REVIEW 3 major objections 4 minor 1 cited by
Fitting the center-of-mass charge to a boosted post-Newtonian template makes numerical-relativity waveform frame fixing much less sensitive to the analyst's choice of fitting window, with median variance improvements up to about a factor of
Reviewed by Pith at T0; open to challenge. T0 means a machine referee read the full paper against a public rubric. the ladder, T0–T4 →
T0 review · deepseek-v4-flash
2026-08-04 05:40 UTC pith:C7ZNX6VO
load-bearing objection Solid, well-scoped improvement to BMS CoM frame-fixing: the PN-boosted fit does reduce window sensitivity, though the variance-ratio metric measures internal consistency, not unbiasedness. the 3 major comments →
Fixing the center-of-mass frame of numerical relativity waveforms using the post-Newtonian center-of-mass charge
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
The authors claim that the numerical CoM charge of a quasicircular, nonprecessing binary, once boosted and translated by the unknown frame parameters, is well fit over the inspiral by the leading-PN analytical form of Eq. (26). That template combines the PN out-spiral of the CoM charge, evaluated with orbital phase taken from the (2,1) strain mode, with a small-boost transformation and nuisance parameters alpha1, alpha2, plus a constant translation Delta. Fitting this template instead of a line yields boost and translation parameters that vary far less as the fitting window is moved or resized, and the method is incorporated into the public frame-fixing code so Cauchy-characteristic-evolutio
What carries the argument
The central object is the boosted CoM charge fitting function, Eq. (26): G' = (1/P^0)[(alpha1 lambda-hat + alpha2 n-hat)|G| - beta x J - u P^0 beta] + Delta. It packages the leading-PN CoM charge |G| (proportional to x^{5/2} sqrt(1-4 nu) nu^2), the spin-weighted orbital direction vectors, a small-boost transformation involving the angular momentum, and nuisance parameters that absorb higher-PN and metadata errors. Its role is to let the fit separate the physical out-spiral from the spurious boost and translation, making the extracted beta and Delta robust to the fitting-window choice.
Load-bearing premise
The template assumes the numerical CoM charge, after rescaling by nuisance amplitudes, is accurately captured by the leading-PN out-spiral with phase taken from the (2,1) mode; junk radiation or higher-PN corrections can violate this and bias the fit.
What would settle it
Take a long, low-junk simulation and compute the boost and translation from the PN fit using windows anchored at early, middle, and late times; also compute the CoM frame independently from apparent-horizon trajectories. If the PN-fit parameters disagree with the horizon-based frame by more than the linear fit's disagreement, or if the variance ratios are near unity once junk-free early data are excluded, the central robustness claim fails.
If this is right
- Frame-fixed Cauchy-characteristic-evolution waveforms from numerical relativity will depend less on which window an analyst chooses, improving reproducibility.
- Users can safely use shorter windows (at least about 1500M) and center them in the inspiral, reducing contamination from junk radiation and merger.
- Waveform models calibrated or compared against numerical-relativity waveforms will see smaller gauge-induced mode mixing, tightening comparisons.
- The z-component of the CoM charge still requires a linear fit, so out-of-plane frame fixing remains as before.
- The method is now incorporated into the public Python package used for asymptotic waveform analysis, making it directly usable by the community.
Where Pith is reading between the lines
- If the PN template's phase mapping from the (2,1) mode is slightly off, the apparent gain could partly reflect overfitting rather than a true frame; a direct test would compare with an independent CoM determination.
- The same template idea should extend to eccentric and precessing binaries once the corresponding PN CoM-charge expressions are derived; the nuisance-parameter structure may need to grow to absorb in-plane higher-PN effects.
- The improvement largely vanishes when the window is anchored at the start of the inspiral, suggesting junk radiation, not the linear-drift baseline, is the main contaminant; treating junk radiation explicitly could yield further gains.
- Higher-PN corrections to the CoM charge might shrink the scatter of the nuisance parameters around (1,0), turning the fit into a diagnostic for PN-numerical-relativity agreement.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes an improved method for fixing the center-of-mass (CoM) part of the BMS frame of numerical relativity waveforms. The authors compute a leading-post-Newtonian expression for the boosted CoM charge, Eq. (26), which includes the physical out-spiraling oscillation of the CoM, and fit this template to the numerically computed CoM charge from CCE waveforms. They compare the new fit to the previous linear fit by varying the fitting-window duration and placement for 20 nonprecessing quasicircular SXS simulations. They report median variance ratios up to ~25 for βx and ~20 for Δx for center-fixed windows, and implement the method in the scri package. The central claim is that the PN-based fit yields boost and translation parameters that are substantially less sensitive to the choice of fitting window.
Significance. If the improvement is real and the fitted parameters are unbiased, the method reduces the user-dependence of BMS frame-fixing for CCE waveforms, which is important for waveform modeling and NR/analytic comparisons. The paper draws on external PN results, uses a reasonable set of simulations, and makes its code available through scri. However, the evidence presented is exclusively about internal consistency across windows; the paper does not demonstrate that the new fit produces the correct CoM frame. The nuisance parameters α1 and α2 can absorb template misspecification, so the robustness gain may be partially an artifact. A validation test against an independent known boost/translation is needed before the method is adopted as a definitive frame-fixing tool.
major comments (3)
- [Section VI, Table II and Fig. 6] The headline improvement is measured as a ratio of variances of fitted parameters over different windows. This is a self-consistency metric; it does not establish that the fitted β and Δ equal the true CoM-frame parameters. Equation (26) contains two nuisance parameters α1 and α2 that can absorb amplitude and directional errors in the leading-PN template. A misspecified template can produce parameters that are stable across windows but biased. The paper's own Fig. 6 shows several simulations with (α1, α2) far from (1,0), and the text says the behavior of these outliers is not addressed. To support the frame-fixing claim, please add a recovery test (e.g., apply a known boost/translation to a waveform and check the fitted parameters) or compare with an independent CoM estimate. Without such validation, the central claim is about precision, not accuracy.
- [Section V, Eqs. (25)–(26)] The error-order statements are inconsistent: Eq. (25) says O(x, β^2) while Eq. (26) says O(x^3, β^2). Since G ~ x^{5/2} and J ~ x^{1/2}, an O(x) remainder in Eq. (25) is not the natural PN order of the neglected terms; the boost terms omitted from Eq. (23) are of order β x^{1/2}. Please clarify the PN counting: what exactly is being neglected in each equation, and why does the remainder improve from O(x) to O(x^3) between the two equations?
- [Section V, Eq. (19)] The coefficient 1142/105 in the total CoM flux is a key input to the template, but its derivation is only summarized in one sentence. If this coefficient is not already present in the cited Compère et al. expression, the authors should show the intermediate steps or give a direct reference. Currently a reader cannot verify the central analytical result on which the fitting function rests.
minor comments (4)
- [Table II] Report the spread (e.g., quartiles or confidence intervals) of the variance ratios across the 20 simulations, not just the median.
- [Fig. 5] The caption states the simulation, but the vertical axis labels are in different units for β and Δ; consider adding a legend or an explicit note about the different scales.
- [Section VI] Typo: 'Conversly' should be 'Conversely'.
- [Section V, Eq. (28)] State the branch of arg and the convention used to unwrap ψ to avoid phase jumps, since the fitting is sensitive to phase continuity.
Circularity Check
No significant circularity: the PN CoM template is externally derived, the nuisance parameters are explicit fits rather than disguised predictions, and the variance-ratio improvement is an empirical comparison, not a constructed equivalence.
full rationale
The paper's central derivation is self-contained rather than circular. The leading-order CoM charge in Eq. (20) is derived from the Compère et al. flux-balance law and Blanchet's equations of motion, both external references, and the paper explicitly notes that the result matches Compère et al. Eq. (4.18b). The fitting function Eq. (26) introduces α1 and α2 as explicit nuisance parameters to absorb amplitude and direction mismatches; these are fitted per simulation and are not presented as predictions. The old linear fit is a special case of Eq. (26) with α1=α2=0, so the new model is a deliberate generalization, not a hidden redefinition of the target. The robustness claim is a measured comparison of the variances of fitted β and Δ across different fitting windows; it does not define the improved parameters in terms of the quantity being predicted. The paper's self-citations ([28,29,34,37]) provide context, prior baseline methods, and software implementation, but the load-bearing PN input comes from external literature. The acknowledged limitations—avoiding equal-mass systems because of the derivative singularity in the ν2√(1−4ν) prefactor, and the reduced improvement in the start-fixed case attributed to junk radiation—are correctness risks, not evidence of circularity. No step in the derivation reduces by construction to its own input, and no fitted parameter is relabeled as a prediction.
Axiom & Free-Parameter Ledger
free parameters (4)
- alpha1 =
per-simulation; Fig. 6 shows scatter roughly in [-2, 2]
- alpha2 =
per-simulation; Fig. 6 shows scatter roughly in [-2, 2]
- boost vector beta (beta_x, beta_y; beta_z from linear fit) =
per-simulation, e.g. SXS:BBH:2115 beta_x ~ 3.15e-6, beta_y ~ -1.38e-5 (Fig. 5)
- translation vector Delta (Delta_x, Delta_y; Delta_z from linear fit) =
per-simulation, e.g. SXS:BBH:2115 Delta_x ~ 3.5e-4, Delta_y ~ -2e-3 (Fig. 5)
axioms (7)
- domain assumption Compere et al. center-of-mass balance law (Eq. 4.14b) is correct and applicable; it seeds Eq. (17).
- domain assumption The leading-order linear momentum flux of a quasicircular nonprecessing binary is Eq. (18) (Wiseman; Mishra et al.), and Blanchet's equations of motion supply the remaining flux terms leading to Eq. (19).
- domain assumption BMS charge definitions (mass, angular momentum, boost, CoM) and their transformation under small boosts, Eqs. (12) and (22)-(23), are taken from prior BMS charge literature.
- domain assumption For quasicircular nonprecessing orbits, the PN CoM charge is aligned with lambda-hat through 2PN, with a 2.5PN nhat contribution absorbed by alpha2 in Eq. (26).
- domain assumption The orbital phase psi, and hence the direction of lambda-hat/nhat, is obtained from the h_2,1 mode phase via Eq. (28), assuming the SpEC/PN sign and phase conventions.
- domain assumption The numerical CCE CoM charge is accurate enough that residual differences from the PN model are dominated by junk radiation and higher-PN terms, not by systematic CCE error.
- domain assumption O(beta^2) terms in the boost transformation are negligible for the small frame boosts in SXS CCE data.
read the original abstract
The Bondi--van der Burg--Metzner--Sachs (BMS) frame of gravitational waves produced by numerical relativity (NR) simulations is crucial for building accurate waveform models. A proper comparison of NR waveforms with other models requires fixing the arbitrary BMS frame. In this work we improve the center-of-mass (CoM) frame fixing for quasicircular, nonprecessing binary systems. Past work approximated the CoM motion with just a linear fit. We compute a post-Newtonian result of the boosted CoM charge to also capture its physical out-spiraling oscillations. We show that using the analytical results improves the robustness of the fit parameters -- translation and boost vectors -- to the choice of duration and time of the fitting window. Our analysis demonstrates a maximum improvement in robustness when the window is placed at the center of the inspiral. We quantified this improvement by computing the ratio of variances of fit parameters when the fit window size is varied. The largest improvement in robustness of parameters is by a factor of $\sim 25$ for the boost vector and $\sim 20$ for the translation vector. Finally, we incorporate this method into the BMS frame-fixing routine of the python package $\texttt{scri}$ for waveforms produced with Cauchy-characteristic evolution.
Figures
Forward citations
Cited by 1 Pith paper
-
The Bondi--Sachs gauge, BMS frames, and memory in black hole perturbation theory
Introduces a gauge transformation framework for BMS frames in multiscale black hole perturbation theory on Kerr that incorporates memory effects and avoids infrared divergences.
Reference graph
Works this paper leans on
-
[5]
LISAcollaboration,Laser Interferometer Space Antenna, 1702.00786
-
[6]
Mroue et al.,Catalog of 174 Binary Black Hole Simulations for Gravitational Wave Astronomy,Phys
A.H. Mroue et al.,Catalog of 174 Binary Black Hole Simulations for Gravitational Wave Astronomy,Phys. Rev. Lett.111(2013) 241104 [1304.6077]
Pith/arXiv arXiv 2013
-
[7]
Boyle et al.,The SXS Collaboration catalog of binary black hole simulations,Class
M. Boyle et al.,The SXS Collaboration catalog of binary black hole simulations,Class. Quant. Grav.36(2019) 195006 [1904.04831]
Pith/arXiv arXiv 2019
-
[8]
Scheel et al.,The SXS Collaboration’s third catalog of binary black hole simulations,2505.13378
M.A. Scheel et al.,The SXS Collaboration’s third catalog of binary black hole simulations,2505.13378
-
[9]
G. Pratten, S. Husa, C. Garcia-Quiros, M. Colleoni, A. Ramos-Buades, H. Estelles et al.,Setting the cornerstone for a family of models for gravitational waves from compact binaries: The dominant harmonic for nonprecessing quasicircular black holes,Phys. Rev. D 102(2020) 064001 [2001.11412]
Pith/arXiv arXiv 2020
-
[10]
S. Bernuzzi, M. Thierfelder and B. Bruegmann,Accuracy of numerical relativity waveforms from binary neutron star mergers and their comparison with post-Newtonian waveforms,Phys. Rev. D85(2012) 104030 [1109.3611]
Pith/arXiv arXiv 2012
-
[11]
T. Hinderer et al.,Periastron advance in spinning black hole binaries: comparing effective-one-body and Numerical Relativity,Phys. Rev. D88(2013) 084005 [1309.0544]
Pith/arXiv arXiv 2013
-
[12]
A. Ramos-Buades, A. Buonanno, H. Estellés, M. Khalil, D.P. Mihaylov, S. Ossokine et al.,Next generation of accurate and efficient multipolar precessing-spin effective-one-body waveforms for binary black holes,Phys. Rev. D108(2023) 124037 [2303.18046]
Pith/arXiv arXiv 2023
-
[13]
R. Gamba, D. Chiaramello and S. Neogi,Toward efficient effective-one-body models for generic, nonplanar orbits,Phys. Rev. D110(2024) 024031 [2404.15408]
Pith/arXiv arXiv 2024
-
[14]
M. Khalil, A. Buonanno, H. Estelles, D.P. Mihaylov, S. Ossokine, L. Pompili et al.,Theoretical groundwork supporting the precessing-spin two-body dynamics of the effective-one-body waveform models SEOBNRv5,Phys. Rev. D108(2023) 124036 [2303.18143]
Pith/arXiv arXiv 2023
-
[15]
V. Varma, S.E. Field, M.A. Scheel, J. Blackman, L.E. Kidder and H.P. Pfeiffer,Surrogate model of hybridized numerical relativity binary black hole waveforms,Phys. Rev. D99(2019) 064045 [ 1812.07865]
Pith/arXiv arXiv 2019
-
[16]
V. Varma, S.E. Field, M.A. Scheel, J. Blackman, D. Gerosa, L.C. Stein et al.,Surrogate models for precessing binary black hole simulations with unequal masses,Phys. Rev. Research.1(2019) 033015 [1905.09300]
Pith/arXiv arXiv 2019
-
[17]
J. Yoo, V. Varma, M. Giesler, M.A. Scheel, C.-J. Haster, H.P. Pfeiffer et al.,Targeted large mass ratio numerical relativity surrogate waveform model for GW190814,Phys. Rev. D106(2022) 044001 [2203.10109]
Pith/arXiv arXiv 2022
-
[18]
Boyle,Transformations of asymptotic gravitational-wave data,Phys
M. Boyle,Transformations of asymptotic gravitational-wave data,Phys. Rev. D93(2016) 084031 [1509.00862]
Pith/arXiv arXiv 2016
-
[19]
Bondi, M.G.J
H. Bondi, M.G.J. van der Burg and A.W.K. Metzner, Gravitational waves in general relativity. 7. Waves from axisymmetric isolated systems,Proc. Roy. Soc. Lond. A 269(1962) 21
1962
-
[20]
Sachs,Gravitational waves in general relativity
R.K. Sachs,Gravitational waves in general relativity. 8. Waves in asymptotically flat space-times,Proc. Roy. Soc. Lond. A270(1962) 103
1962
-
[21]
Sachs,Asymptotic symmetries in gravitational theory, Phys
R. Sachs,Asymptotic symmetries in gravitational theory, Phys. Rev.128(1962) 2851
1962
-
[22]
Bondi,Gravitational Waves in General Relativity, Nature186(1960) 535
H. Bondi,Gravitational Waves in General Relativity, Nature186(1960) 535
1960
-
[23]
Sachs,Gravitational waves in general relativity
R.K. Sachs,Gravitational waves in general relativity. 6. The outgoing radiation condition,Proc. Roy. Soc. Lond. A264(1961) 309
1961
-
[24]
C.J. Woodford, M. Boyle and H.P. Pfeiffer,Compact Binary Waveform Center-of-Mass Corrections,Phys. Rev. D100(2019) 124010 [1904.04842]
Pith/arXiv arXiv 2019
-
[25]
J. Moxon, M.A. Scheel, S.A. Teukolsky, N. Deppe, N. Fischer, F. Hébert et al.,SpECTRE Cauchy-characteristic evolution system for rapid, precise waveform extraction,Phys. Rev. D107(2023) 064013 [2110.08635]
Pith/arXiv arXiv 2023
-
[26]
J. Moxon, M.A. Scheel and S.A. Teukolsky,Improved Cauchy-characteristic evolution system for high-precision numerical relativity waveforms,Phys. Rev. D102(2020) 044052 [2007.01339]
Pith/arXiv arXiv 2020
-
[27]
D.A.B. Iozzo, M. Boyle, N. Deppe, J. Moxon, M.A. Scheel, L.E. Kidder et al.,Extending gravitational wave extraction using Weyl characteristic fields,Phys. Rev. D103(2021) 024039 [2010.15200]
Pith/arXiv arXiv 2021
-
[28]
Mitman et al.,Fixing the BMS frame of numerical relativity waveforms,Phys
K. Mitman et al.,Fixing the BMS frame of numerical relativity waveforms,Phys. Rev. D104(2021) 024051 [2105.02300]
Pith/arXiv arXiv 2021
-
[29]
Mitman et al.,Fixing the BMS frame of numerical relativity waveforms with BMS charges,Phys
K. Mitman et al.,Fixing the BMS frame of numerical relativity waveforms with BMS charges,Phys. Rev. D 106(2022) 084029 [2208.04356]
Pith/arXiv arXiv 2022
-
[30]
D.A.B. Iozzo et al.,Comparing Remnant Properties from Horizon Data and Asymptotic Data in Numerical Relativity,Phys. Rev. D103(2021) 124029 [2104.07052]
Pith/arXiv arXiv 2021
-
[31]
É.É. Flanagan and D.A. Nichols,Conserved charges of the extended Bondi-Metzner-Sachs algebra,Phys. Rev. D 95(2017) 044002 [1510.03386]
Pith/arXiv arXiv 2017
-
[32]
O.M. Moreschi and S. Dain,Rest frame system for asymptotically flat space-times,J. Math. Phys.39(1998) 6631 [gr-qc/0203075]
Pith/arXiv arXiv 1998
-
[33]
S. Ossokine, F. Foucart, H.P. Pfeiffer, M. Boyle and B. Szilágyi,Improvements to the construction of binary black hole initial data,Class. Quant. Grav.32(2015) 245010 [1506.01689]
Pith/arXiv arXiv 2015
-
[34]
A. Khairnar, L.C. Stein and M. Boyle,Approximate helical symmetry in compact binaries,Phys. Rev. D111 (2025) 024072 [2410.16373]
Pith/arXiv arXiv 2025
-
[35]
M. Boyle,Angular velocity of gravitational radiation from precessing binaries and the corotating frame,Phys. Rev. D87(2013) 104006 [1302.2919]
Pith/arXiv arXiv 2013
-
[36]
R. O’Shaughnessy, B. Vaishnav, J. Healy, Z. Meeks and D. Shoemaker,Efficient asymptotic frame selection for binary black hole spacetimes using asymptotic radiation, Phys. Rev. D84(2011) 124002 [1109.5224]
Pith/arXiv arXiv 2011
-
[37]
M. Boyle, D. Iozzo, L. Stein, A. Khairnar, H. Rüter, M. Scheel et al.,scri, June, 2025. 10.5281/zenodo.15693419
-
[38]
Moreschi,Supercenter of Mass System at Future Null Infinity,Class
O.M. Moreschi,Supercenter of Mass System at Future Null Infinity,Class. Quant. Grav.5(1988) 423
1988
-
[39]
G. Compère, R. Oliveri and A. Seraj,The Poincaré and BMS flux-balance laws with application to binary systems, JHEP10(2020) 116 [1912.03164]
Pith/arXiv arXiv 2020
-
[40]
L. Blanchet and G. Faye,Flux-balance equations for linear momentum and center-of-mass position of self-gravitating post-Newtonian systems,Class. Quant. Grav.36(2019) 085003 [1811.08966]. 12
Pith/arXiv arXiv 2019
-
[41]
L. Blanchet, G. Faye and D. Trestini,Gravitational radiation reaction for compact binary systems at the fourth-and-a-half post-Newtonian order,Class. Quant. Grav.42(2025) 065015 [2407.18295]
Pith/arXiv arXiv 2025
-
[42]
L. Bernard, L. Blanchet, G. Faye and T. Marchand, Center-of-Mass Equations of Motion and Conserved Integrals of Compact Binary Systems at the Fourth Post-Newtonian Order,Phys. Rev. D97(2018) 044037 [1711.00283]
Pith/arXiv arXiv 2018
-
[43]
Wiseman,Coalescing binary systems of compact objects to (post)5/2 Newtonian order
A.G. Wiseman,Coalescing binary systems of compact objects to (post)5/2 Newtonian order. 2. Higher order wave forms and radiation recoil,Phys. Rev. D46(1992) 1517
1992
-
[44]
C.K. Mishra, K.G. Arun and B.R. Iyer,The 2.5PN linear momentum flux and associated recoil from inspiralling compact binaries in quasi-circular orbits: Nonspinning case,Phys. Rev. D85(2012) 044021 [1111.2701]
Pith/arXiv arXiv 2012
-
[45]
Blanchet,Post-Newtonian theory for gravitational waves,Living Rev
L. Blanchet,Post-Newtonian theory for gravitational waves,Living Rev. Rel.27(2024) 4
2024
-
[46]
D. Trestini,Schott term in the binding energy for compact binaries on circular orbits at fourth post-Newtonian order,Phys. Rev. D112(2025) 024076 [2504.13245]
Pith/arXiv arXiv 2025
-
[47]
N. Deppe, W. Throwe, L.E. Kidder, N.L. Vu, K.C. Nelli, C. Armaza et al., “SpECTRE v2025.01.30.” 10.5281/zenodo.14774916, 1, 2025. 10.5281/zenodo.14774916
-
[48]
M. Boyle, L.E. Kidder, S. Ossokine and H.P. Pfeiffer, Gravitational-wave modes from precessing black-hole binaries,1409.4431
discussion (0)
Sign in with ORCID, Apple, or X to comment. Anyone can read and Pith papers without signing in.