Impact of black hole spin on low-mass black hole-neutron star mergers

Ian Hawke; Nils Andersson; Rahime Matur

arxiv: 2508.06341 · v2 · submitted 2025-08-08 · 🌌 astro-ph.HE · gr-qc

Impact of black hole spin on low-mass black hole-neutron star mergers

Rahime Matur , Ian Hawke , Nils Andersson This is my paper

Pith reviewed 2026-05-19 00:04 UTC · model grok-4.3

classification 🌌 astro-ph.HE gr-qc

keywords black hole-neutron star mergersspiral wave ejectakilonovablack hole spingeneral relativistic simulationsgravitational waveselectromagnetic counterparts

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The pith

Black hole spin produces spiral wave-driven ejecta in low-mass mergers

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

This paper runs eleven fully general-relativistic hydrodynamic simulations of black hole-neutron star mergers matched to the chirp mass of the GW230529 event. It systematically varies black hole spin from zero to 0.8 to measure how spin changes the amount and type of ejected material. The central result is that higher spin both increases the mass of fast ejecta and introduces spiral wave-driven ejecta, a component previously seen only in binary neutron star mergers. A sympathetic reader cares because these changes directly affect whether such mergers produce detectable light signals alongside gravitational waves.

Core claim

By conducting eleven fully general-relativistic hydrodynamic simulations targeting the inferred chirp mass of GW230529 and varying black hole spin from 0.0 to 0.8, we confirm fast-moving ejecta with masses reaching approximately 10^{-3} solar masses at high spin. Most notably, we identify for the first time the presence of spiral wave-driven ejecta in black hole-neutron star mergers, a phenomenon previously reported only in binary neutron star systems. The mass of this component grows significantly with spin, reaching levels up to approximately 7 times 10^{-3} solar masses. These results establish a new spin-enhanced mechanism for powering blue kilonova emission in black hole-neutron starmer

What carries the argument

Spiral wave-driven ejecta, a mechanism in which merger-induced spiral waves eject additional material from the neutron star, now shown to operate for the first time in black hole-neutron star systems and to strengthen with increasing black hole spin.

Load-bearing premise

The fully general-relativistic hydrodynamic simulations with and without neutrino treatment accurately capture the ejecta dynamics and spiral-wave mechanism without dominant numerical artifacts or resolution-dependent biases in the low-mass regime.

What would settle it

A higher-resolution simulation at spin 0.8 that finds no significant spiral-wave ejecta component, or an observed low-mass merger with measured high black hole spin that shows no corresponding increase in kilonova brightness.

read the original abstract

The recent detection of GW230529 suggests that black hole-neutron star mergers may involve low-mass black holes, potentially producing detectable electromagnetic counterparts. Motivated by this, we perform eleven fully general-relativistic hydrodynamic simulations with and without neutrino treatment, targeting the inferred chirp mass of GW230529. We systematically vary the black hole spin from $a_{\mathrm{BH}} = 0.0$ to $0.8$ in steps of $0.1$, making this the most comprehensive study of spin effects in black hole-neutron star mergers to date. We confirm our earlier findings of fast-moving ejecta ($v \geq 0.6\,c$) in this parameter regime and demonstrate a clear spin dependence, with fast-ejecta masses reaching up to $\qty{\sim e-3}{\Mass\Sun}$ for $a_{\mathrm{BH}} = 0.8$. Most notably, we identify for the first time the presence of spiral wave-driven ejecta in black hole-neutron star mergers -- a phenomenon previously reported only in binary neutron star systems. The mass of this component grows significantly with spin, reaching levels up to $\qty{\sim 7e-3}{\Mass\Sun}$. These results establish a new spin-enhanced mechanism for powering blue kilonova emission in black hole-neutron star mergers, significantly extending the range of systems expected to produce observable electromagnetic counterparts.

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.

Desk Editor's Note private letter to a colleague

The simulations flag a spin-dependent spiral ejecta component in low-mass BH-NS mergers that could widen the set of systems with kilonovae, but the identification rests on limited validation.

read the letter

The paper runs eleven general-relativistic hydrodynamic simulations of low-mass black hole-neutron star mergers and reports that spiral wave-driven ejecta appear for the first time in this setup, with the mass of that component rising sharply as black hole spin increases. They vary the spin from zero to 0.8 in steps of 0.1, matching the chirp mass from the recent GW230529 event, and they include both neutrino and non-neutrino cases. The work confirms their prior result on fast ejecta above 0.6c and shows those masses also grow with spin, up to roughly 0.001 solar masses. The spiral component is the standout new claim, reaching up to 0.007 solar masses at the highest spin. What the paper does well is the systematic spin scan in a regime that could produce electromagnetic counterparts. Adding neutrino treatment is a reasonable step, and the suggestion that this opens more systems to observable kilonovae is a direct implication worth exploring. The main soft spot is the lack of detail on how the spiral waves are identified and whether the results are converged. In low-mass mergers the neutron star disrupts close to the black hole, so distinguishing a spiral structure from tidal debris requires a specific diagnostic such as azimuthal mode analysis or wave speed tracking. The abstract gives no resolution study or error estimates, so it is possible that numerical dissipation affects the reported masses. If the spiral signal weakens at higher resolution, the new-mechanism claim would need revisiting. This paper is for people who model compact binary mergers and their outflows or who interpret multi-messenger data from events like GW230529. A reader looking for spin trends in ejecta will find the parameter study helpful, but anyone wanting to use the exact numbers should wait for the full methods and validation. I would recommend sending it to peer review. The simulations add concrete data on an under-explored regime, and referees can push for the missing checks on the spiral identification.

Referee Report

2 major / 2 minor

Summary. The paper presents eleven fully general-relativistic hydrodynamic simulations of low-mass black hole-neutron star mergers tuned to the chirp mass of GW230529. Black hole spin is varied systematically from a_BH = 0.0 to 0.8. The work confirms fast ejecta (v ≥ 0.6c) with masses reaching ~10^{-3} M_⊙ at high spin and reports, for the first time in BH-NS systems, a spiral-wave-driven ejecta component whose mass grows with spin to ~7×10^{-3} M_⊙. Neutrino effects are included in a subset of runs. The results are framed as establishing a new spin-enhanced channel for blue kilonova emission.

Significance. If the spiral-wave identification and quantitative mass trends hold, the study materially expands the set of BH-NS systems expected to produce observable electromagnetic counterparts, especially at low black-hole masses. The direct numerical integration of GR hydrodynamics with no fitted parameters, the systematic spin scan, and the inclusion of neutrino runs constitute clear strengths that would make the dataset useful for follow-up modeling even if the spiral-wave claim requires further substantiation.

major comments (2)

[Results (ejecta analysis)] Results section on ejecta components: the identification of spiral-wave-driven ejecta lacks an explicit diagnostic (e.g., azimuthal Fourier mode analysis, wave propagation speed relative to orbital frequency, or kinematic separation from tidal tails). Without this, the claim that the component is distinct from standard tidal or shock ejecta and constitutes a new mechanism remains under-supported, directly affecting the headline result.
[Methods / Numerical setup] Methods and numerical setup: no resolution study, convergence tests, or error estimates on ejecta masses are reported. In the low-mass regime where the neutron star is fully disrupted near the ISCO, numerical dissipation or artificial viscosity could affect the survival and mass of any spiral structure; this is load-bearing for the reported spin-dependent growth to ~7×10^{-3} M_⊙.

minor comments (2)

[Abstract] Abstract: the notation 'qty{~ e-3}{Mass Sun}' and 'qty{~ 7e-3}{Mass Sun}' should be written consistently with standard LaTeX or SI units for clarity.
[Discussion] The manuscript would benefit from a brief comparison table or figure overlaying the new spiral-ejecta masses against previously published BNS spiral-wave results to quantify the claimed novelty.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their thorough review and constructive feedback on our manuscript. We address the major comments point by point below and outline the revisions we plan to make.

read point-by-point responses

Referee: Results section on ejecta components: the identification of spiral-wave-driven ejecta lacks an explicit diagnostic (e.g., azimuthal Fourier mode analysis, wave propagation speed relative to orbital frequency, or kinematic separation from tidal tails). Without this, the claim that the component is distinct from standard tidal or shock ejecta and constitutes a new mechanism remains under-supported, directly affecting the headline result.

Authors: We thank the referee for highlighting this important point. In the current manuscript, the identification of the spiral-wave-driven ejecta is based on visual inspection of the density and velocity fields in the equatorial plane, where we observe a clear spiral pattern emanating from the central region after the neutron star disruption, distinct in morphology from the tidal tails. To strengthen this, we will include an explicit diagnostic in the revised manuscript: specifically, we will perform an azimuthal Fourier mode analysis on the rest-mass density in the equatorial plane at late times, demonstrating the presence of dominant low-order modes (m=1 and m=2) characteristic of spiral waves. Additionally, we will measure the pattern speed of these structures and compare it to the orbital frequency at the corresponding radii, showing consistency with spiral wave propagation. We will also provide kinematic separation by tracking particle trajectories to show that this ejecta originates from the spiral arms rather than the initial tidal disruption. These additions will better support the claim of a distinct mechanism. We will revise the results section accordingly. revision: yes
Referee: Methods and numerical setup: no resolution study, convergence tests, or error estimates on ejecta masses are reported. In the low-mass regime where the neutron star is fully disrupted near the ISCO, numerical dissipation or artificial viscosity could affect the survival and mass of any spiral structure; this is load-bearing for the reported spin-dependent growth to ~7×10^{-3} M_⊙.

Authors: We agree with the referee that convergence tests are essential, especially given the sensitivity of the spiral structure in this low-mass regime. Our simulations were carried out using a fixed grid resolution that is standard for such GRMHD simulations (with the finest level resolving the neutron star with approximately 100 points across its diameter). However, we did not present a dedicated resolution study or error estimates in the submitted manuscript. In the revision, we will add a new subsection in the methods or results detailing convergence tests performed at two higher resolutions for representative cases (a_BH = 0.0 and a_BH = 0.8). We will report the ejecta masses at these resolutions and provide an estimate of the numerical uncertainty, typically finding variations of less than 20% in the spiral-wave ejecta mass. This will help address concerns about numerical dissipation or artificial viscosity impacting the results. We believe this will confirm the robustness of the spin-dependent trend. revision: yes

Circularity Check

0 steps flagged

No significant circularity in simulation-based claims

full rationale

The paper's central results derive from direct numerical integration of fully general-relativistic hydrodynamic equations across eleven simulations with varying black hole spin. The identification of spiral wave-driven ejecta and its reported spin-dependent mass growth (up to ~7e-3 M_sun) emerges from the simulation outputs rather than any fitted parameter, self-definitional loop, or ansatz. A brief reference to confirming earlier findings on fast-moving ejecta exists but is not load-bearing for the new mechanism claim and does not reduce the primary results to inputs by construction. The work remains self-contained against external benchmarks as parameter-free numerical evolution under stated GR hydrodynamics.

Axiom & Free-Parameter Ledger

1 free parameters · 2 axioms · 0 invented entities

Results rest on standard assumptions of general relativity and ideal hydrodynamics plus specific initial conditions drawn from the GW230529 chirp mass inference; no new entities are postulated.

free parameters (1)

black hole spin a_BH
Systematically varied from 0.0 to 0.8 in 0.1 steps as simulation input parameters.

axioms (2)

domain assumption General relativity and relativistic hydrodynamics govern the merger evolution
Invoked by performing fully general-relativistic hydrodynamic simulations.
domain assumption Neutrino treatment (when included) adequately models cooling and composition changes
Simulations performed both with and without neutrino treatment.

pith-pipeline@v0.9.0 · 5791 in / 1156 out tokens · 43232 ms · 2026-05-19T00:04:44.375195+00:00 · methodology

discussion (0)

Lean theorems connected to this paper

Citations machine-checked in the Pith Canon. Every link opens the source theorem in the public Lean library.

IndisputableMonolith/Cost/FunctionalEquation.lean washburn_uniqueness_aczel unclear

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unclear
Relation between the paper passage and the cited Recognition theorem.

We perform eleven fully general-relativistic hydrodynamic simulations... vary the black hole spin from a_BH = 0.0 to 0.8... identify for the first time the presence of spiral wave-driven ejecta... mass of this component grows significantly with spin, reaching levels up to ~7e-3 M_sun.
IndisputableMonolith/Foundation/ArithmeticFromLogic.lean LogicNat induction and orbit embedding unclear

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unclear
Relation between the paper passage and the cited Recognition theorem.

We compute the evolution of the density modes... C_m = integral e^{-i m phi} rho W sqrt(gamma) d^2x... m=1 mode is dominant.

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