 68% of first-year Einstein Probe extragalactic transients have no detected gamma-ray counterpart, yet their properties are consistent with a low-energy extension of the classical gamma-ray burst (GRB)…

The reading

The paper systematically searches for gamma-ray counterparts to 72 extragalactic transients detected by Einstein Probe (EP) during its first year. It finds that 23 events (32%) have a gamma-ray counterpart, while 49 (68%) do not, despite most being visible to Fermi/GBM. For the 49 without counterparts, simultaneous 3-$\sigma$ flux upper limits imply low spectral peak energies (E_peak < 30–100 keV) and, for the 14 with redshifts, low isotropic luminosities (L_iso < $10^{48}$–9×$10^{51}$ erg/s). If classical GRB correlations hold, these limits imply low bulk Lorentz factors (Gamma_0 < 100), as confirmed for a few events by afterglow modeling. The paper concludes that many EP transients without gamma-ray counterparts could be compatible with the GRB population, extending it toward softer spectra, lower luminosities, and less relativistic jets.

What carries the argument

The Fermi/GBM Targeted Search pipeline, adapted for long durations and fixed sky positions, provides simultaneous 3-sigma flux upper limits in the 10–1000 keV band over the X-ray duration of each EP event. These upper limits are combined with EP/WXT 0.5–4 keV fluxes to constrain the spectral peak energy E_peak via a Band-function model. For events with redshifts, the Yonetoku relation (L_iso–E_peak) and the Ghirlanda relation (Gamma_0–L_iso) are used to infer isotropic luminosity and bulk Lorentz factor limits.

Extended reading notes

Core claim

Over two-thirds of the first-year Einstein Probe extragalactic transients lack a detected gamma-ray counterpart, yet their X-ray fluxes, durations, and derived upper limits on peak energy and luminosity are statistically consistent with being drawn from the low-energy tail of the classical GRB distribution. The gamma-ray non-detections are not evidence of a distinct class; instead, they suggest that EP is revealing a previously hidden population of low-luminosity, soft-spectrum GRBs with mildly relativistic jets (Gamma_0 ≲ 100). This extends the known GRB population to regimes that gamma-ray instruments alone cannot access.

Load-bearing premise

The paper assumes that the Yonetoku (L_iso–E_peak) and Ghirlanda (Gamma_0–L_iso) correlations established for classical GRBs hold unchanged in the low-luminosity, low-E_peak regime where the EP FXTs lie; if these relations have different slopes or break down, the inference that these events are low-energy GRBs with mildly relativistic jets would be unreliable.

Editorial extensions

If this is right

  • If correct, EP is uncovering a continuous distribution of GRB properties down to E_peak ~30 keV, L_iso ~10^48 erg/s, and Gamma_0 ~20–100, bridging the gap between classical GRBs and X-ray flashes.
  • The detection of associated Type Ic-BL supernovae in several of these events supports a collapsar origin, linking low-luminosity GRBs to massive star core collapse.
  • The lack of GeV emission from Fermi/LAT places first constraints on the synchrotron self-Compton component in this soft, low-power regime, favoring low magnetic fields and Lorentz factors.
  • The finding that X-ray brightness alone does not predict gamma-ray detectability implies that spectral shape and jet dynamics, not just flux, govern whether the high-energy counterpart is observable.
  • Future larger EP samples, combined with CTAO observations, can test whether these events form a smooth extension of the GRB population or require additional physical channels (e.g., shock breakout).

Reading between the lines

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

  • The low Lorentz factors (Gamma_0 < 100) inferred for many events suggest that the jet is only mildly relativistic, potentially consistent with relativistic shock breakout or cocoon emission rather than a clean fireball, a scenario that could be tested with detailed afterglow polarization measurements.
  • The high fraction (68%) of gamma-ray-quiet EP transients implies that the GRB luminosity function at low energies is severely undersampled by gamma-ray missions alone, and that soft X-ray surveys are essential to complete the picture.
  • If the empirical correlations (Yonetoku, Ghirlanda) break down at low luminosities, the inferred low Lorentz factors could be spurious; a direct measurement of compactness from gamma-ray upper limits could independently constrain Gamma_0 without assuming correlations.
Original abstract

While the connection of Fast X-ray Transients (FXTs) detected by Einstein Probe (EP) with Gamma-ray Bursts (GRBs) is increasingly compelling, most lack gamma-ray counterparts, leaving their relation to GRBs unclear. We performed a systematic search for gamma-ray counterparts to the 72 transients detected by EP in its first year, using targeted searches in Fermi/GBM and Fermi/LAT data, complemented by SVOM/GRM, SVOM/ECLAIRs, Swift/BAT and Konus-Wind. For the visible events without a gamma-ray counterpart, we derived simultaneous $3\sigma$ flux upper limits in the 10-1000 keV and the 0.1-1 GeV band. We find that 23 events (32%) have a gamma-ray counterpart (seven sub-threshold), while 49 events (68%) have none, despite most being visible. For events with a gamma-ray counterpart, the X-ray flux over the gamma-ray $T_{\rm 90}$ is systematically higher than over the full X-ray duration, with flux ratios $1 \lesssim F_{\rm [10-1000\,keV]}/F_{\rm [0.5-4\,keV]} \lesssim 100$, suggesting that gamma-rays are preferentially detected during the brightest X-ray phases. For events without gamma-ray counterpart, the most stringent Fermi/GBM upper limits imply $E_{\rm peak} \lesssim 30$-100 keV and, for the 14 events with measured redshifts, $L_{\rm iso}\lesssim 10^{48}-9\times10^{51}$ erg/s. If classical GRB correlations hold in this regime, these lower energetics imply lower bulk Lorentz factors ($\Gamma_0 \lesssim 100$), as confirmed in a few cases by afterglow constraints. No significant GeV emission was detected by Fermi/LAT, yielding $1 \lesssim F_{0.1-1\,{\rm GeV}}/F_{0.5-4\,{\rm keV}} \lesssim 10$, placing the first constraints on a potential synchrotron self-Compton component for this new population. Although individual origins remain uncertain, many EP FXTs could be compatible with the GRB population, extending it toward lower $E_{\rm peak}$, luminosities, and bulk Lorentz factors.

The compiled record

The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.

Compiled by classifier-1.13.0 via TypeSafe API, question bank abstract-p1-20260923, 2026-10-22: 1 request, 7,686 input tokens.

What the abstract itself states, as probabilities: claims first 0.11 · states limitation 0.87 · human participants 0.01 · controlled comparison 0.13 · randomized assignment 0.01 · states quantitative result 0.99 · compares with prior results 0.47 · mentions code or data availability 0.02.

The abstract, sentence by sentence

arXiv listing abstract v1. Results and the main claim are underlined; .

While the connection of Fast X-ray Transients (FXTs) detected by Einstein Probe (EP) with Gamma-ray Bursts (GRBs) is increasingly compelling, most lack gamma-ray counterparts, leaving their relation to GRBs unclear. We performed a systematic search for gamma-ray counterparts to the 72 transients detected by EP in its first year, using targeted searches in Fermi/GBM and Fermi/LAT data, complemented by SVOM/GRM, SVOM/ECLAIRs, Swift/BAT and Konus-Wind. For the visible events without a gamma-ray counterpart, we derived simultaneous $3\sigma$ flux upper limits in the 10-1000 keV and the 0.1-1 GeV band. We find that 23 events (32%) have a gamma-ray counterpart (seven sub-threshold), while 49 events (68%) have none, despite most being visible. For events with a gamma-ray counterpart, the X-ray flux over the gamma-ray $T_{\rm 90}$ is systematically higher than over the full X-ray duration, with flux ratios $1 \lesssim F_{\rm [10-1000\,keV]}/F_{\rm [0.5-4\,keV]} \lesssim 100$, suggesting that gamma-rays are preferentially detected during the brightest X-ray phases. For events without gamma-ray counterpart, the most stringent Fermi/GBM upper limits imply $E_{\rm peak} \lesssim 30$-100 keV and, for the 14 events with measured redshifts, $L_{\rm iso}\lesssim 10^{48}-9\times10^{51}$ erg/s. If classical GRB correlations hold in this regime, these lower energetics imply lower bulk Lorentz factors ($\Gamma_0 \lesssim 100$), as confirmed in a few cases by afterglow constraints. No significant GeV emission was detected by Fermi/LAT, yielding $1 \lesssim F_{0.1-1\,{\rm GeV}}/F_{0.5-4\,{\rm keV}} \lesssim 10$, placing the first constraints on a potential synchrotron self-Compton component for this new population. Although individual origins remain uncertain, many EP FXTs could be compatible with the GRB population, extending it toward lower $E_{\rm peak}$, luminosities, and bulk Lorentz factors.

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#sentencerolethe authors' own workstates the main conclusionstrength
s0 While the connection of Fast X-ray Transients (FXTs) detected by Einstein Probe (EP) with Gamma-ray Bursts (GRBs) is increasingly compelling, most lack gamma-ray counterparts, leaving their relation to GRBs unclear. gap 0.53 0.04 0.05 asserted 0.89
s1 We performed a systematic search for gamma-ray counterparts to the 72 transients detected by EP in its first year, using targeted searches in Fermi/GBM and Fermi/LAT data, complemented by SVOM/GRM, SVOM/ECLAIRs, Swift/BAT and Konus-Wind. method 0.92 0.97 0.03 asserted 0.99
s2 For the visible events without a gamma-ray counterpart, we derived simultaneous $3\sigma$ flux upper limits in the 10-1000 keV and the 0.1-1 GeV band. method 0.87 0.97 0.03 asserted 1.00
s3 We find that 23 events (32%) have a gamma-ray counterpart (seven sub-threshold), while 49 events (68%) have none, despite most being visible. result 1.00 0.97 0.44 asserted 1.00
s4 For events with a gamma-ray counterpart, the X-ray flux over the gamma-ray $T_{\rm 90}$ is systematically higher than over the full X-ray duration, with flux ratios $1 \lesssim F_{\rm [10-1000\,keV]}/F_{\rm [0.5-4\,keV]} \lesssim 100$, suggesting that gamma-rays are preferentially detected during the brightest X-ray phases. result 0.94 0.95 0.09 asserted 0.70
s5 For events without gamma-ray counterpart, the most stringent Fermi/GBM upper limits imply $E_{\rm peak} \lesssim 30$-100 keV and, for the 14 events with measured redshifts, $L_{\rm iso}\lesssim 10^{48}-9\times10^{51}$ erg/s. result 1.00 0.95 0.07 asserted 0.97
s6 If classical GRB correlations hold in this regime, these lower energetics imply lower bulk Lorentz factors ($\Gamma_0 \lesssim 100$), as confirmed in a few cases by afterglow constraints. interpretation 0.99 0.39 0.11 hedged possibility 0.50
s7 No significant GeV emission was detected by Fermi/LAT, yielding $1 \lesssim F_{0.1-1\,{\rm GeV}}/F_{0.5-4\,{\rm keV}} \lesssim 10$, placing the first constraints on a potential synchrotron self-Compton component for this new population. result 0.98 0.95 0.08 asserted 0.98
s8 Although individual origins remain uncertain, many EP FXTs could be compatible with the GRB population, extending it toward lower $E_{\rm peak}$, luminosities, and bulk Lorentz factors. interpretation 1.00 0.40 0.72 hedged possibility 1.00

What kind of paper the classifier reports this is: empirical study 1.00 · evidence from analysis of existing data 0.87.

What was examined, and what was not

sources read
  • arXiv listing abstract v1
not examined
  • full text
  • figures and tables
  • references and citing sentences
  • relations to other papers
levels run
  • P abstract
not run
  • S sections
  • T body sentences
  • C citations
  • R cross-paper relations

Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

Figures
Figure 1. Comparison between average fluxes in γ-ray [10-1000 keV] and in X-ray energy bands [0.5-4 keV] for the sample of EP events analysed in this work. EP sources with a γ-ray counter￾parts are represented by squares: light beige when the X-ray flux is measured over the total X-ray duration (T100,X−ray), and orange when it is measured simultaneously to the γ-ray emission (i.e. during T90,γ). Events that did not trigger Fermi/GBM but were recovered via the Targeted Search are marked with a red contour. In case of non-detections, the GBM 3-σ upper limits simultane￾ous to T100,X−ray are…
Figure 2. Same as Figure 1, but with peak fluxes computed at the
Figure 3. Time-averaged flux in the GeV band compared to the
Figure 5. For EP transients associated with a γ-ray counterpart (left panel), we report both the total (light beige squares) and the simultaneous (orange squares) time-averaged X-ray flux mea￾sured by EP/WXT in the 0.5–4 keV energy range, together with the corresponding average γ-ray flux measured in the 10–1000 keV band (light blue squares). For events within the Fermi/LAT field of view, the corresponding 0.1–1 GeV flux upper limits are shown as green downward arrows. These broadband SEDs are 10 9 10 8 Peak X-ray Flux [0.5-4 keV] (erg cm 2 s 1 ) 10 8 10 7 10 6 P e a k L A T Flu x [0.1-1 G e…
Figure 7. Expected fluxes in the GBM 10–1000 keV band and
Figure 8. Comparing them with the simultaneous X-ray fluxes
Figure 9. Comparison in the Γ0–Liso plane between the EP sources analyzed in this work and the GRB sample with well-constrained prompt emission properties and measured afterglow onset times from the ’gold’ sample of Ghirlanda et al. 2018 (red points). The dashed red line shows their reported best-fit Γ0–Liso cor￾relation, with the shaded region indicating the 1σ scatter. For the EP sources with and without γ-ray counterpart, orange solid lines indicate Liso measurements, while grey dashed lines repre￾sent upper limits, respectively. Two EP events, EP241026b (van Dalen et al. 2026) and…
Figure 10. SSC modelling of a low-luminosity GRB-like scenario.

Showing 8 of the paper's 10 figures. See all figures in the PDF ↗ Figures reproduced from arXiv:2610.28734 by the authors.

References

Discussion

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