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Physics of Pair Producing Gaps in Black Hole Magnetospheres II -- General Relativity

T0 review · 3 major / 4 minor · reviewed 2026-08-14 · deepseek-v4-flash

Pith's one-line read Pair gaps may explain M87's day-scale TeV flares

desk verdict A solid GR PIC study of black hole gaps with a real resolution claim, but the M87 flare link relies on the most favorable soft-photon optical depth. read the letter →

arxiv 1908.06919 v1 pith:I4CX6BAB submitted 2019-08-19 astro-ph.HE

classification astro-ph.HE
keywords blackholemagnetospherespaircascadesparticle-in-cellsimulationinverseComptonscatteringgamma-rayflaresM87Kerrspacetimeelectrostaticgaps
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

This paper argues that the region above a rotating black hole's magnetic pole does not settle into a steady discharge; instead, whenever the local pair density drops, a macroscopic electric gap opens near the null surface, accelerates particles, and then screens itself in a quasi-periodic burst of electron-positron pairs and gamma rays. The authors simulate this process with fully general-relativistic one-dimensional particle-in-cell methods that include inverse Compton scattering and photon tracking, and they find the cycle repeats in almost all parameter regimes they try. If this picture is right, low-luminosity accreting black holes naturally produce time-variable high-energy radiation, and the day-scale TeV flares of M87 can be explained by the same gap physics with a low soft-photon optical depth. The paper therefore supplies a concrete, first-principles mechanism linking black hole spin, pair creation, and observed gamma-ray variability.

What carries the argument

The load-bearing object is the one-dimensional flux tube along a magnetic field line in Kerr spacetime, solved in 3+1 form with a tortoise radial coordinate, with the deviation from the background force-free configuration as the dynamical electric field $D_\xi$. On this tube the simulation follows electrons, positrons, and photons with full inverse Compton scattering, including the Klein-Nishina cross section, and $\gamma\gamma$ pair production, and it uses the pair multiplicity $M = |\rho_+-\rho_-|\alpha c\sqrt{g_{rr}}/j^r_{\rm ff}$ as the criterion for gap formation: whenever $M<1$, the parallel electric field grows. This setup is what turns the qualitative idea of a screening gap into a quantitative, parameter-dependent prediction for gap power, timescale, and photon cutoff.

What would settle it

Measure or tightly constrain the soft-photon density and spectrum within a few gravitational radii of M87's black hole, for example through energy-dependent gamma-ray absorption features in the flare spectra; if the implied optical depth is $\tau_0 \sim 5\times 10^3$ rather than $\sim 10$, the predicted gap power drops by orders of magnitude and the model no longer matches the observed TeV flare luminosity.

Watch

Extended reading notes

Core claim

The central discovery is that a pair-producing gap in a slowly accreting black hole magnetosphere is intrinsically time-dependent: the gap opens quasi-periodically from the null surface where the force-free charge density vanishes, reaches a macroscopic size up to about the gravitational radius, and is then screened by pairs created when inverse-Compton photons collide with the soft photon background. In the deep Klein-Nishina regime, screening is delayed because the upscattered photons with the shortest mean free path are emitted before the primary particles reach their highest energies, so the gap grows larger and the primary particles are accelerated to Lorentz factors an order of magnitude above the radiation-reaction limit. The simulations give an outgoing photon spectrum that is a power law ending near $0.1/\tilde{\epsilon}_{\min}$, and the gap power scales approximately as $(\tilde{B}_0\tilde{\epsilon}_{\min})^{-1}$ at fixed $\tau_0$, depending only on the product rather than the two parameters separately. Rescaled to M87 parameters with $\tau_0 \sim 10$, the predicted gap power is $10^{40}$–$10^{41}$ erg s$^{-1}$ with a photon cutoff near 25 TeV, which the authors argue is consistent with the observed TeV flares.

Load-bearing premise

The argument collapses if the soft photon field in the gap region is not an isotropic, radius-independent power law that dominates locally emitted radiation, and in particular if M87's optical depth is near the upper end (~5e3) rather than the low value ~10 used for the flare comparison.

Editorial extensions

If this is right

  • Gap activity is cyclic: after an initial transient that depends on initial conditions, the system settles into quasi-periodic opening and screening with recurrence times of several $r_g/c$.
  • The gap power and screening timescale depend on the product $\tilde{B}_0\tilde{\epsilon}_{\min}$ rather than on either parameter separately, so runs with vastly different field strengths and soft-photon energies collapse onto the same behavior.
  • For $\tau_0 \lesssim 3$ the gap is no longer screened in the simulations: particles are accelerated so far into the Klein-Nishina regime that screening fails, marking a regime boundary.
  • Outgoing photon spectra from the gap form power laws that are harder in the high state and cut off near $0.1/\tilde{\epsilon}_{\min}$, corresponding to about 25 TeV for M87.
  • Rescaling to M87 with low optical depth ($\tau_0 \sim 10$) gives a gap power of $10^{40}$–$10^{41}$ erg s$^{-1}$, within reach of the observed TeV flare luminosity.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • If the gap cycles persist in two dimensions and with synchrotron and curvature losses included, the model predicts that M87's gamma-ray flares should show quasi-periodic substructure on timescales of several $r_g/c$, roughly a day for M87, rather than a single impulsive event.
  • The same mechanism, with different scaling, could apply to Sgr A* and other low-luminosity nuclei; the paper's dependence on the product $\tilde{B}_0\tilde{\epsilon}_{\min}$ gives a direct target for testing against their quiescent and flaring spectra.
  • A testable extension is to include radiation from secondary pairs and triplet pair production; if those are important, the gap luminosity could be higher than the present estimate, possibly accounting for the strongest $10^{42}$ erg s$^{-1}$ flares.
  • The resolution dependence found here suggests that earlier quasi-steady gap results may have been numerical artifacts of under-resolving the plasma skin depth; a dedicated convergence study at fixed physical parameters would settle which regime is physical.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

3 major / 4 minor

Summary. This paper presents time-dependent, fully general-relativistic 1D particle-in-cell simulations of pair-producing gaps in low-luminosity black hole magnetospheres, extending the authors' earlier flat-spacetime work (CY18) to include Kerr geometry, full inverse Compton scattering in the Klein-Nishina regime, and photon tracking. The central numerical finding is that the gap opens and is screened quasi-periodically near the null surface, producing bursts of pairs and high-energy photons. The authors measure the gap power as a function of the parameters B0~eps_min and tau0, obtain an empirical scaling L/L0 ~ (B0~eps_min)^-1 and a tau0 dependence that steepens around tau0~50, and then rescale these results to M87, concluding that the observed TeV flares could potentially be explained under certain parameter assumptions. The paper also reports resolution and particle-per-cell studies showing that the quasi-periodic state requires sufficient numerical resolution.

Significance. If the central result holds, this is a substantial advance: it demonstrates with a self-consistent kinetic model that low-luminosity BH magnetospheres can produce repeated, quasi-periodic pair cascades, and it provides a concrete physical mechanism for day-scale TeV variability in sources like M87. The technical strengths are real: the implementation of full GR particle motion, KN cross sections, and photon transport are described carefully; convergence and resolution tests are shown; and the code is publicly available. The empirical scaling relation is a useful organizing tool, though it is not derived from first principles. The significance for M87 specifically is moderate rather than definitive, because the application rests on an unconstrained soft-photon optical depth and on extrapolating the empirical scaling far beyond the reliably simulated parameter range.

major comments (3)
  1. [Section 3.2 and Figure 4] The M87 claim depends on choosing tau0 ~ 10, while the authors' own estimate gives an upper bound tau0 ~ 5e3 from u_s ~ 0.1 erg/cm^3. They state that using tau0 ~ 5e3 in CY18 yields L ~ 3e39 erg/s, more than two orders of magnitude below the observed TeV flare luminosity L ~ 1e42 erg/s. Since Eq. (22) assumes a spatially uniform, isotropic soft-photon power law and the actual photon density near the null surface is acknowledged to be highly uncertain and radius-dependent, the abstract's statement that the observed flares 'could potentially be explained' is conditional on the most favorable end of an essentially unconstrained parameter. To make the M87 connection load-bearing, the authors should either provide an observationally or theoretically motivated constraint on tau0 in the gap region, or present the resulting L_gap as a function of tau0 over the full range 10 to 5e3 and explicitly state that only the lowest end is consistent with the observed flares.
  2. [Section 3.2 and Figure 4] The empirical scaling L/L0 ~ (B0~eps_min)^-1 is inferred from a small number of simulations by eye (green dashed line), with no quoted uncertainties, no goodness-of-fit measure, and no stated range of validity. The extrapolation to M87 uses B0~eps_min = 2.4e5, which is a factor of eight beyond the largest reliably simulated value (~3e4), and the authors themselves note in Section 3.3 that numerical heating makes the highest-product simulations unreliable (e.g., the last point in the left panel of Figure 4). A one-order-of-magnitude error in the scaling exponent at these extrapolated values changes L_gap by orders of magnitude and breaks the claimed consistency with the TeV flares. The paper should quantify the uncertainty in the fitted scaling, restrict the M87 extrapolation to a defensible range, or provide additional simulations at intermediate products to test the power law.
  3. [Section 5] The paper's own Discussion concedes that the 1D approximation is not completely valid once the gap grows to a size comparable to rg, and that synchrotron and curvature radiation, as well as triplet pair production, are neglected but could be significant. These omissions bear directly on the M87 application in Section 4: the predicted photon spectrum and luminosity are computed without these processes, yet the spectral comparison to the observed TeV flare (power law up to ~25 TeV) is presented without carrying forward these caveats. The central simulation result is not invalidated, but the observational claims in the abstract and Section 4 should be explicitly framed as contingent on the 1D geometry and on the neglected radiation processes, or the relevant robustness tests should be performed.
minor comments (4)
  1. [Section 3.1 and Figure 3] The low-energy cutoff of the outgoing photon spectrum is due to the artificial removal of photons below ~10^3 m_e c^2 at creation; this should be stated in the figure caption and in Section 4, since it affects the shape of the spectrum that is later compared with observations.
  2. [Section 3.2] The notation for the product of normalized field and photon energy is inconsistent: the text switches between 'B0~eps_min', '˜B˜ϵmin', and 'B~eps' in Section 3.2 and Figure 4. Please use a single, clearly defined symbol throughout.
  3. [Section 4] The text contains several typographical errors, including 'vincinity' (twice) and 'the spectrum of the radio flux seems to peak around 1.2 meV' without an explicit reference for this value; please polish the language and add the reference.
  4. [Section 5] The discussion of the resolution dependence in Section 3.3 is important and should be moved closer to the main results, perhaps with a sentence in Section 3.1 noting that the quasi-periodic state is only recovered when the plasma skin depth is resolved and the particle noise is sufficiently low.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the M87 application is an extrapolation of simulation-derived scaling, not a fit to the observed flare.

full rationale

The paper's central derivation is self-contained: it uses a general-relativistic PIC code with standard radiative cross sections (inverse Compton and gamma-gamma pair production) and an assumed background soft-photon power law. The gap-power scaling L/L0 versus B0*epsilon_min and tau0 is measured from the authors' own simulations (Figure 4), not fitted to M87 data. The M87 parameters are inferred independently: B0 ~ 200 G from the jet power, epsilon_min from the radio SED, and tau0 ~ 10 as a low-opacity assumption within the paper's stated upper bound tau0 ~ 5e3. The conclusion is explicitly conditional ('could potentially be explained by this model under certain parameter assumptions'), and Section 4 flags the uncertainty in the soft-photon optical depth. The self-citation of CY18 for the Thomson-regime scaling is not load-bearing: equation (35) is re-expressed in this paper's units, and the product scaling is supported both by the dimensionless equations and by the simulation scan in Figure 4. No step reduces by construction to its inputs; the unconstrained tau0 value and the 1D approximation are physical and correctness risks, not circularity.

Assumptions & free parameters 3 free parameters · 8 assumptions · 0 invented entities

The central claims rest on a small number of chosen simulation parameters and several stated modeling simplifications; the M87 conclusion is especially sensitive to the assumed soft-photon field because the scaling relation is empirical and the extrapolation spans many orders of magnitude. No new particles or forces are introduced.

free parameters (3)
  • Soft photon spectral index alpha = not specified in text
    The background photon spectrum n(epsilon) proportional to (epsilon/epsilon_min)^(alpha-1) in equation (22) is assumed, and the value sets IC and pair-production rates; no value is given in the paper.
  • Soft photon optical depth tau0 at M87 = 10 assumed; 5e3 upper bound
    M87 application uses tau0 ~ 10 to enter the simulated regime; a value near the inferred upper bound would change the gap power by orders of magnitude (Section 4).
  • Dimensionless magnetic field and photon peak energy product at M87 = B0 ~ 200 G, epsilon_min ~ 2e-9, product 2.4e5
    Chosen from jet-power and SED estimates; uncertain by orders of magnitude and directly sets the predicted gap power.
assumptions (8)
  • standard math Kerr metric in 3+1 formalism with tortoise coordinates describes the spacetime near the black hole.
    Used throughout Section 2.1 for the field equations and particle equations of motion; standard general relativity.
  • domain assumption The background magnetosphere is a force-free split-monopole solution with a specified flux tube.
    The background charge and current densities come from this configuration, described in Appendix A and taken from Yuan et al. (2019).
  • domain assumption Particles are well magnetized and move only along magnetic field lines.
    Assumed in Section 2.1; this reduces the problem to 1D and excludes cross-field motion.
  • domain assumption The soft photon field is isotropic, a radius-independent power law, and much denser than photons emitted in the gap.
    Equation (22) and the surrounding text in Section 2.2; this controls IC scattering and gamma-gamma pair production.
  • domain assumption Photon motion can neglect the theta component of the four-velocity.
    Stated in Section 2.1 as a good approximation for the nearly radial poloidal field lines used.
  • domain assumption Gamma-gamma pair production is the only pair-creation mechanism; synchrotron and curvature radiation are ignored.
    Stated in Section 5; the authors note triplet pair production (Petropoulou et al. 2019) and synchrotron/curvature losses could change the gap physics.
  • domain assumption The 1D approximation along the magnetic flux tube is valid for gap dynamics.
    Assumed in Section 2.1; the authors acknowledge in Section 5 that gaps often grow to sizes comparable to rg, at which point the approximation is not completely valid.
  • domain assumption Low-energy photons below a cutoff can be removed at creation without affecting the results.
    Section 3.1 states photons below 10^3 m_e c^2 are removed at creation for computational feasibility, which truncates the low-energy spectrum.

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Pith. "Pith review of Physics of Pair Producing Gaps in Black Hole Magnetospheres II -- General Relativity." pith.science (2026). https://pith.science/paper/I4CX6BAB

@misc{pith2026190806919,
  author       = {Pith},
  title        = {Pith review of: Physics of Pair Producing Gaps in Black Hole Magnetospheres II -- General Relativity},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/I4CX6BAB}},
  note         = {Machine review of arXiv:1908.06919}
}
abstract

This is the second paper in a series where we examine the physics of pair producing gaps in low-luminosity accreting supermassive black hole systems. In this paper, we carry out time-dependent self-consistent fully general relativistic 1D PIC simulations of the gap, including full inverse Compton scattering and photon tracking. Similar to the previous paper, we find a highly time-dependent solution where a macroscopic vacuum gap can open quasi-periodically, producing bursts of $e^\pm$ pairs and high energy radiation. We present the light curve, particle and photon spectra from this process. Using an empirical scaling relation, we rescale the parameters to the inferred values at the base of the jet in M87, and find that the observed TeV flares could potentially be explained by this model under certain parameter assumptions.

Figures

Figures reproduced from arXiv: 1908.06919 by the authors.

Figure 1
Figure 1. Time evolution of the gap. From left to right are snapshots at labeled times. The 4 panels from top to bottom are: 1) Phase space plots for electrons (blue), positrons (orange), and photons (black). The green line is electric field and its scale is on the right. 2) Pair multiplicity M defined in equation (34). Orange dashed line marks M = 1. 3) Current j in blue and its background value jff in orange. 4) Spectrum of… view at source ↗
Figure 2
Figure 2. Left panel: Dissipation rate as a function of time, for 3 different simulations with the same τ0 = 10, normalized to the jet power. The three runs, despite having orders of magnitude difference in B˜0 and ˜min, exhibit roughly the same gap screening and recurrence time scales, as well as similar amount of overall dissipation. The second run (orange line) has a different initial condition with higher multiplicity, h… view at source ↗
Figure 3
Figure 3. compares the spectra of the high state (when photon flux is highest) and low state (when photon flux is lowest). Both spectra form a power law, with the high state having a harder spectral index. The pho￾ton spectrum tends to extend a bit above 0.1/˜min, at which point it is strongly absorbed. The lower end of the spectrum is somewhat artificial, since we need to remove some low energy photons in the simulation in … view at source ↗
Figures from the paper (3 more)
Figure 4
Figure 4. Figure 4: Scaling of the gap power with respect to B˜0˜min (left) and τ0 (right). The green dashed lines are added to illustrate potential scaling relationship. The luminosities L are measured at the peak averaged over several cycles. 3.2. Dependence on Parameters In the high τ…
Figure 5
Figure 5. Figure 5: Quasi-steady state vs. time-dependent gap for different numerical parameters. Left panel: Light curves of two simulations with B˜0 = 107 , ˜min = 10−3 , and τ0 = 10, but different initial particles per cell. Right panel: Light curves of two simulations with B˜0 = 108 …
Figure 6
Figure 6. Figure 6: Force-free solution of a monopolar magnetosphere around a Kerr black hole with spin a = 0.99. The thin black lines show the flux surfaces with constant ψ, and the white line indicates the specific flux tube we used as the back￾ground for our kinetic simulations present…

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Works this paper leans on

34 extracted references · 26 canonical work pages

  1. [1]

    , " * write output.state after.block = add.period write newline

    ENTRY address archivePrefix author booktitle chapter doi edition editor eprint howpublished institution journal key month note number organization pages publisher school series title misctitle type volume year version url label extra.label sort.label short.list INTEGERS output.state before.all mid.sentence after.sentence after.block FUNCTION init.state.co...

  2. [2]

    write newline

    " write newline "" before.all 'output.state := FUNCTION format.doi doi empty "" "doi:" doi * if FUNCTION format.url url empty "" new.block "" url * "" * if FUNCTION format.eprint eprint empty "" archivePrefix empty "" archivePrefix ":" * if eprint field.or.null * if FUNCTION format.pid eprint empty format.doi format.eprint if FUNCTION n.dashify 't := "" t...

  3. [3]

    , " * write output.state after.block = add.period write newline

    ENTRY address archivePrefix author booktitle chapter doi edition editor eprint howpublished institution journal key month note number organization pages publisher school series title misctitle type volume year version url adsurl label extra.label sort.label short.list INTEGERS output.state before.all mid.sentence after.sentence after.block FUNCTION init.s...

  4. [4]

    write newline

    " write newline "" before.all 'output.state := FUNCTION format.doi doi empty "" "doi:" doi * if FUNCTION format.url url empty "" new.block "" url * "" * if FUNCTION format.eprint eprint empty "" archivePrefix empty "" archivePrefix ":" * if eprint field.or.null * if FUNCTION format.pid eprint empty format.doi format.eprint if FUNCTION n.dashify 't := "" t...

  5. [5]

    [7 v&V9 t\ 5?pܛ [; :•e \ ǻ E [y WHb ]_əбf^빴f.0rl;#7 Gm=| ? V8 1[f5TzvHOEӛ NѸbSK _ 8W[eФ4 F\^ 3 ڱi hTGxձۃ -*|Ӯ1eY0c` ? DD <H fO Ne '

    thebibliography [1] 20pt to REFERENCES 6pt =0pt 10pt plus 3pt =0pt =0pt =1pt plus 1pt =0pt =0pt -12pt =13pt plus 1pt =20pt =13pt plus 1pt \@M =10000 =-1.0em =0pt =0pt 0pt =0pt =1.0em @enumiv\@empty 10000 10000 `\.\@m \@noitemerr \@latex@warning Empty `thebibliography' environment \@ifnextchar \@reference \@latexerr Missing key on reference command Each re...

  6. [6]

    2012, , 746, 151

    Abramowski , A., Acero , F., Aharonian , F., et al. 2012, , 746, 151

  7. [7]

    A., Aliu , E., Arlen , T., et al

    Acciari , V. A., Aliu , E., Arlen , T., et al. 2009, Science, 325, 444

  8. [8]

    A., Barkov , M

    Aharonian , F. A., Barkov , M. V., & Khangulyan , D. 2017, , 841, 61

Show all 34 references
  1. [9]

    A., et al

    Aleksi \'c , J., Ansoldi , S., Antonelli , L. A., et al. 2014, Science, 346, 1080

  2. [10]

    S., Istomin , Y

    Beskin , V. S., Istomin , Y. N., & Parev , V. I. 1992, , 36, 642

  3. [11]

    D., & Znajek , R

    Blandford , R. D., & Znajek , R. L. 1977, , 179, 433

  4. [12]

    R., & Gould , R

    Blumenthal , G. R., & Gould , R. J. 1970, Reviews of Modern Physics, 42, 237

  5. [13]

    E., & Tchekhovskoy , A

    Broderick , A. E., & Tchekhovskoy , A. 2015, , 809, 97

  6. [14]

    Y., Yuan , Y., & Yang , H

    Chen , A. Y., Yuan , Y., & Yang , H. 2018, , 863, L31

  7. [15]

    2019, , 875, L1

    Event Horizon Telescope Collaboration , Akiyama , K., Alberdi , A., et al. 2019, , 875, L1

  8. [16]

    L., Keenan , B

    Ford , A. L., Keenan , B. D., & Medvedev , M. V. 2018, , 98, 063016

  9. [17]

    J., & Schr\'eder, G

    Gould, R. J., & Schr\'eder, G. P. 1967, Phys. Rev., 155, 1404. https://link.aps.org/doi/10.1103/PhysRev.155.1404

  10. [18]

    1998, , 497, 563

    Hirotani , K., & Okamoto , I. 1998, , 497, 563

  11. [19]

    2016, , 818, 50

    Hirotani , K., & Pu , H.-Y. 2016, , 818, 50

  12. [20]

    C.-C., et al

    Hirotani , K., Pu , H.-Y., Lin , L. C.-C., et al. 2016, , 833, 142

  13. [21]

    2017, , 845, 77

    ---. 2017, , 845, 77

  14. [22]

    Jones , F. C. 1968, Physical Review, 167, 1159

  15. [23]

    Katsoulakos , G., & Rieger , F. M. 2018, , 852, 112

  16. [24]

    Komissarov , S. S. 2004, , 350, 427

  17. [25]

    2018, , 616, A184

    Levinson , A., & Cerutti , B. 2018, , 616, A184

  18. [26]

    2011, , 730, 123

    Levinson , A., & Rieger , F. 2011, , 730, 123

  19. [27]

    2017, , 96, 123006

    Levinson , A., & Segev , N. 2017, , 96, 123006

  20. [28]

    C., Tchekhovskoy , A., & Blandford , R

    McKinney , J. C., Tchekhovskoy , A., & Blandford , R. D. 2012, , 423, 3083

  21. [29]

    2019, , 122, 035101

    Parfrey , K., Philippov , A., & Cerutti , B. 2019, , 122, 035101

  22. [30]

    Y., & Mastichiadis , A

    Petropoulou , M., Yuan , Y., Chen , A. Y., & Mastichiadis , A. 2019, arXiv e-prints, arXiv:1907.03175

  23. [31]

    2016, , 593, A8

    Ptitsyna , K., & Neronov , A. 2016, , 593, A8

  24. [32]

    B., & Lightman, A

    Rybicki, G. B., & Lightman, A. P. 1986, Radiative Processes in Astrophysics (Wiley-VCH). http://adsabs.harvard.edu/abs/1986rpa..book.....R

  25. [33]

    N., & Arons , J

    Timokhin , A. N., & Arons , J. 2013, , 429, 20

  26. [34]

    D., & Wilkins , D

    Yuan , Y., Blandford , R. D., & Wilkins , D. R. 2019, , 484, 4920

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