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First search for GeV neutrinos from bright gamma-ray solar flares using the IceCube Neutrino Observatory

T0 review · 3 major / 4 minor · reviewed 2026-08-14 · deepseek-v4-flash

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Pith's one-line read A first search for GeV neutrinos from bright gamma-ray solar flares with IceCube finds no significant signal and sets upper limits that constrain optimistic models.

desk verdict First IceCube GeV solar-flare neutrino search: solid null result, plausible Fargion exclusion, but the limit calculation is under-specified in the proceedings text. read the letter →

arxiv 1908.08300 v1 pith:WOOKJYYE submitted 2019-08-22 astro-ph.HE astro-ph.SR

Gwenhaël de Wasseige (for the IceCube Collaboration) This is my paper · ORCID
classification astro-ph.HEastro-ph.SR
keywords solarflaresneutrinosgammaraysIceCubeDeepCoremulti-messengerastronomyprotonaccelerationupperlimits
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 tries to establish whether the Sun emits GeV neutrinos during bright solar flares and to set the first constraints on that emission using IceCube data. It argues that pion production in the lower solar atmosphere should produce gamma rays and neutrinos together, so the brightest pion-decay flares seen by Fermi-LAT are the best targets. A new IceCube event selection lowered the detector's energy reach into the GeV range, and a rate-monitoring search found no significant excess in five flares from 2012 to 2017. The resulting upper limits rule out an optimistic theoretical prediction for solar-flare neutrino emission, while a more conservative prediction remains below the reach of the current analysis.

What carries the argument

The analysis is carried by the hadronic pion-production chain that links solar-flare gamma rays and neutrinos: accelerated protons collide with chromospheric plasma to produce pions whose decays yield both gamma rays and neutrinos. Fermi-LAT observations of pion-decay-dominated gamma-ray emission select five bright flares and define the impulsive-phase time windows. In IceCube, a new GeV event selection based on detected photoelectrons and causality between optical modules reduces the data rate by five orders of magnitude while keeping more than 40% of GeV neutrino events, and a rate-monitoring approach (rather than direction reconstruction) searches for an excess in each flare window. Significance is computed with the Li and Ma statistic, and the excess implied by $N_{\mathrm{on}} - \alpha N_{\mathrm{off}}$ is converted into flux limits through the effective area of the selection.

What would settle it

A future, more sensitive search that records a statistically significant excess of GeV events in coincidence with a gamma-ray-bright solar flare at a flux above the upper limits reported here would refute the conclusion that solar-flare neutrinos are absent at this level; conversely, a gamma-ray-bright flare with no coincident excess in such a detector would tighten the limits.

Watch

Extended reading notes

Core claim

None of the five selected solar flares shows a statistically significant increase in the rate of GeV-like events in IceCube when the flux is compared with an eight-hour off-source window. The paper therefore derives the first upper limits on the solar-flare neutrino flux between 0.5 and 5 GeV as a function of the neutrino spectral index, using the effective area of the new low-energy event selection. For all flares except the September 6, 2017 long-duration window, these limits exclude the optimistic Fargion prediction with an average neutrino energy of 140 MeV; the 500 MeV prediction sits just below the current sensitivity, and the more conservative model considered in the paper cannot be tested yet.

Load-bearing premise

The analysis assumes that the flares that produce bright pion-decay gamma rays are the same flares that produce detectable neutrinos; if the accelerated proton populations for the two messengers differ, the search could miss the relevant events and the limits would not apply to solar-flare neutrinos in general.

Editorial extensions

If this is right

  • No solar-flare neutrino burst brighter than the derived upper limits occurred in the five gamma-ray-bright flares studied, so hadronic proton acceleration in those events did not produce a GeV neutrino fluence above this level.
  • The combination of Fermi-LAT gamma-ray selection with IceCube rate monitoring provides a working template for future GeV solar-flare neutrino searches.
  • The upper limits constrain models that channel a large fraction of flare energy into high-energy protons, while models with smaller energy fractions remain consistent with the data.
  • The September 6, 2017 long-duration emission window is set aside from the model comparison, so long-duration solar-flare neutrino emission is not constrained by this analysis.
  • With the IceCube-Upgrade's lower threshold and improved reconstruction, the same search strategy has a defined path toward sensitivity at the 500 MeV prediction line.

Reading between the lines

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

  • If the selection assumption is relaxed, the reported limits should not be read as a general bound on solar-flare neutrinos; they apply specifically to gamma-ray-bright, impulsive-phase flares.
  • A natural extension not explored in the paper is to stack many smaller flares over the next solar cycle, trading per-flare sensitivity for integrated exposure and testing the same hadronic-emission hypothesis.
  • One implicit consequence is that the null result mainly constrains the high-energy tail of the accelerated proton spectrum; the lower-energy neutrino flux predicted by conservative models remains entirely untested.
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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 / 4 minor

Summary. This proceedings paper reports the first IceCube search for GeV neutrinos from solar flares, using a low-threshold DeepCore event selection and a rate-monitoring approach rather than directional reconstruction. Six solar flares selected by Fermi-LAT pion-decay gamma-ray emission are analyzed; Table 2 gives the on/off counts and Li-Ma significances, all below 0.9 sigma, so no significant excess is claimed. The paper then derives upper limits on the neutrino flux using the counts and effective areas, and compares these with theoretical predictions, stating that the limit constrains Fargion's 140 MeV model while the 500 MeV model and the prediction of [13] are below the current reach. The paper emphasizes the novelty of the GeV-scale search and the prospects for the next solar cycle with the IceCube-Upgrade.

Significance. The central null result is well supported: for all five flares the Li-Ma significances are below 0.9 sigma, and the observed on-source counts are consistent with the expected background counts. The paper's main strengths are the innovative use of a gamma-ray-selected flare sample, the explicit description of the fixed 8-hour background window, and the transparent tabulation of event counts and significances. However, the more ambitious physical claim that the derived upper limits constrain Fargion's 140 MeV prediction is not independently verifiable from the manuscript, because the upper-limit calculation is not specified in enough detail (no confidence level, no treatment of negative signal counts, no propagation of the stated 13% background uncertainty). If these details are supplied, the paper would be a solid and useful contribution; as it stands, the model-exclusion statement goes beyond what the text demonstrates.

major comments (3)
  1. [Section 4, upper-limit derivation] The derivation of the neutrino flux upper limit is not specified: the text only states that the limit is obtained from Non - alpha Noff and the effective areas, but does not give the confidence level, the statistical prescription (e.g., Poisson, Feldman-Cousins, or likelihood), or the treatment of cases where Non - alpha Noff is negative, such as September 10, 2017 (Non = 5, alpha Noff = 6.57). The 13% uncertainty on the background rate stated earlier in the same section is also not propagated into the limit. Without this information, the claimed exclusion of Fargion's 140 MeV model cannot be independently verified.
  2. [Section 4 and Figure 4] There is an inconsistency between the text and the figure caption: the text states that Figure 4 shows the upper limit for September 10, 2017, while the caption says 'derived for March 7th, 2012'. Please clarify which flare is shown and state whether the comparison with Fargion's prediction applies equally to all flares, given that the September 6 flare is excluded from the comparison.
  3. [Section 2.1] The search assumes that neutrino emission is temporally coincident with significant pion-decay gamma-ray signals detected by Fermi-LAT. If the proton population that produces the impulsive gamma rays is not the same as the population that produces detectable neutrinos, the selection could miss the relevant flares, and the derived upper limits would not constrain solar flare neutrino emission in general. The paper should state this caveat explicitly when interpreting the limits.
minor comments (4)
  1. [Section 2.1] The phrase 'are reported, are reported' in the first paragraph of Section 2.1 contains a duplicated phrase; please remove the repetition.
  2. [Section 1] The word 'perfomed' in the first paragraph of Section 1 should be spelled 'performed'.
  3. [Table 2] The column header 'Non-expected' would be clearer as 'N_expected' to match the notation used in the text.
  4. [Section 2.2] In the sentence 'withδ = 3', a space is missing between 'with' and 'δ'; also, 'E f l = 1032 erg' appears without superscript formatting for the exponent.

Circularity Check

0 steps flagged · score 0.0 of 10

No circular derivation: the null result and upper limits are computed from IceCube on/off event counts and simulated effective areas, with an external Fargion benchmark.

full rationale

The paper's central claim is an experimental null result: Table 2 lists on-source and off-source event counts and Li & Ma significances, all below 1 sigma. The upper limit is then derived from the excess counts (Non - alpha Noff) and the detector effective area from Section 3.1. This is a direct data-to-limit chain with no fitted parameter renamed as a prediction. The selection of flares is motivated by the physical assumption that neutrino emission accompanies pion-decay gamma rays observed by Fermi-LAT; this is an external multimessenger hypothesis, not an input that is later recovered as the output. The comparison with Fargion uses an independent formula, Eq. 2.11 of reference [14], with stated assumptions, so the model exclusion is an external benchmark rather than a construction of the search. The self-citations to the author's PhD thesis [13] supply an assumed neutrino spectral index range (4-6) and an additional theoretical prediction, but neither enters the significance calculation nor is fitted to the IceCube flare data; the same-author prediction is not used to define the limit. The manuscript does omit the detailed statistical prescription for converting excess counts into an upper limit and does not propagate the stated 13% background uncertainty, but that is a missing-support limitation, not circularity. No equation in the paper reduces to its own input by definition, and no load-bearing argument relies solely on a self-citation.

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

The analysis relies on standard assumptions about solar flare proton spectra, detector simulation, and the gamma-ray/neutrino coincidence. No new physical entities are introduced, and the free parameters are external model inputs or assumed spectral indices, not fitted to the IceCube data.

free parameters (5)
  • Assumed neutrino spectral index delta_nu = 4 to 6, estimated from [13]
    Used to set the shape of the neutrino spectrum for the flux upper limit; the limit is presented in the (delta_nu, C) parameter space.
  • Assumed proton spectral index delta_p = 3.2 for March 7, 2012 (from gamma-ray observations)
    Used in the simulation of neutrino yield for the theoretical comparison in Figure 4.
  • Solar flare energy E_fl = 10^32 erg
    Input parameter from Fargion's model for the theoretical flux prediction.
  • Average neutrino energy <E_nu e> = 140 MeV and 500 MeV
    Input parameters from Fargion's model for the theoretical flux prediction; the 140 MeV case is constrained by the limits.
  • Upper cutoff E_max in proton spectrum = 3, 5, 7, 10 GeV
    Illustrative values for showing the neutrino yield in Figure 2; not directly used in the final limits.
assumptions (5)
  • domain assumption Neutrino emission is coincident with significant pion-decay gamma-ray signals detected by Fermi-LAT.
    Section 2.1 states this assumption explicitly, which is the basis for selecting the five flares.
  • domain assumption The accelerated proton flux is a power law, dphi/dE = A E^-delta H(E_max - E).
    Section 2.2 defines the proton spectrum model used for the yield calculations.
  • domain assumption The background event rate during a flare equals the rate measured in the 8-hour off-source window, after excluding the immediate pre-flare period.
    Section 4 uses this to estimate the expected number of background events in the Li-Ma test.
  • domain assumption The solar atmosphere composition is 75% hydrogen and 25% helium for the proton-nucleus interaction simulation.
    Footnote in Section 4 states this standard solar-surface assumption, which affects the neutrino yield calculation.
  • domain assumption The GENIE 2.8.6 simulation with KNO and PYTHIA hadronization accurately models IceCube's response to GeV neutrinos.
    Section 3.1 relies on these simulations to compute effective areas and selection efficiency.

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Cite this review

Pith. "Pith review of First search for GeV neutrinos from bright gamma-ray solar flares using the IceCube Neutrino Observatory." pith.science (2026). https://pith.science/paper/WOOKJYYE

@misc{pith2026190808300,
  author       = {Pith},
  title        = {Pith review of: First search for GeV neutrinos from bright gamma-ray solar flares using the IceCube Neutrino Observatory},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/WOOKJYYE}},
  note         = {Machine review of arXiv:1908.08300}
}
read the original abstract

In response to a reported increase in the total neutrino flux in the Homestake experiment in coincidence with solar flares at the end of the eighties, solar neutrino detectors have searched for solar flare signals. Solar flares convert magnetic energy into thermal energy of plasma and kinetic energy of charged particles such as protons. As a consequence of magnetic reconnection, protons are injected downwards from the coronal acceleration region and can interact with dense plasma in the lower solar atmosphere, producing mesons that will subsequently decay into gamma rays and neutrinos at O(MeV-GeV) energies. The main motivation to search for solar flare neutrinos comes from their hadronic origin. As inherent products of high-energy proton collisions with the chromosphere, they are a direct probe of the proton accelerated towards the chromosphere. Using a multi-messenger approach, it is therefore possible to constrain the proton acceleration taking place in the solar flares, including the spectral index of the accelerated flux and its shape. We present the results of the first search for GeV neutrinos emitted during solar flares carried out with the IceCube Neutrino Observatory. We present a new approach which allows us to strongly lower the energy threshold of IceCube, originally designed to detect 10 GeV - PeV neutrinos. We compare the results with theoretical estimates of the corresponding flux.

Figures

Figures reproduced from arXiv: 1908.08300 by the authors.

Figure 1
Figure 1. Selected time window (orange points) compared to the initial gamma-ray light curve of [PITH_FULL_IMAGE:figures/full_fig_p004_1.png] view at source ↗
Figure 2
Figure 2. Energy distribution for neutrinos depending on the upper cutoff in the accelerated proton [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. Effective area for νe and νµ at the final level of the event selection. The plot shows that the effective area is constant in the zenith band considered in the analysis [PITH_FULL_IMAGE:figures/full_fig_p006_3.png] view at source ↗
Figures from the paper (1 more)
Figure 4
Figure 4. Figure 4: Comparison of the experimental upper limit derived for March 7th, 2012 and the corre [PITH_FULL_IMAGE:figures/full_fig_p007_4.png]

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Reference graph

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