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arxiv: 2605.21580 · v1 · pith:GZ2I5NESnew · submitted 2026-05-20 · 🌌 astro-ph.HE · astro-ph.GA· astro-ph.SR

Massquerade: Impacts of Mass Ratio Reversals on Binary Black Hole Merger Rates and Mass Distributions

Tyler B. Smith , Floor Broekgaarden , Sasha Levina , Amedeo Romagnolo , Manasvini Komandur , Melanie Santiago , Kyle A. Rocha This is my paper

Pith reviewed 2026-05-22 09:25 UTC · model grok-4.3

classification 🌌 astro-ph.HE astro-ph.GAastro-ph.SR
keywords binary black holesmass ratio reversalpopulation synthesisgravitational wave astronomystellar evolutionmerger ratescompact objects
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The pith

Mass ratio reversal lets the initially less massive star become the more massive black hole in many mergers.

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

The paper examines how mass ratio reversal affects binary black hole mergers in two population synthesis codes. In one code, systems where the lighter star ends up forming the heavier black hole dominate the high-mass end of the merger distribution. This means the observed primary black hole masses reflect a mix of different formation paths rather than simply the masses of the first-formed stars. The result matters because it changes how we interpret gravitational wave data to learn about the birth masses of stars.

Core claim

Mass ratio reversal systems, where the secondary star forms the primary black hole, dominate the merger rate density at high primary masses above 12 solar masses, high secondary masses above 20 solar masses, and mass ratios above 0.6 in COMPAS simulations, but remain subdominant in SEVN. This shows that the primary-mass distribution is a superposition of physically distinct evolutionary populations. Three pathways lead to these reversals, with core-growth via stable mass transfer being the dominant one when weighted by merger rates, and most systems forming from massive stars at low metallicity below 0.1 solar metallicity.

What carries the argument

Mass ratio reversal (MRR) through core-growth, PPISN-shrinking, or asymmetric core-collapse supernova mass loss, which allows the initially lighter star to form the heavier compact object.

If this is right

  • MRR systems dominate high-mass and high mass-ratio mergers in some models, altering the shape of the observed mass distribution.
  • The core-growth channel accounts for nearly all weighted MRR mergers.
  • MRR systems form preferentially from massive low-metallicity progenitors below 0.1 solar metallicity.
  • Accounting for MRR is required to connect future observations to the physics of massive binary evolution.

Where Pith is reading between the lines

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

  • If MRR is common, then the high-mass end of the black hole mass function may be shaped more by binary interactions than by the initial mass function alone.
  • Observations of the mass ratio distribution at high masses could distinguish between population synthesis models that differ on MRR importance.

Load-bearing premise

The prescriptions for stable mass transfer, pulsational pair-instability supernovae, and core-collapse supernova mass loss in COMPAS and SEVN accurately capture the relevant stellar physics.

What would settle it

Measuring the fraction of binary black hole mergers with mass ratio greater than 0.6 at primary masses above 12 solar masses in LIGO-Virgo-KAGRA data and comparing it to the predicted dominance in one model versus the other.

Figures

Figures reproduced from arXiv: 2605.21580 by Amedeo Romagnolo, Floor Broekgaarden, Kyle A. Rocha, Manasvini Komandur, Melanie Santiago, Sasha Levina, Tyler B. Smith.

Figure 1
Figure 1. Figure 1: Redshift evolution of the BBH merger rate density R(z) expected by the fiducial COMPAS (left) and SEVN (right) models. The total merger rate (black) is decomposed into contributions from MRR (purple) and non-MRR (green) systems, and compared to the LVK O4a B-spline population model (gray band; Abac et al. 2025). In both models, MRR systems contribute approximately one-third of the total BBH merger rate acr… view at source ↗
Figure 2
Figure 2. Figure 2: Intrinsic BBH merger rate density as a function of primary mass M1 (top), secondary mass M2 (middle), and mass ratio q (bottom) at z ≈ 0.2, for SEVN (left) and SEVN (right). The total population (black) is decomposed into MRR (purple) and non-MRR (green) contributions, and compared to LVK-inferred constraints (gray shaded regions; Abac et al. 2025; Sadiq et al. 2024). COMPAS predicts that MRR systems domin… view at source ↗
Figure 3
Figure 3. Figure 3: Two-dimensional BBH merger rate density (z ≈ 0) in the (M1, M2) plane (top row) and corresponding MRR fraction (bottom row) for COMPAS (left) and SEVN (middle), compared to LVK inference (right). Contours indicate lines of constant mass ratio q. COMPAS exhibits a concentrated region at high masses and q ≳ 0.6 where MRR systems dominate, while SEVN shows no comparable region of dominance and instead display… view at source ↗
Figure 4
Figure 4. Figure 4: Sensitivity of the primary black hole mass distribution, M1, to key uncertainties in massive binary evolution within COMPAS. Each panel shows a single-parameter variation relative to the fiducial model, with the M1 distribution decomposed into MRR (dashed) and non-MRR (solid) contributions. Variations in the common-envelope efficiency αCE, supernova engine prescription, and black-hole natal kick dispersion… view at source ↗
Figure 5
Figure 5. Figure 5: Fraction of systems undergoing mass ratio reversal (MRR) for COMPAS (left two columns) and SEVN (right two columns), weighted by their merger rate contribution at z ≈ 0.2. For each population synthesis code, the leftmost column shows the MRR fraction in the plane of ZAMS quantities and metallicity, while the right panels show the corresponding BH masses in the "LVK" defined view, i.e. M1 defined as the mos… view at source ↗
Figure 6
Figure 6. Figure 6: Representative evolutionary pathways leading to MRR in binary systems. Shown are the total stellar masses of the initially more massive star (black) and initially less massive star (red), together with their corresponding core masses (magenta and green), as a function of time. Vertical dotted lines mark the supernova events of the primary (SN1) and secondary (SN2). First: Core-growth channel (COMPAS). Stab… view at source ↗
Figure 7
Figure 7. Figure 7: Initial parameter distributions of the distinct MRR channels in COMPAS (top-row) and SEVN (bottom-row). The distributions are normalized jointly across each codes channels, so that the area under the sum of all curves equals unity per panel. From left to right the columns are: ZAMS primary mass, ZAMS secondary mass, initial mass ratio, and metallicity. The channels for COMPAS are core-growth (pink) and PPI… view at source ↗
Figure 8
Figure 8. Figure 8: Massquerading in the component-mass distributions for COMPAS (left) and SEVN (right). The results from COMPAS show that depending on whether the primary black hole is classified based on being the larger black hole or stemming from the initially larger progenitor can have significant effects on the high-end regime in the mass distributions. Conversely, this definition has little effect on the results from … view at source ↗
Figure 9
Figure 9. Figure 9: The mass ratio reversal contribution to the merger rate density across cosmic time. The black line is the fiducial COMPAS model, while the colored lines correspond to single parameter variations. Throughout cosmic evolution all variations show a slight decline in MRR fraction, hinting at a dependence on metallicity and/or delay-time. [ negligible spin in up to 25% of BBH systems and that the MRR systems ca… view at source ↗
Figure 10
Figure 10. Figure 10: The same as [PITH_FULL_IMAGE:figures/full_fig_p022_10.png] view at source ↗
Figure 11
Figure 11. Figure 11: The chirp-mass distribution is shown for variations in binary parameters relative to the fiducial model, following Broekgaarden et al. (2022a). Each panel compares the MRR and non-MRR contributions for the fiducial model and a single parameter variation. While individual prescriptions modify the overall normalization and relative MRR contribution, the low-mass peak, dominated by non-MRR systems, remains r… view at source ↗
Figure 12
Figure 12. Figure 12: Same as [PITH_FULL_IMAGE:figures/full_fig_p025_12.png] view at source ↗
Figure 13
Figure 13. Figure 13: MRR and non-MRR contributions as a function of initial progenitor semimajor axis for COMPAS (left) and SEVN (right). Included is the evolutionary channel decomposition of the MRR contribution. This distribution is weighted by the formation efficiency (Equation 2). [ [PITH_FULL_IMAGE:figures/full_fig_p025_13.png] view at source ↗
Figure 14
Figure 14. Figure 14: Same as [PITH_FULL_IMAGE:figures/full_fig_p026_14.png] view at source ↗
Figure 15
Figure 15. Figure 15: Variations of the massquerading effect based on single-parameter variations in COMPAS. The effect occurs mostly for ≳ 20 M⊙ as this is where MRR constitutes a dominant fraction of systems. See [PITH_FULL_IMAGE:figures/full_fig_p027_15.png] view at source ↗
Figure 16
Figure 16. Figure 16: Same as [PITH_FULL_IMAGE:figures/full_fig_p028_16.png] view at source ↗
read the original abstract

We investigate the role of mass ratio reversal (MRR), in which the initially less massive star in a binary forms the more massive compact object, in shaping the astrophysical binary black hole (BBH) merger rate and mass distribution inferred by LIGO-Virgo-KAGRA, comparing simulation outcomes from population synthesis frameworks COMPAS and SEVN. We find that the observational imprint of MRR differs qualitatively between the two models. In COMPAS, MRR systems dominate the merger rate density at high primary masses ( $\gtrsim$ 12 M$_\odot$), high secondary masses ( $\gtrsim$ 20 M$_\odot$), and high mass ratios ($q>0.6$), whereas in SEVN, MRR systems remain subdominant across the BBH mass distribution. This implies that the initially less massive star can massquerade as the observed primary black hole, such that the primary-mass distribution is not a direct tracer of the initially more massive stars, but instead a superposition of physically distinct evolutionary populations. We identify in the simulations three distinct evolutionary pathways leading to MRR systems: core-growth, in which stable mass transfer increases the helium-core mass of the secondary; PPISN-shrinking, where pulsational pair-instability episodes reduce the primary remnant mass; and asymmetric-CCSN, where differential supernova mass loss drives the reversal. When weighted by the local BBH merger-rate density, the core-growth channel dominates almost exclusively. MRR systems predominantly originate from massive ($\gtrsim$ 50 M$_\odot$), low-metallicity progenitors, with most of the systems forming below 0.1 $Z_\odot$. Our results demonstrate that MRR is a physically distinct and potentially observable feature of isolated binary evolution. Accounting for MRR will be important for robustly connecting future gravitational-wave observations to the physics of massive binary evolution and compact-object formation.

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 / 2 minor

Summary. The manuscript examines the impact of mass ratio reversals (MRR) on binary black hole (BBH) merger rates and mass distributions using population synthesis codes COMPAS and SEVN. It reports that in COMPAS, MRR systems dominate the merger rate density at high primary masses (≳12 M⊙), high secondary masses (≳20 M⊙), and high mass ratios (q > 0.6), while in SEVN, MRR systems are subdominant. The paper concludes that the primary-mass distribution is a superposition of distinct evolutionary populations, with the core-growth channel dominating when weighted by local merger-rate density, and that MRR systems primarily originate from massive, low-metallicity progenitors.

Significance. If the central claims hold, this work is significant for gravitational-wave astrophysics as it shows that MRR can substantially alter the mapping between observed BBH masses and the initial stellar masses, implying that future LIGO-Virgo-KAGRA data may require accounting for MRR to accurately infer massive binary evolution physics. The comparison across two codes and the identification of three specific pathways (core-growth, PPISN-shrinking, and asymmetric-CCSN) provide a framework for understanding how different evolutionary channels contribute to the observed population. The emphasis on low-metallicity progenitors aligns with expectations for high-mass BBHs.

major comments (2)
  1. [Simulation methodology and results comparison] The qualitative difference in MRR dominance between COMPAS and SEVN is central to the claim that the primary-mass distribution is a superposition of populations. However, the paper employs fixed prescriptions for stable mass transfer, pulsational pair-instability supernovae (PPISN), and core-collapse supernovae (CCSN) mass loss without reporting sensitivity tests or cross-code swaps of these prescriptions. If the dominance flips under modest changes to the PPISN mass-loss function or supernova fallback fraction, the reported contrast would be code-specific rather than a general feature of isolated binary evolution.
  2. [Results on channel weighting] The assertion that the core-growth channel dominates almost exclusively when weighted by the local BBH merger-rate density is load-bearing for downplaying the other two pathways. Details on the exact weighting procedure, including the assumed metallicity distribution and star formation history, are needed to assess robustness, as different assumptions could change which channel appears dominant.
minor comments (2)
  1. [Abstract] The abstract introduces the term 'massquerade' without a brief explanation, which may confuse readers unfamiliar with the pun on 'masquerade'.
  2. [Throughout manuscript] Ensure consistent use of solar mass symbol (M_⊙) and mass ratio q notation across figures and text for clarity.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their constructive and detailed feedback. We address each major comment below and indicate where revisions will be incorporated to improve the manuscript.

read point-by-point responses

  1. Referee: [Simulation methodology and results comparison] The qualitative difference in MRR dominance between COMPAS and SEVN is central to the claim that the primary-mass distribution is a superposition of populations. However, the paper employs fixed prescriptions for stable mass transfer, pulsational pair-instability supernovae (PPISN), and core-collapse supernovae (CCSN) mass loss without reporting sensitivity tests or cross-code swaps of these prescriptions. If the dominance flips under modest changes to the PPISN mass-loss function or supernova fallback fraction, the reported contrast would be code-specific rather than a general feature of isolated binary evolution.

    Authors: We agree that the absence of explicit sensitivity tests limits the generality of the reported contrast. The manuscript's intent is to compare results from the standard, publicly released versions of COMPAS and SEVN using their default prescriptions, thereby illustrating how differences in code implementations can affect conclusions about MRR. In the revised manuscript we will add a dedicated paragraph in the Discussion section acknowledging this limitation, stating that the qualitative differences are specific to the default settings employed, and noting that a full parameter-sweep or cross-code prescription swap lies beyond the present scope but would be a valuable extension. revision: yes

  2. Referee: [Results on channel weighting] The assertion that the core-growth channel dominates almost exclusively when weighted by the local BBH merger-rate density is load-bearing for downplaying the other two pathways. Details on the exact weighting procedure, including the assumed metallicity distribution and star formation history, are needed to assess robustness, as different assumptions could change which channel appears dominant.

    Authors: We thank the referee for this observation. The weighting follows the standard local merger-rate density construction that combines the Madau & Dickinson star-formation-rate density with a redshift-dependent metallicity distribution taken from cosmological simulations (log-normal scatter of 0.5 dex). We will expand the Methods section and add an appendix that reproduces the precise functional forms, parameter values, and references used for both the star-formation history and the metallicity distribution, allowing readers to recompute the channel weights under alternative assumptions. revision: yes

Circularity Check

0 steps flagged

No circularity: results are direct tallies from population synthesis simulations

full rationale

The paper reports outcomes from running the COMPAS and SEVN population synthesis codes under fixed prescriptions for mass transfer, PPISN, and CCSN. The central claims (MRR dominance at high masses and q>0.6 in COMPAS but subdominance in SEVN; core-growth channel dominating when weighted by merger-rate density; three pathways identified) are obtained by counting and classifying simulation realizations, not by any algebraic derivation, parameter fit, or self-referential definition that reduces the output to the input. No equations are presented whose right-hand side is constructed from the left-hand side, and no load-bearing step relies on a self-citation whose content is itself unverified or fitted to the same data. The analysis is therefore self-contained against external benchmarks and receives the default non-circularity finding.

Axiom & Free-Parameter Ledger

3 free parameters · 2 axioms · 0 invented entities

The claims depend on the internal physics modules and parameter choices of two population synthesis codes whose detailed implementations are not provided in the abstract; many standard binary-evolution parameters remain free or calibrated within each code.

free parameters (3)
  • mass-transfer efficiency and stability criteria
    These control whether the core-growth channel operates and are known to vary between codes.
  • pulsational pair-instability supernova mass-loss prescription
    Directly affects the PPISN-shrinking pathway and remnant masses.
  • core-collapse supernova mass-loss and kick distributions
    Drive the asymmetric-CCSN reversal channel.
axioms (2)
  • domain assumption Isolated binary evolution is the dominant channel for the BBH mergers considered
    The study focuses exclusively on isolated binaries and does not include dynamical formation channels.
  • domain assumption The local merger-rate density weighting accurately reflects the astrophysical population
    Used to determine that core-growth dominates when results are weighted.

pith-pipeline@v0.9.0 · 5924 in / 1608 out tokens · 40249 ms · 2026-05-22T09:25:23.915191+00:00 · methodology

discussion (0)

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