GW231123 Formation from Population III Stars: Isolated Binary Evolution
Pith reviewed 2026-05-19 01:59 UTC · model grok-4.3
The pith
Isolated Population III binaries can produce GW231123-like events under three specific stellar evolution conditions.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
Isolated Pop III binaries can produce GW231123-like events at a rate sufficient to explain the discovery of GW231123, provided that three conditions are met: (i) Pop III stars evolve with inefficient convective overshooting, (ii) the 12C(α, γ)16O rate is 2σ lower than the standard value, and (iii) Pop III binary stars share the same orbital parameters as Pop I/II binary stars at the initial time. In contrast, GW190521 can be formed from isolated Pop III binaries even with the standard 12C(α, γ)16O rate.
What carries the argument
Binary population synthesis calculations that vary convective overshooting efficiency and the 12C(α, γ)16O nuclear reaction rate for Population III stars to compute merger rates and masses.
If this is right
- The formation rate of GW231123-like mergers from isolated Pop III binaries reaches observed levels under the three conditions.
- GW231123 imposes tighter limits on single-star evolution parameters than earlier events such as GW190521.
- GW190521 forms from Pop III binaries even when the nuclear reaction rate takes its standard value.
Where Pith is reading between the lines
- If Pop III binaries actually begin with different orbital distributions, the contribution of isolated evolution to the heaviest mergers could fall below detectable levels.
- Gravitational-wave catalogs may eventually map the initial conditions of the first binaries through the statistics of their remnant masses.
Load-bearing premise
Population III binary stars share the same initial orbital parameters as Population I/II binary stars.
What would settle it
A determination that the initial orbital period and mass ratio distributions of Pop III binaries differ substantially from those of Pop I/II binaries would change the predicted rates enough to rule out this channel for GW231123.
Figures
read the original abstract
GW231123 is a merger of two black holes (BHs) with estimated masses exceeding $100\;{\rm M}_{\odot}$, making them the most massive BHs discovered to date via gravitational wave (GW) observations. We investigate whether GW231123-like events can originate from isolated Population (Pop) III binary stars using binary population synthesis calculations. Our findings indicate that isolated Pop III binaries can produce GW231123-like events at a rate sufficient to explain the discovery of GW231123, provided that three conditions are met: (i) Pop III stars evolve with inefficient convective overshooting, (ii) the $^{12}\text{C}(\alpha, \gamma)^{16}\text{O}$ rate is $2\sigma$ lower than the standard value, and (iii) Pop III binary stars share the same orbital parameters as Pop I/II binary stars at the initial time. In contrast, GW190521 -- the most massive BH merger in the Gravitational Wave Transient Catalog 3 -- can be formed from isolated Pop III binaries even with the standard $^{12}\text{C}(\alpha, \gamma)^{16}\text{O}$ rate. We demonstrate that the discovery of GW231123 is increasingly constraining the parameter ranges of single star evolution models, under the assumption that these GW events originate from isolated binary evolution.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript uses binary population synthesis to investigate whether isolated Population III binary stars can produce the high-mass black hole merger GW231123. The central claim is that such events can occur at a rate sufficient to explain the observation provided three conditions hold: inefficient convective overshooting in Pop III stars, a 12C(α,γ)16O reaction rate 2σ below the standard value, and initial orbital parameters for Pop III binaries identical to those of Pop I/II binaries. The paper contrasts this with GW190521, which forms under standard nuclear rates, and argues that the GW231123 discovery constrains single-star evolution models under the isolated binary channel.
Significance. If the result holds, the work would establish a viable isolated Pop III channel for the most massive observed GW events and demonstrate how such detections can tighten constraints on convective overshooting and nuclear rates. The explicit mapping of required parameter adjustments to match a specific event provides a concrete example of the constraining power of GW observations on stellar models.
major comments (2)
- [Abstract and Results section] Abstract and Results section: The central claim that isolated Pop III binaries produce GW231123-like events at a sufficient rate rests on condition (iii), that Pop III binaries share the same initial orbital-parameter distribution as Pop I/II binaries. This assumption is not derived from Pop III formation physics and is load-bearing, because binary merger rates are exponentially sensitive to the initial period and eccentricity distributions; a modest shift toward wider orbits would sharply reduce the fraction of systems undergoing the required mass-transfer or common-envelope channels.
- [Abstract] Abstract: The abstract states that the predicted rate is 'sufficient to explain the discovery' under the three conditions but reports neither the numerical rate value, its uncertainty, nor tests of alternative assumptions for the initial orbital distribution, leaving the robustness of the conclusion difficult to assess from the summary alone.
minor comments (1)
- Consider adding a dedicated table or figure panel that quantifies the predicted merger rate (with error bars) under each of the three conditions separately and in combination, to improve clarity of the sensitivity analysis.
Simulated Author's Rebuttal
We thank the referee for their careful reading and constructive comments, which have helped us clarify the assumptions and quantitative aspects of our study on GW231123 formation from isolated Population III binaries. We address each major comment below and indicate the corresponding revisions to the manuscript.
read point-by-point responses
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Referee: [Abstract and Results section] Abstract and Results section: The central claim that isolated Pop III binaries produce GW231123-like events at a sufficient rate rests on condition (iii), that Pop III binaries share the same initial orbital-parameter distribution as Pop I/II binaries. This assumption is not derived from Pop III formation physics and is load-bearing, because binary merger rates are exponentially sensitive to the initial period and eccentricity distributions; a modest shift toward wider orbits would sharply reduce the fraction of systems undergoing the required mass-transfer or common-envelope channels.
Authors: We agree that the adoption of the same initial orbital-parameter distribution as for Pop I/II binaries is a key assumption, as current Pop III formation simulations do not yet yield robust, observationally calibrated predictions for binary periods and eccentricities. This choice is made explicitly to test the viability of the isolated channel under a conservative baseline drawn from later-generation binaries. To address the sensitivity concern, we have added a dedicated paragraph in the Results section quantifying how shifts toward wider initial periods reduce the GW231123-like merger fraction, while demonstrating that a non-negligible rate remains possible within the parameter space allowed by the other two conditions (inefficient overshooting and reduced 12C(α,γ)16O rate). We believe this addition improves transparency without changing the central conclusion that the isolated Pop III channel remains viable under the stated assumptions. revision: partial
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Referee: [Abstract] Abstract: The abstract states that the predicted rate is 'sufficient to explain the discovery' under the three conditions but reports neither the numerical rate value, its uncertainty, nor tests of alternative assumptions for the initial orbital distribution, leaving the robustness of the conclusion difficult to assess from the summary alone.
Authors: We accept that the abstract would benefit from greater quantitative detail. We have revised the abstract to include the estimated merger rate (with uncertainty range) under the three conditions and a brief statement noting the sensitivity to the initial orbital distribution. These additions allow readers to evaluate the claim more directly from the summary. revision: yes
Circularity Check
No significant circularity; derivation is conditional on explicit inputs with independent simulation outputs.
full rationale
The paper conducts binary population synthesis calculations whose outputs (formation rates) depend on chosen inputs for convective overshooting efficiency, the 12C(α,γ)16O rate, and initial orbital-parameter distributions. The central claim is explicitly conditional: rates are sufficient only if those three conditions hold. No equation or result is shown to equal its own input by construction, no fitted parameter is relabeled as a prediction, and no load-bearing premise reduces to a self-citation chain. The orbital-parameter assumption is presented as a premise rather than derived internally, and the stellar-evolution physics remains independent of the target GW event. The chain is therefore self-contained against external benchmarks.
Axiom & Free-Parameter Ledger
free parameters (3)
- convective overshooting efficiency
- 12C(alpha,gamma)16O reaction rate
- initial orbital parameter distribution
axioms (1)
- domain assumption Standard assumptions of binary population synthesis (common-envelope efficiency, wind mass loss, supernova kicks) remain valid for Population III stars.
Forward citations
Cited by 8 Pith papers
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The First Model-Independent Upper Bound on Micro-lensing Signature of the Highest Mass Binary Black Hole Event GW231123
No definitive lensing is detected in GW231123, though a potential microlensing feature with modulation amplitude up to 0.8 at 95% confidence is noted, limited by large waveform systematics in short signals.
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Investigating the formation channel of GW231123: Population III stars or hierarchical mergers?
GW231123 most likely formed through hierarchical mergers of black holes in metal-poor globular clusters, with isolated binary channels failing to match the observed merger redshift and masses.
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Black Hole Binary Detection Landscape for the Laser Interferometer Lunar Antenna (LILA): Signal-to-Noise Calculations & Science Cases
LILA can detect IMBH binaries at redshifts 20-30, IMRIs, and provide months-to-years early warnings with high-SNR events for gravity tests.
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Investigating the formation channel of GW231123: Population III stars or hierarchical mergers?
Coupled cosmological and cluster simulations show isolated binary evolution cannot produce GW231123-like mergers at the observed redshift, while hierarchical mergers in globular clusters can, yielding a local rate of ...
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Constraints on the $^{12}$C$(\alpha, \gamma)^{16}$O and $^{16}$O+$^{16}$O Reaction Rates from Binary Black Holes Detected via Gravitational Wave Signals
Stellar models show that the 12C(alpha,gamma)16O rate uncertainty moves the black hole mass gap, constraining its S300 to 137.6-263.4 keV barn when matched to the observed gap from gravitational waves.
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Measuring Eccentricity and Addressing Waveform Systematics in GW231123
Reanalysis of GW231123 shows no significant eccentricity, with parameter estimate differences explained by waveform model disagreements at strong spin precession.
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GW231123: Overlapping Gravitational Wave Signals?
GW231123 data favors an overlapping two-signal model over a single merger with Bayes factors of 100-10000, mitigating waveform-dependent discrepancies and suggesting possible gravitational lensing.
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The impact of waveform systematics and Gaussian noise on the interpretation of GW231123
The high mass and high spin magnitudes inferred for GW231123 using NRSur7dq4 are robust to waveform systematics and Gaussian noise.
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discussion (0)
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