REVIEW 3 major objections 4 minor 1 cited by
Resonances of compressible stars in precessing orbits around a spinning black hole
T0 review · 3 major / 4 minor · reviewed 2026-08-08 · deepseek-v4-flash
Pith's one-line read A compressible star in a precessing orbit can hit the same tidal resonance as an incompressible one, for stiff equations of state.
desk verdict A plausible but oversold extension: the new polytropic resonance calculation actually forces a volume-preserving (incompressible) response, so the title's claim about compressible stars is only true for the background. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The affine star model, which represents the star as a uniform linear deformation of a spherical reference state through a $3\times3$ matrix $b_{ia}$, together with the conformal correspondence theorem that rescales $b_{ia}$ and the orbital angular velocity by $\alpha=(b_1b_2b_3)^{1/3}$, thereby mapping a polytropic compressible star onto the same virial equations used for incompressible ellipsoids. The perturbative expansion in the inclination angle $\varepsilon=\theta-\pi/2$ separates the tidal forcing into the geodesic precession frequency $\omega_\theta$ and the star's normal modes; the resonance appears when a mode frequency, here an r-mode set by the Coriolis force and self-gravity, coincides with $\omega_\theta$. The growth rate is obtained by projecting the tidal source $C_\alpha$ onto the resonant eigenvector of the linear operator $M_{\alpha\beta}$ in Eq. (59), which also defines the normal modes of the star.
What would settle it
Run a general-relativistic hydrodynamics simulation of a $0.6\,M_\odot$, $10^4$ km white dwarf modeled as a $\Gamma=5/3$ polytrope on a slightly inclined spherical orbit at $r/m\approx8$ around a $10^5\,M_\odot$ Kerr black hole with $a/m=0.8$: the r-mode should grow with the rate from Eq. (63) over 10–100 orbits and reach surface velocities of order the escape speed; if no such growth appears, the resonance is an artifact of the affine model's closure assumptions.
Extended reading notes
Core claim
The paper establishes that the tidal resonance identified for incompressible stars is not an artifact of incompressibility: a compressible polytropic star in a slightly inclined spherical orbit around a Kerr black hole undergoes the same first-order r-mode resonance, driven by the off-diagonal gravitoelectric component of the tidal tensor coupling to the Coriolis force, provided the polytropic index satisfies $n \lesssim 2$ (adiabatic index $\Gamma \gtrsim 3/2$). The second-order resonance, however, disappears for $n=2$ ($\Gamma=3/2$). The resonance condition, Eq. (57), and the growth-rate formula, Eq. (63), are derived analytically by expanding the affine-model equations to first and second order in the inclination angle and closing them with an adiabatic pressure relation; the growth-rate formula is checked against numerical integration of the linearized equations in Fig. 5. The same formalism also yields the zero-order Roche limit for each polytrope, and the paper shows that the first-order resonance can be reached before that limit for white dwarfs of typical compactness.
Load-bearing premise
The calculation keeps the scale factor $\alpha=(b_1b_2b_3)^{1/3}$ fixed at its equilibrium value during the perturbation; if first-order volume changes feed back through $\alpha$, the normal-mode frequencies and the resonance condition derived from them would shift.
Editorial extensions
If this is right
- For stiff polytropic white dwarfs ($n=1$ to $3/2$), the first-order resonance occurs at a larger $r/m$ than for incompressible stars and can still lie outside the Roche limit, so it can act before tidal disruption.
- A $0.6\,M_\odot$, $10^4$ km white dwarf around a $10^5\,M_\odot$ Kerr black hole with $a/m=0.8$ enters the resonance near $r/m\approx8$, where the predicted growth over 10–100 orbits drives surface velocities toward the escape speed, suggesting mass ejection and eventual disruption.
- The gravitational-wave signal from such an inspiral would terminate at an orbital frequency slightly below the value expected from tidal disruption alone, because resonant mass ejection reduces the star's compactness beforehand.
- For polytropic index $n=2$ ($\Gamma=3/2$), the second-order resonance no longer exists, so only the first-order resonance matters for softer stars, and it is triggered closer to the black hole.
- The same perturbative framework suggests that weakly eccentric, precessing orbits, as expected from three-body interactions or tidal capture events, would also show these resonances for not-too-soft equations of state.
Reading between the lines
- An extension the paper leaves implicit: if the resonance grows as predicted, the ejected mass carries away orbital angular momentum and changes the inspiral rate, which would advance or delay the gravitational-wave phase relative to a point-particle inspiral and could be measurable with a space-based detector.
- A testable extension beyond the paper's formalism is to allow $\alpha=(b_1b_2b_3)^{1/3}$ to respond to the first-order volume change instead of being fixed; this would directly show whether the resonance distance shifts significantly for soft equations of state.
- The same resonance mechanism should operate for any star in an inclined, precessing orbit around a compact spinning primary—including neutron stars in extreme-mass-ratio inspirals—where the polytropic index and spin set the resonant radius; the growth-rate formula derived here is a natural tool for those cases.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper extends a previously reported tidal resonance for incompressible stars in slightly inclined spherical orbits around a Kerr black hole to stars with polytropic equations of state. Using the affine star model and the Carter-Luminet conformal correspondence, the authors derive a first-order resonance condition and a growth-rate formula, apply these to white dwarfs around a 10^5 solar-mass black hole, and find that the first-order resonance survives for relatively stiff equations of state while the second-order resonance disappears for polytropic index n=2. They also estimate the number of orbits needed for the resonance to affect the star and discuss possible gravitational-wave and electromagnetic signatures.
Significance. If the result holds, it identifies a new astrophysical channel for tidal resonances before disruption in white-dwarf/black-hole systems, with potentially observable consequences for LISA-band sources. The paper is largely self-contained, presents an explicit analytical growth-rate formula, and checks it against a numerical integration of the same linear ODE in Fig. 5. The use of the conformal correspondence to translate polytropic backgrounds into the incompressible framework is elegant and useful. However, the central claim that the resonances occur for genuinely compressible stars is undermined by the assumption that the scale factor alpha is held fixed during the perturbation, which effectively reduces the perturbed dynamics to the incompressible case.
major comments (3)
- [Sec. III A, Eqs. (41), (49), and (54)] The assumption that alpha stays constant and equal to (b1 b2 b3)^{1/3}, together with the definition of alpha in Eq. (41), forces the first-order perturbation to be volume-preserving: tr(\hat b0^{-1} \hat beta) = 0, which is exactly Eq. (54). Combining this with Eq. (52) gives delta P = 0 and, from Eq. (53), delta Pi = 0. The perturbed star therefore obeys the same divergence-free equations as in the incompressible case, and the polytropic index enters only through the equilibrium background. This contradicts the text in the same section, which states that 'contrary to the incompressible case, we cannot assume that the star has a fixed volume.' The paper gives no estimate of the error arising from dropping the delta-alpha terms that would appear if the volume were allowed to change. Consequently, the central claim that the resonance is extended to compressible stars is not actually demonstrated; what is shown is an incompressible-mode calculation on a polytropic background.
- [Sec. III C] The second-order resonance condition is never displayed. The text describes that one should recast the equations in matrix form and find where the determinant vanishes, but neither the matrix, the determinant condition, nor the resulting resonant-distance equation is given. Since the disappearance of the second-order resonance for n=2 is a quantitative claim shown only in Fig. 2, the derivation is not verifiable from the manuscript as written.
- [Sec. III B, Eqs. (57), (61), and (63)] The resonance is identified as an r-mode driven by the Coriolis force, but the eigenvector rho_alpha of Eq. (61) is not computed or displayed. Given the alpha-constant closure, one cannot verify that the resonant mode has vanishing divergence and vanishing pressure perturbation, nor compare its frequency with known r-mode frequencies of compressible polytropes. Without such a check, the claim that the resonance mechanism is genuinely the same in compressible stars remains an assertion rather than a demonstrated result.
minor comments (4)
- [Abstract and Introduction] The abstract says 'We give further credence to the result previously given,' which is vague; the introduction's phrasing 'previously found' is clearer. The sentence should be reworded to state what new evidence is provided.
- [Fig. 5 caption] The text says delta is approximately -7.2 and 'close to 7'; it should say 'close to -7' to avoid a sign ambiguity.
- [Sec. II C] The statement that the formalism cannot be extended to Gamma = 4/3 is mentioned in passing but never discussed; since the paper considers Gamma = 2, 5/3, and 3/2, a brief comment on the physical meaning of the Gamma = 4/3 exclusion would be helpful.
- [Sec. IV A, Fig. 4] The figure uses epsilon_0 = 0.5, which is not a small inclination angle; the linear perturbation expansion in epsilon may therefore be quantitatively unreliable at this amplitude. The authors should state more carefully that this value is chosen for illustration and that the linear prediction is only indicative.
Circularity Check
The α-constant assumption makes the perturbed dynamics volume-preserving, so the 'compressible-star' resonance is the incompressible r-mode on a polytropic background; the central claim reduces to this input.
-
self definitional
[Sec. III A, around Eqs. (48)-(49); admission in Sec. III B after Eq. (56)]
"To facilitate the subsequent computations, we assume that α stays constant and equal to (b1b2b3)1/3. Thus, we have at first order ``` ``` ``` ```"
α=(b1b2b3)^{1/3} is the cube root of the deformation determinant, so "α stays constant" forces tr(b0^{-1}β)=0: the first-order density change vanishes and, via Eqs. (51)-(52), δP=0. The paper then states (Sec. III B) that the off-diagonal perturbation equations giving the r-mode resonance "are the same as the ones for the incompressible model." Thus the claimed compressible-star resonance is the incompressible linear response on a polytropic background; the polytropic index changes only the equilibrium shape, Roche limit, α, and τ*, not the perturbed compressible dynamics. A genuine compressible mode would have nonvanishing divergence and modified frequencies in Eqs. (57) and (63). The central claim therefore reduces by construction to the α-constant input.
full rationale
The only substantive circularity is the α-constant assumption: since α=(b1b2b3)^{1/3}, fixing α during the perturbation fixes the first-order volume (and hence density) perturbation to zero, and the paper itself notes that the off-diagonal equations that produce the resonance are the same as in the incompressible model. Thus the existence of the resonance in 'compressible' stars is not a new dynamical result; it is the incompressible r-mode of the previous paper [20] evaluated on a polytropic equilibrium. The polytropic background calculation (Lane-Emden solution, conformal rescaling, Roche limit, resonant-distance shifts) is independent and not circular. Citations of [20] and of Carter-Luminet are continuity statements, not load-bearing uniqueness claims; no fitted parameter is relabeled as a prediction, and Fig. 5 is an internal consistency check of Eq. (58) against Eq. (63). The score of 6 reflects that one central 'prediction'—the persistence of the resonance—reduces by construction to the volume-preserving input, while the EOS-dependent background content remains substantive.
Assumptions & free parameters
free parameters (2)
- epsilon_0 =
0.5
- delta =
-7.2
assumptions (8)
- domain assumption Affine model: the star's deformation is a spatially uniform linear map r_i = q_ia rhat_a (Eq. 1).
- domain assumption Polytropic equation of state P = kappa rho^(1+1/n).
- domain assumption Newtonian self-gravity for the star; general relativistic self-gravity neglected.
- domain assumption Corotating star with zero vorticity, lambda_ab = 0.
- ad hoc to paper Scale factor alpha remains constant during the perturbation.
- domain assumption Adiabatic pressure-density relation delta P/P = Gamma delta rho/rho.
- domain assumption Geodesic orbit fixed; tidal back-reaction on the orbit is neglected.
- standard math Carter-Luminet conformal correspondence theorem.
Cite this review
Pith. "Pith review of Resonances of compressible stars in precessing orbits around a spinning black hole." pith.science (2026). https://pith.science/paper/HDANK5BC
@misc{pith2026250206998,
author = {Pith},
title = {Pith review of: Resonances of compressible stars in precessing orbits around a spinning black hole},
year = {2026},
howpublished = {\url{https://pith.science/paper/HDANK5BC}},
note = {Machine review of arXiv:2502.06998}
}
read the original abstract
In our previous paper, we reported the presence of a new resonance of an incompressible star orbiting a spinning black hole and showed that it can set in before the tidal disruption limit if the star has an inclined spherical orbit around the black hole. Using the affine model developed by Carter and Luminet, we extend our result to the stars with polytropic equations of state. We give further credence to the result previously given. We also derive the formula for the growth rate of the resonant motion, which is useful for checking the results of hydrodynamics simulations.
Figures
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Reference graph
Works this paper leans on
-
[1]
G. I. Ogilvie, Tidal Dissipation in Stars and Giant Plan- ets, ARA&A 52, 171 (2014), arXiv:1406.2207 [astro- ph.SR]. 14
arXiv 2014
-
[2]
Gezari, Tidal Disruption Events, ARA&A 59, 21 (2021), arXiv:2104.14580 [astro-ph.HE]
S. Gezari, Tidal Disruption Events, ARA&A 59, 21 (2021), arXiv:2104.14580 [astro-ph.HE]
arXiv 2021
-
[3]
J. Souchay, S. Mathis, and T. Tokieda, Tides in Astron- omy and Astrophysics , Vol. 861 (2013)
work page 2013
-
[4]
D. E. Cartwright, Tides : a scientific history (1999)
work page 1999
-
[5]
V. Deparis, H. Legros, and J. Souchay, Investigations of Tides from the Antiquity to Laplace, in Lecture Notes in Physics, Berlin Springer Verlag , Vol. 861, edited by J. Souchay, S. Mathis, and T. Tokieda (2013) p. 31
work page 2013
-
[6]
P. Bodenheimer, D. N. C. Lin, and R. A. Mardling, On the Tidal Inflation of Short-Period Extrasolar Planets, ApJ 548, 466 (2001)
work page 2001
-
[7]
L. Ibgui and A. Burrows, Coupled Evolution with Tides of the Radius and Orbit of Transiting Giant Planets: General Results, ApJ 700, 1921 (2009), arXiv:0902.3998 [astro-ph.EP]
arXiv 2009
-
[8]
C. S. Kochanek, Coalescing Binary Neutron Stars, ApJ 398, 234 (1992)
work page 1992
Show all 43 references
-
[9]
Bildsten and C
L. Bildsten and C. Cutler, Tidal Interactions of Inspiral- ing Compact Binaries, ApJ 400, 175 (1992)
1992
-
[10]
D. Lai, F. A. Rasio, and S. L. Shapiro, Hydrodynamic Instability and Coalescence of Close Binary Systems, ApJ 406, L63 (1993)
1993
-
[11]
D. Lai, F. A. Rasio, and S. L. Shapiro, Hydrodynamic Instability and Coalescence of Binary Neutron Stars, ApJ 420, 811 (1994), arXiv:astro-ph/9304027 [astro-ph]
1994 arXiv
-
[12]
´E. ´E. Flanagan and T. Hinderer, Constraining neutron- star tidal Love numbers with gravitational-wave detec- tors, Phys. Rev. D 77, 021502 (2008), arXiv:0709.1915 [astro-ph]
2008 arXiv
-
[13]
L. G. Fishbone, The Relativistic Roche Problem, ApJ 175, L155 (1972)
1972
-
[14]
L. G. Fishbone, The relativistic Roche problem. II. Sta- bility theory., ApJ 195, 499 (1975)
1975
-
[15]
M. Shibata, Relativistic Roche-Riemann Problems around a Black Hole, Progress of Theoretical Physics 96, 917 (1996), https://academic.oup.com/ptp/article- pdf/96/5/917/5226638/96-5-917.pdf
1996
-
[16]
Wiggins and D
P. Wiggins and D. Lai, Tidal Interaction between a Fluid Star and a Kerr Black Hole in Circular Orbit, ApJ 532, 530 (2000), arXiv:astro-ph/9907365 [astro-ph]
2000 arXiv
-
[17]
Ishii, M
M. Ishii, M. Shibata, and Y. Mino, Black hole tidal prob- lem in the Fermi normal coordinates, Phys. Rev. D 71, 044017 (2005), arXiv:gr-qc/0501084
2005 arXiv
-
[18]
Banerjee, S
P. Banerjee, S. Paul, R. Shaikh, and T. Sarkar, Tidal effects away from the equatorial plane in Kerr backgrounds, Physics Letters B 795, 29 (2019), arXiv:1812.08642 [gr-qc]
2019 arXiv
-
[19]
Chandrasekhar, Ellipsoidal figures of equilibrium (1969)
S. Chandrasekhar, Ellipsoidal figures of equilibrium (1969)
1969
-
[20]
Stockinger and M
M. Stockinger and M. Shibata, Relativistic Roche prob- lem for stars in precessing orbits around a spinning black hole, Phys. Rev. D 110, 043038 (2024), arXiv:2403.06834 [astro-ph.HE]
2024 arXiv
-
[21]
Shibata, Effects of tidal resonances in coalescing com- pact binary systems, Prog
M. Shibata, Effects of tidal resonances in coalescing com- pact binary systems, Prog. Theor. Phys. 91, 871 (1994)
1994
-
[22]
Lai, Resonant Oscillations and Tidal Heating in Coa- lescing Binary Neutron Stars, MNRAS 270, 611 (1994), arXiv:astro-ph/9404062 [astro-ph]
D. Lai, Resonant Oscillations and Tidal Heating in Coa- lescing Binary Neutron Stars, MNRAS 270, 611 (1994), arXiv:astro-ph/9404062 [astro-ph]
1994 arXiv
-
[23]
Reisenegger and P
A. Reisenegger and P. Goldreich, Excitation of Neutron Star Normal Modes during Binary Inspiral, ApJ 426, 688 (1994)
1994
-
[24]
K. D. Kokkotas and G. Schafer, Tidal and tidal-resonant effects in coalescing binaries, MNRAS 275, 301 (1995), arXiv:gr-qc/9502034 [gr-qc]
1995 arXiv
-
[25]
´E. ´E. Flanagan and ´E. Racine, Gravitomagnetic reso- nant excitation of Rossby modes in coalescing neutron star binaries, Phys. Rev. D 75, 044001 (2007), arXiv:gr- qc/0601029 [gr-qc]
2007
-
[26]
Xu and D
W. Xu and D. Lai, Resonant tidal excitation of oscilla- tion modes in merging binary neutron stars: Inertial- gravity modes, Phys. Rev. D 96, 083005 (2017), arXiv:1708.01839 [astro-ph.HE]
2017 arXiv
-
[27]
Poisson, Gravitomagnetic tidal resonance in neutron- star binary inspirals, Phys
E. Poisson, Gravitomagnetic tidal resonance in neutron- star binary inspirals, Phys. Rev. D 101, 104028 (2020), arXiv:2003.10427 [gr-qc]
2020 arXiv
-
[28]
Astoul and A
A. Astoul and A. J. Barker, Do nonlinear effects disrupt tidal dissipation predictions in convective en- velopes?, arXiv e-prints , arXiv:2310.05980 (2023), arXiv:2310.05980 [astro-ph.SR]
2023 arXiv
-
[29]
H. Yu, P. Arras, and N. N. Weinberg, Dynamical tides during the inspiral of rapidly spinning neutron stars: Solutions beyond mode resonance, Phys. Rev. D 110, 024039 (2024), arXiv:2404.00147 [gr-qc]
2024 arXiv
-
[30]
Carter and J
B. Carter and J. P. Luminet, Mechanics of the affine star model, MNRAS 212, 23 (1985)
1985
-
[31]
Carter and J
B. Carter and J. P. Luminet, Tidal compression of a star by a large black hole. I Mechanical evolution and nuclear energy release by proton capture, A&A 121, 97 (1983)
1983
-
[32]
J. P. Luminet and J. A. Marck, Tidal squeezing of stars by Schwarzschild black holes, MNRAS 212, 57 (1985)
1985
-
[33]
J. P. Luminet and B. Carter, Dynamics of an Affine Star Model in a Black Hole Tidal Field, ApJS 61, 219 (1986)
1986
-
[34]
Chandrasekhar, An introduction to the study of stellar structure (1939)
S. Chandrasekhar, An introduction to the study of stellar structure (1939)
1939
-
[35]
J. A. Marck, Solution to the Equations of Parallel Trans- port in Kerr Geometry; Tidal Tensor, Proceedings of the Royal Society of London Series A 385, 431 (1983)
1983
-
[36]
E. N. Parker, Tensor Virial Equations, Physical Review 96, 1686 (1954)
1954
-
[37]
S. L. Shapiro and S. A. Teukolsky, Black holes, white dwarfs and neutron stars. The physics of compact objects (1983)
1983
-
[38]
Amaro-Seoane, H
P. Amaro-Seoane, H. Audley, S. Babak, J. Baker, E. Ba- rausse, P. Bender, E. Berti, P. Binetruy, M. Born, D. Bor- toluzzi, J. Camp, C. Caprini, V. Cardoso, M. Colpi, J. Conklin, N. Cornish, C. Cutler, K. Danzmann, R. Dolesi, L. Ferraioli, V. Ferroni, E. Fitzsimons, J. Gair, L....
2017 arXiv
-
[39]
Tejeda, E
E. Tejeda, E. Gafton, S. Rosswog, and J. C. Miller, 15 Tidal disruptions by rotating black holes: relativistic hy- drodynamics with Newtonian codes, MNRAS 469, 4483 (2017), arXiv:1701.00303 [astro-ph.HE]
2017 arXiv
-
[40]
Gafton and S
E. Gafton and S. Rosswog, Tidal disruptions by rotating black holes: effects of spin and impact parameter, MN- RAS 487, 4790 (2019), arXiv:1903.09147 [astro-ph.HE]
2019 arXiv
-
[41]
Maguire, M
K. Maguire, M. Eracleous, P. G. Jonker, M. MacLeod, and S. Rosswog, Tidal Disruptions of White Dwarfs: Theoretical Models and Observational Prospects, Space Sci. Rev. 216, 39 (2020), arXiv:2004.00146 [astro-ph.HE]
2020 arXiv
-
[42]
Carter, Global Structure of the Kerr Family of Grav- itational Fields, Physical Review 174, 1559 (1968)
B. Carter, Global Structure of the Kerr Family of Grav- itational Fields, Physical Review 174, 1559 (1968)
1968
-
[43]
J. M. Bardeen, W. H. Press, and S. A. Teukolsky, Ro- tating Black Holes: Locally Nonrotating Frames, Energy Extraction, and Scalar Synchrotron Radiation, ApJ 178, 347 (1972)
1972
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