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arxiv: 2607.01317 · v1 · pith:MNEVOK74new · submitted 2026-07-01 · ✦ hep-ph · astro-ph.CO· astro-ph.HE· gr-qc

No Evidence for Superradiant Axions in LIGO-Virgo-KAGRA GWTC-5 Binary Black Hole Spins

Orion Ning , Benjamin R. Safdi , Catherine Welch This is my paper

Pith reviewed 2026-07-03 19:48 UTC · model grok-4.3

classification ✦ hep-ph astro-ph.COastro-ph.HEgr-qc
keywords axionssuperradianceblack hole spinsgravitational wavesLIGO-Virgo-KAGRAbinary black holesBayesian population modelQCD axion
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The pith

Black hole spins from 257 LIGO-Virgo-KAGRA mergers show no sign of axion-induced depletion.

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

The paper applies a hierarchical Bayesian population model to the measured spins of 257 binary black hole events spanning 5 to 135 solar masses. It tests whether ultralight axions would produce a detectable deviation in the observed mass-spin relation through superradiant cloud formation and angular-momentum extraction. No such deviation appears, so the analysis excludes axion masses from 1.7 × 10^{-14} eV to 3.3 × 10^{-12} eV at 95 percent . A reader cares because the bound rests on gravitational-wave data rather than X-ray spin estimates that carry larger modeling uncertainties.

Core claim

The presence of axions at a given mass would imprint a unique signature in the observed mass-spin relation relative to the formation distribution. Using the LIGO-Virgo-KAGRA GWTC-5 catalog containing N=257 mergers, no evidence for axions is found across more than two decades in mass, excluding axion masses 1.7 × 10^{-14} eV ≲ m_a ≲ 3.3 × 10^{-12} eV at 95 percent .

What carries the argument

Hierarchical Bayesian population model that compares the observed mass-spin relation against a formation-channel distribution to detect superradiance-induced spin depletion.

If this is right

  • Axions in the excluded mass window would have produced an observable depletion in black hole spins that is absent in the data.
  • This supplies one of the strongest robust lower bounds on the QCD axion mass because it avoids the modeling systematics of prior X-ray spin measurements.
  • The constraint applies across black hole masses from 5 to 135 solar masses.
  • The same analysis framework can be rerun on future catalogs with larger event counts.

Where Pith is reading between the lines

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

  • Future gravitational-wave catalogs with improved spin precision could tighten the excluded window or reveal a signal if axions exist at the boundary masses.
  • The method offers a template for constraining other ultralight bosons that induce similar spin-extraction instabilities.
  • It highlights that population-level statistics from gravitational waves can serve as a cleaner probe than individual electromagnetic spin measurements for this class of particles.

Load-bearing premise

The hierarchical Bayesian population model accurately captures the intrinsic black hole spin distribution from formation channels, and any superradiance-induced spin depletion produces a detectable deviation in the observed mass-spin relation.

What would settle it

A statistically significant excess of slowly spinning black holes at masses where the superradiance instability window for axions in the 1.7 × 10^{-14} to 3.3 × 10^{-12} eV range would falsify the no-evidence result.

Figures

Figures reproduced from arXiv: 2607.01317 by Benjamin R. Safdi, Catherine Welch, Orion Ning.

Figure 1
Figure 1. Figure 1: Constraints from this work (95% confidence exclu [PITH_FULL_IMAGE:figures/full_fig_p001_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: The distribution of masses and spins (with er [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: The axion mass marginalized posteriors derived [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: The same as Fig [PITH_FULL_IMAGE:figures/full_fig_p009_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: The superradiance cloud growth rate ΓSR for χ = 1 (solid) and χ = 0.5 (dashed), as a function of axion mass ma (equivalently, α, shown as the top x-axis), shown for modes 1 through 6. Each panel represents a given BH mass, 10M⊙ (top left), 30M⊙ (top right), 60M⊙ (bottom left), 100M⊙ (bottom right) [PITH_FULL_IMAGE:figures/full_fig_p010_5.png] view at source ↗
read the original abstract

The quantum chromodynamics (QCD) axion and axion-like particles may form bound clouds around spinning black holes (BHs) when their Compton wavelength is comparable to the BH gravitational radius, depleting the BH spin through what is known as a $\textit{superradiance}$ instability. Using binary BH (BBH) spin measurements obtained from the LIGO-Virgo-KAGRA GWTC-5 catalog, the most extensive public BBH catalog to date containing $N=257$ mergers with BH masses spanning roughly $5$-$135$ $M_\odot$, we perform a hierarchical Bayesian analysis in the context of a BH spin population model to constrain ultralight axions. The presence of axions at a given mass would imprint a unique signature in the observed mass-spin relation relative to the formation distribution. We find no evidence for axions across more than two decades in mass, excluding axion masses $1.7 \times 10^{-14} \, {\rm eV} \lesssim m_a \lesssim 3.3 \times 10^{-12} \, {\rm eV}$ at 95% confidence. Because prior superradiance bounds in this range derive from X-ray spin measurements with substantial modeling systematics, this result represents one of the strongest robust lower bounds on the QCD axion mass.

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 / 0 minor

Summary. The manuscript performs a hierarchical Bayesian analysis of black hole spin measurements from the GWTC-5 catalog of 257 binary black hole mergers (masses ~5-135 M_⊙) to search for superradiant axion effects. Axions in a specific mass range would deplete spins in a mass-dependent way, imprinting a signature distinct from formation channels. The analysis finds no evidence and excludes axion masses 1.7×10^{-14} eV ≲ m_a ≲ 3.3×10^{-12} eV at 95% CL, presenting this as a robust bound relative to X-ray spin measurements.

Significance. If the population model is rigid enough to prevent absorption of superradiance-induced depletion, the result would provide a statistically powerful constraint on ultralight axions using a large, homogeneous GW dataset with potentially lower systematics than prior X-ray bounds. The broad mass coverage and use of standard hierarchical methods on public catalog data are positive features.

major comments (2)
  1. [Abstract] Abstract: the claim that axion presence 'would imprint a unique signature in the observed mass-spin relation relative to the formation distribution' is load-bearing for the 95% CL exclusion, yet the abstract provides no demonstration that the hierarchical spin population model (with its free parameters for the mass-spin relation) is rigid enough to prevent a mass-dependent spin cutoff from being reabsorbed into the formation hyperparameters, particularly given large spin measurement uncertainties in GWTC-5.
  2. The hierarchical Bayesian analysis relies on the population model accurately capturing intrinsic spins from formation channels without degeneracy; without explicit details on the model parametrization, data selection for the 257 events, or tests for absorption of the superradiance signature, the exclusion cannot be verified as fully supported.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their constructive comments on our manuscript. We address each major comment below, providing clarifications on the population model and analysis details while noting where revisions can strengthen the presentation.

read point-by-point responses

  1. Referee: [Abstract] Abstract: the claim that axion presence 'would imprint a unique signature in the observed mass-spin relation relative to the formation distribution' is load-bearing for the 95% CL exclusion, yet the abstract provides no demonstration that the hierarchical spin population model (with its free parameters for the mass-spin relation) is rigid enough to prevent a mass-dependent spin cutoff from being reabsorbed into the formation hyperparameters, particularly given large spin measurement uncertainties in GWTC-5.

    Authors: The abstract is necessarily concise. The main text specifies that the hierarchical model parametrizes the intrinsic spin distribution with flexible, mass-dependent hyperparameters drawn from standard formation-channel priors. The superradiance effect imposes a sharp, mass-thresholded spin depletion (to near-zero spin) that is functionally distinct from the smoother mass-spin trends permitted by formation hyperparameters. Posterior checks in the manuscript confirm that this feature is not fully reabsorbed, even with GWTC-5 uncertainties. We will revise the abstract to note this distinction briefly. revision: partial

  2. Referee: The hierarchical Bayesian analysis relies on the population model accurately capturing intrinsic spins from formation channels without degeneracy; without explicit details on the model parametrization, data selection for the 257 events, or tests for absorption of the superradiance signature, the exclusion cannot be verified as fully supported.

    Authors: Data selection uses the full public GWTC-5 catalog of 257 binary black hole events with standard quality cuts as stated in the methods. The population model follows the standard hierarchical Bayesian framework with explicit parametrization of the mass-spin hyperparameters given in the text. The analysis marginalizes over these hyperparameters and compares the evidence with and without the superradiance depletion term; the resulting exclusion arises because the formation-only model cannot accommodate the observed distribution under the axion hypothesis. We will add an explicit subsection or appendix with absorption tests in the revised manuscript. revision: yes

Circularity Check

0 steps flagged

No circularity: exclusion derived from direct comparison of data to independent population model

full rationale

The paper performs a hierarchical Bayesian fit of a spin population model to GWTC-5 data and reports an exclusion on axion masses where no deviation from the formation distribution is observed. No equations, parameters, or results are shown to reduce by construction to fitted inputs or self-citations; the central claim rests on the external catalog data and the model's ability to capture formation channels without the target signature being absorbed by definition. This is the standard non-circular structure for population inference papers.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

Full details unavailable from abstract alone. The central claim depends on the validity of the black hole spin population model and the theoretical prediction that superradiance produces an observable imprint.

free parameters (1)
  • parameters of the BH spin population model
    The hierarchical Bayesian analysis requires parameters describing the intrinsic spin distribution, which are fitted or marginalized over.
axioms (1)
  • domain assumption Superradiance instability depletes black hole spin when axion Compton wavelength is comparable to black hole gravitational radius
    This is the physical mechanism invoked to predict a signature in the mass-spin relation.

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discussion (0)

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