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Search for very-high-energy gamma-ray counterparts of gravitational waves with HAWC

T0 review · 0 major / 6 minor · reviewed 2026-08-14 · deepseek-v4-flash

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Pith's one-line read HAWC searched for TeV gamma-ray flashes tied to LIGO/Virgo events and found none, a result consistent with background.

desk verdict A robust first null-result search for VHE gamma-ray counterparts of LIGO/Virgo events; the non-detection stands even if the background calibration is off, but the constraints are limited by small sky coverage and missing systematics. read the letter →

arxiv 1908.06122 v1 pith:AEYMIP6G submitted 2019-08-16 astro-ph.HE

Israel Martinez-Castellanos (for the HAWC Collaboration) This is my paper · ORCID
classification astro-ph.HE
keywords gravitationalwavesgamma-rayburstsvery-high-energygammaraysHAWCmulti-messengerastronomyLIGO/Virgotransientsearchupperlimits
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 asks whether any of the gravitational-wave events detected by LIGO and Virgo in runs O1, O2, and O3 were accompanied by very-high-energy gamma-ray emission in HAWC's sky. The answer it defends is no: after searching eight time windows and all overlapping sky regions, every result is consistent with background, with the largest test statistic giving a trials-corrected p-value of 1.0. The paper also derives an upper limit on late-time TeV emission from GW170817. The point of the exercise is that HAWC's wide field of view and nearly continuous operation let a single instrument check the prompt VHE phase of nearby mergers. A detection would pin down jet physics; an upper limit sharpens what future joint searches must beat.

What carries the argument

The load-bearing instrument is HAWC itself plus the internal analysis chain called ZEBRA: a binned likelihood search over a HEALPix grid at Nside = 512, with events binned by the fraction of detector channels hit ($f_{\mathrm{hit}}$), background estimated by 'direct integration' (convolving a stable local arrival direction probability with the total event rate), and eight logarithmically spaced time windows sliding from $t_0 - 5\Delta t$ to $t_0 + 10\Delta t$. The null-hypothesis test-statistic distribution is produced by randomly fluctuating the expected background through the same analysis, which accounts for pixel and time-window correlations; per-timescale false-alarm rates are then combined to yield the trials-corrected p-value. This machinery is what lets a wide-field survey claim either detection or meaningful constraints on prompt TeV emission.

What would settle it

Take archived HAWC data for the analyzed gravitational-wave events, inject artificial VHE bursts of known flux into the same time windows, and rerun ZEBRA: if the injection recovery fraction differs substantially from the claimed sensitivity at the same test-statistic threshold, the null calibration is wrong. Alternatively, a future binary neutron star merger fully inside HAWC's field of view that is seen in TeV by an independent instrument would directly settle whether the search misses real counterparts.

Watch

Extended reading notes

Core claim

For every LIGO/Virgo gravitational-wave event with at least partial HAWC sky coverage during O1, O2, and O3, a search over time windows from 0.3 to 1000 seconds and over the sky region containing 95% of the localization probability found no excess above background. The largest test statistic at any location, time, event, or timescale was $TS = 26.0$, corresponding to a p-value of 1.0 after trial correction. For GW170817, whose merger time fell outside HAWC's field of view, HAWC observed the later transit 8.20 to 10.23 hours after coalescence and set an upper limit of $1.5 \times 10^{-8}\,\mathrm{erg\,cm^{-2}\,s^{-1}}$ between 100 GeV and 1 TeV. The conclusion is that no very-high-energy gamma-ray counterpart above HAWC's sensitivity was emitted by the analyzed events.

Load-bearing premise

The background estimate and the simulated null-hypothesis test-statistic distribution are correctly calibrated, including all correlations between nearby pixels and overlapping time windows; if those simulations miss a systematic effect, the reported p-value and upper limit would shift.

Editorial extensions

If this is right

  • For nearby binary neutron star and neutron star-black hole mergers inside HAWC's field of view, the search will either detect a VHE counterpart or place an upper limit that constrains prompt-emission models.
  • Starting in O3, HAWC's automatic follow-up of gravitational-wave alerts can provide a localization to about 0.5 degrees within minutes to an hour, helping other telescopes target the counterpart.
  • The non-detection across O1-O3 sets a benchmark: any model predicting bright TeV prompt emission for typical LIGO/Virgo events is disfavoured at HAWC's sensitivity.
  • Continued running increases exposure, so the same pipeline naturally tightens these limits as more gravitational-wave events accumulate.

Reading between the lines

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

  • One extension the paper leaves implicit: apply the same pipeline to short gamma-ray bursts without gravitational-wave triggers, where larger samples could calibrate the VHE detection rate even if the GW association is absent.
  • The GW170817 upper limit covers only the later transit, not the prompt phase; interpreting it as a constraint on prompt emission would require an afterglow model, since the prompt emission was outside the field of view.
  • A sub-second time window below 0.3 seconds could be tried, since Fermi-LAT shows a fraction of short GRB emission concentrated in much shorter intervals than the search's shortest window.
  • An obvious test, not discussed in the paper: run the same search on fake GW alerts placed at random times and locations; the distribution of maximum test statistics should match the claimed null distribution, validating the false-alarm-rate formula.
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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

0 major / 6 minor

Summary. The paper reports a search for very-high-energy (VHE) gamma-ray emission coincident with gravitational-wave (GW) events detected by LIGO/Virgo during O1, O2, and part of O3, using the HAWC observatory. The search is performed on a HEALPix grid over the 95% localization region, with eight time-window widths from 0.3 s to 1000 s, and uses a binned likelihood test statistic. All analyzed events are consistent with background; the maximum test statistic anywhere is TS = 26.0, which corresponds to a post-trial p-value of 1.0. For GW170817, the source was outside the field of view at merger, and an upper limit of 1.5 x 10^-8 erg cm^-2 s^-1 (100 GeV - 1 TeV) is set using the later transit. The paper concludes that no significant VHE counterpart was observed and that HAWC can provide competitive constraints for events in its field of view.

Significance. The result is a null search with no significant excess, which is valuable for constraining VHE emission from compact binary mergers. The paper's strengths are the wide field of view and continuous operation of HAWC, and the explicit post-trial p-value of 1.0 after a large trials correction. However, the quantitative constraints are limited by the small sky coverage for O3 events (25%, 28%, and 4% of localization probability) and the degraded sensitivity for GW170817. The paper does not include systematic uncertainties on the upper limit or full event-by-event results, but these do not affect the central null result, which is robust to plausible errors in the trials factor and background estimation.

minor comments (6)
  1. [Section 2, Eq. (2.3)] The formula for the expected number of false positives should explicitly include the number of gravitational-wave events searched, and the conversion from <n> to a p-value should be stated for the case <n> >= 1 (e.g., p = 1 - exp(-<n>)).
  2. [Section 4] A table listing all analyzed GW events with the observed maximum TS, location, time window, and searched sky fraction would improve reproducibility and is standard for such searches.
  3. [Section 4] The GW170817 upper limit should be accompanied by an estimate of systematic uncertainties from the background normalization and the assumed spectral index (alpha = -2).
  4. [Section 2] The text contains the typo 'precense' (for 'presence'), and the expansion of ZEBRA should read 'Zenith' instead of 'ZEnith.'
  5. [Section 2] The statement that the search is only weakly dependent on the assumed spectral index is not supported with a reference or figure; a brief justification or citation would help.
  6. [Section 3, Figures 1 and 2] The sensitivity curves are compared to Fermi-LAT spectra, but the caption does not state which energy range of the HAWC sensitivity is shown; restating the energy range in the caption would improve clarity.

Circularity Check

0 steps flagged · score 0.0 of 10

No circularity: the search is a direct experimental analysis whose null result is derived from HAWC data and independent background simulation, not from the quantities it reports.

full rationale

This paper reports a targeted search for very-high-energy gamma-ray counterparts of gravitational-wave events. The central result is a null observation: the maximum test statistic over all pixels, events, time windows, and timescales is TS = 26.0 with a post-trials p-value of 1.0. There is no fitted parameter that is then renamed as a prediction, and no theoretical claim is derived from an input that already contains the answer. The assumed spectral index alpha = -2 is a search sensitivity input, not an output of the analysis, and the upper limit on GW170817 is obtained from observed counts and a background estimate, not from the assumed spectrum. The background estimate and null-hypothesis TS distribution are standard statistical calibrations for the search itself; even if imperfect, they do not make the non-detection circular because the conclusion is that no significant excess was found under the stated background model. The comparisons with Fermi-LAT short GRB spectra are illustrative sensitivity references, not predictions derived from HAWC inputs. Citations to prior HAWC analyses describe the likelihood implementation and are not used as a substitute for the data analysis or as justification of the claimed null result. No step in the paper reduces by construction to its own inputs, and no self-citation is load-bearing. The honest finding is therefore no significant circularity.

Assumptions & free parameters 3 free parameters · 4 assumptions · 0 invented entities

The analysis rests on standard statistical assumptions and domain models from previous literature; no new entities are introduced. The main free choices are the spectral index and search geometry.

free parameters (3)
  • Spectral index alpha = -2 (assumed)
    The search assumes a power-law spectrum with index alpha = -2 for all sources. The paper states sensitivity is weakly dependent on the index, but this choice affects the expected signal count.
  • Zenith angle cutoff = 45 degrees
    The search is limited to locations within 45 degrees of zenith because sensitivity degrades at larger angles. This affects sky coverage but does not bias the null result.
  • Time window widths = 8 values from 0.3 s to 1000 s
    Eight time windows are tested, chosen from short GRB emission durations. These are analysis choices, not fitted parameters, but they influence the trials factor.
assumptions (4)
  • domain assumption Direct integration background estimation accurately models cosmic-ray background to 1e-3
    Used in Section 2 to estimate background; if inaccurate, p-values and limits would be wrong.
  • domain assumption The source spectrum is a simple power law and EBL attenuation follows Gilmore et al. (2012)
    Used in the likelihood model; if the true spectrum differs, sensitivity estimates change, but the null result is robust.
  • domain assumption The TS distribution under the null is correctly calibrated by fluctuating the expected background
    Section 2; the trials correction and p-value rely on this simulation being accurate.
  • domain assumption HAWC was in normal operation during all analyzed GW events
    Section 4 states this without showing run quality details.

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Cite this review

Pith. "Pith review of Search for very-high-energy gamma-ray counterparts of gravitational waves with HAWC." pith.science (2026). https://pith.science/paper/AEYMIP6G

@misc{pith2026190806122,
  author       = {Pith},
  title        = {Pith review of: Search for very-high-energy gamma-ray counterparts of gravitational waves with HAWC},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/AEYMIP6G}},
  note         = {Machine review of arXiv:1908.06122}
}
read the original abstract

The High-Altitude Water Cherenkov Observatory (HAWC) is a large field of view (~2 sr) continuously operating experiment sensitive to very-high energy (VHE) gamma rays (~0.3-100 TeV). These characteristics make it well suited for observing or constraining the VHE emission of rapid transients such as some gravitational waves progenitors. Of special interest are the events at low redshift where the attenuation due to the extragalactic background light is minimal. This is the case for binary neutron star mergers in the horizon of the LIGO and Virgo experiments, for which HAWC can either detect or place constraining limits on events occurring in our field of view. We report on our search for counterparts of the gravitational waves detected by LIGO and Virgo.

Figures

Figures reproduced from arXiv: 1908.06122 by the authors.

Figure 1
Figure 1. HAWC quasi-differential sensitivity to 1s bursts as a function of the zenith angle, defined as the mean flux in a given half-decade that would result in at least a 5σ detection half of the time. For reference we include the spectrum for the short GRBs detected by the Fermi-LAT as measured in the Fermi-GBM T90 window, which is of the order of 1s, reported in [20]. The redshift is known only for GRB 090510 (z = 0.9); … view at source ↗
Figure 2
Figure 2. HAWC quasi-differential sensitivity to 100s bursts. We show for reference the spectra measured by the Fermi-LAT for GRBs with a duration >70 s (the longest being 170 s). See description of [PITH_FULL_IMAGE:figures/full_fig_p005_2.png] view at source ↗
Figure 3
Figure 3. HAWC 95 % C.L. upper bound (blue) on GW170817 between 100 GeV and 1 TeV from ∆t = t −t0 = 8.20 hrs to ∆t = 10.23 hrs where t0 is the time of coalescence. Fermi-LAT UL between 100 MeV and 100 GeV from ∆t =1153 s to ∆t =2027 s are shown in green. For comparison we show the HAWC sensitivity (0.1–1 TeV) a function of the emission duration ∆t and Fermi-LAT measurements of previously detected short GRBs (red) [23]. The Fe… view at source ↗

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Cited by 1 Pith paper

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