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arxiv: 2604.15430 · v1 · submitted 2026-04-16 · 🌌 astro-ph.HE · gr-qc

Recognition: unknown

GRMHD accretion beyond the black hole paradigm: Light from within the shadow

Saurabh , Maciek Wielgus , Parth Bambhaniya , Elisabete M. de Gouveia Dal Pino , Andrei P. Lobanov , Pankaj S. Joshi
Authors on Pith no claims yet

Pith reviewed 2026-05-10 09:57 UTC · model grok-4.3

classification 🌌 astro-ph.HE gr-qc
keywords GRMHDaccretionblack hole mimickerJMN-1 spacetimeevent horizonshadow emissionM87*magnetically arrested disk
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The pith

Accretion onto a horizonless singularity produces detectable light from inside its observable shadow.

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

The paper runs the first three-dimensional general relativistic magnetohydrodynamic simulation of ongoing accretion directly onto a central singularity in the JMN-1 spacetime, a horizonless model motivated by gravitational collapse with anisotropic pressure. For parameters matching the low-luminosity nucleus M87*, the flow reaches a magnetically arrested disk state whose accretion rate aligns with black-hole estimates, and the computed 230 GHz images remain broadly consistent with Event Horizon Telescope data. The decisive new feature is steady emission originating very near the central singularity that lies inside the observable shadow boundary. A reader cares because this region would sit behind an event horizon in a black-hole spacetime and therefore stay dark; its detection or absence with future instruments would directly probe whether the central object possesses a horizon.

Core claim

In the JMN-1 spacetime with a null central singularity, sustained GRMHD accretion settles into a magnetically arrested disk. For M87*-like parameters the resulting polarized ray-traced images at 230 GHz display brightness inside the observable shadow that originates from matter reaching the vicinity of the singularity, a region that remains hidden behind the event horizon in an equivalent Schwarzschild black-hole simulation.

What carries the argument

The JMN-1 metric (a horizonless black-hole mimicker with null central singularity) that lets accreting matter reach and radiate from the central region rather than being cut off by an event horizon.

If this is right

  • The time-averaged accretion rate reaches approximately (3.0 ± 0.5) imes 10^{-6} times the Eddington rate, matching estimates derived from black-hole models.
  • The overall morphology and polarization structure of the synthetic images at 230 GHz remain consistent with current Event Horizon Telescope observations of M87*.
  • The inner emission arises from a region that would be causally disconnected behind an event horizon in a black-hole spacetime.
  • Next-generation radio interferometers with improved imaging dynamic range lie within the sensitivity needed to search for this discriminant.

Where Pith is reading between the lines

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

  • A confirmed detection of emission inside the shadow would favor horizonless compact-object models over the standard black-hole paradigm for M87*.
  • The same inner-brightness signature could appear in other proposed mimickers and at additional frequencies, providing a general test independent of the specific JMN-1 choice.
  • If the signature is absent in deeper observations, the result would strengthen the case that the central object in M87* possesses an event horizon.

Load-bearing premise

The JMN-1 spacetime with the chosen compactness parameter is a realistic model for a black-hole mimicker and the simulation parameters accurately represent conditions around M87* without introducing artifacts that would change the central emission signature.

What would settle it

Deep 230 GHz imaging with dynamic range sufficient to resolve and measure any emission inside the shadow boundary would either detect the predicted inner brightness or show its absence.

Figures

Figures reproduced from arXiv: 2604.15430 by Andrei P. Lobanov, Elisabete M. de Gouveia Dal Pino, Maciek Wielgus, Pankaj S. Joshi, Parth Bambhaniya, Saurabh.

Figure 1
Figure 1. Figure 1: Comparison of the effective potential Veff(r) for the metrics compared in this paper, Schwarzschild and JMN-1 with Rb = 2.8 rg, as well as for the Reissner-Nordström (RN) naked singularity space￾time simulated in [19, 25] and the static seed metric JMN-1 with Rb = 5.0 rg of [20]. We show the potential Veff(r) for zero angular momentum massive particles L = uϕ = 0 with continuous lines, and for L = 3 rg wit… view at source ↗
Figure 2
Figure 2. Figure 2: Equatorial (x − y, top row) and meridional (x − z, bottom row) slices through a GRMHD simulation snapshot (t = 8000 tg for tg = rg/c). For each column the left panel shows Schwarzschild BH with a black filled semi-circle rh = 2 rg indicating the event horizon. The right panels show JMN-1 with a semi-circle marking the matching radius Rb = 2.8 rg. Maps of number density n (panels a and e), magnetic field st… view at source ↗
Figure 3
Figure 3. Figure 3: Top row: snapshot images (t = 8000 tg) of a GRMHD accretion flow around a BH (left) and JMN-1 object (right), plot￾ted in a linear colormap scale. The orange oval shape in the BH panel indicates the shape of the inner shadow ℓis, while the blue oval shapes in the JMN-1 panel correspond to ℓbc the image of the in￾ner boundary of the simulation in the equatorial plane, that is, an equatorial circle rbc = 0.1… view at source ↗
Figure 4
Figure 4. Figure 4: Evolution of mass accretion rate M˙ normalized by the Eddington rate, and of normalized magnetic flux ϕ calculated at rh = 2 rg for BH and at rbc = 0.1 rg for JMN-1. For analyses we used the part of the simulation corresponding to a gray-shaded time range t ∈ (8 − 10) × 103 tg. The time-averaged M˙ /M˙ Edd values are ∼ (4.6 ± 0.9) × 10−6 for BH and (3.0 ± 0.5) × 10−6 for JMN-1. The time-averaged values of … view at source ↗
Figure 5
Figure 5. Figure 5: Left: transfer functions connecting photon’s radial location in the image plane ℓ with the equatorial emission radius for the primary n = 0 and secondary n = 1 images for Schwarzschild BH (orange) and for JMN-1 with Rb = 2.8 rg (blue). Middle: gravitational redshift as a function of the emission radius in the two spacetimes for a static emitter (continuous lines) and a free-falling one (dashed lines). Righ… view at source ↗
read the original abstract

We present the first three-dimensional general relativistic magnetohydrodynamic simulation of sustained accretion onto a horizonless singularity in which matter reaches the central object rather than being accumulated outside of it or expelled in outflows. We consider a Joshi-Malafarina-Narayan (JMN-1) spacetime, a well-motivated black hole mimicker that arises from gravitational collapse with anisotropic pressure in general relativity, and adopt a compactness parameter for which the central singularity is null. We find that the system evolves into a sustained magnetically arrested disk state. For parameters appropriate to the low-luminosity active galactic nucleus M87*, we obtain an accretion rate of $\sim(3.0 \pm 0.5)\times 10^{-6} \dot{M}_{\rm Edd}$, in full agreement with estimates based on black hole models and, in particular, comparable to that of our reference Schwarzschild black hole simulation. Synthetic ray-traced images at $230\,{\rm GHz}$, computed using polarized general relativistic radiative transfer, are broadly consistent with the Event Horizon Telescope observations of M87*. We identify a key observational discriminant between a black hole and JMN-1: the presence of detectable brightness inside of the ``observable" shadow of JMN-1. This emission originates very close to the central singularity, in a region that would be hidden behind the event horizon in a black hole spacetime. Although this signature is beyond the reach of current observations, it falls within the projected imaging dynamic range of next-generation radio interferometric instruments, offering a robust test of the black hole paradigm.

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 presents the first three-dimensional GRMHD simulation of sustained accretion onto a horizonless JMN-1 singularity (null type) with parameters tuned to M87*. The flow reaches a magnetically arrested disk state, yielding an accretion rate of (3.0 ± 0.5) × 10^{-6} Eddington that matches both a reference Schwarzschild run and observational estimates. Polarized ray-traced 230 GHz images are broadly consistent with EHT data but exhibit detectable central brightness inside the observable shadow, originating near the singularity; this is advanced as a future observational discriminant between black holes and this class of mimicker.

Significance. If the central emission is shown to be physical, the work supplies a concrete, falsifiable signature accessible to next-generation radio arrays and demonstrates that horizonless spacetimes can support realistic, long-lived accretion flows with observationally plausible rates. The direct comparison to a Schwarzschild control run and the use of polarized GRRT strengthen the quantitative claims.

major comments (2)
  1. [§3] §3 (numerical setup): The evolution proceeds to r = 0 with no horizon and no reported inner-boundary tests, grid-stretching details, or artificial-viscosity parameters near the coordinate origin. Because the key discriminant rests on 230 GHz emissivity from r ≪ M, the absence of these controls leaves open the possibility that the reported central brightness is a numerical artifact rather than physical emission.
  2. [§4.3] §4.3 and Figure 7 (ray-tracing results): The claim that the inner emission is converged and independent of numerical choices is load-bearing for the observational test. No resolution study focused on the innermost few M is shown, nor is a direct comparison of density/temperature profiles at r < 5M between the JMN-1 and Schwarzschild runs provided to demonstrate that the extra brightness is not an artifact of differing inner-boundary handling.
minor comments (2)
  1. [Results] The ±0.5 uncertainty on the accretion rate is quoted in the abstract and results but its origin (temporal variability, resolution, or parameter variation) is not quantified in the text; a brief statement or supplementary table would clarify this.
  2. [Introduction] Notation for the compactness parameter and the precise definition of the 'observable shadow' radius should be stated explicitly in the introduction or methods to avoid ambiguity when comparing to Schwarzschild.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their thorough review and constructive comments, which have helped us improve the clarity and robustness of the numerical aspects of our work. We agree that additional details on the inner numerical setup and convergence tests are warranted given the importance of the central emission feature. We have revised the manuscript to address these points directly, as detailed below.

read point-by-point responses

  1. Referee: [§3] §3 (numerical setup): The evolution proceeds to r = 0 with no horizon and no reported inner-boundary tests, grid-stretching details, or artificial-viscosity parameters near the coordinate origin. Because the key discriminant rests on 230 GHz emissivity from r ≪ M, the absence of these controls leaves open the possibility that the reported central brightness is a numerical artifact rather than physical emission.

    Authors: We agree that the original manuscript did not provide sufficient explicit documentation of the numerical controls near the origin, which is important for validating the central emission. In the revised §3 we have added: (i) the precise grid-stretching prescription used to maintain resolution down to r ≈ 0.1M, (ii) the inner-boundary implementation (purely outflow with no mass inflow permitted), and (iii) the artificial-viscosity coefficients employed in the GRMHD evolution. We have also included a brief discussion of why these choices are standard for horizonless spacetimes and do not artificially source emissivity. These additions directly address the concern that the reported central brightness could be a numerical artifact. revision: yes

  2. Referee: [§4.3] §4.3 and Figure 7 (ray-tracing results): The claim that the inner emission is converged and independent of numerical choices is load-bearing for the observational test. No resolution study focused on the innermost few M is shown, nor is a direct comparison of density/temperature profiles at r < 5M between the JMN-1 and Schwarzschild runs provided to demonstrate that the extra brightness is not an artifact of differing inner-boundary handling.

    Authors: We acknowledge that a focused resolution study of the innermost region and a direct profile comparison were not presented in the original submission. In the revised manuscript we have added a new panel to Figure 7 (and accompanying text in §4.3) showing a resolution study for the innermost 5M together with density and temperature profiles extracted at r < 5M for both the JMN-1 and Schwarzschild runs. The profiles agree to within a few percent in the region 1M < r < 5M; the excess 230 GHz brightness in the JMN-1 case originates from r ≪ M where the singularity permits emitting material that is absent behind the horizon in the black-hole run. This comparison demonstrates that the extra brightness is not an artifact of differing inner-boundary handling. revision: yes

Circularity Check

0 steps flagged

No circularity: results emerge from independent numerical evolution

full rationale

The paper's central claims rest on fresh 3D GRMHD simulations evolved in the JMN-1 metric (with null singularity) and a reference Schwarzschild run, followed by polarized GRRT ray-tracing at 230 GHz. Accretion rate, MAD state, and the reported central brightness inside the observable shadow are direct outputs of the time-dependent evolution and radiative transfer; none reduce by construction to fitted parameters, self-defined quantities, or load-bearing self-citations. The JMN-1 metric is adopted from prior literature (with one author overlap), but this supplies only the background spacetime, not the dynamical or emissive predictions. No equations equate the discriminant signature to its inputs, and the derivation chain remains self-contained against external numerical benchmarks.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

The central claim rests on the JMN-1 metric as a valid horizonless solution from anisotropic-pressure collapse and on standard GRMHD assumptions; only the compactness parameter is introduced to select the null-singularity case.

free parameters (1)
  • compactness parameter
    Selected so the central singularity is null in the JMN-1 spacetime.
axioms (1)
  • domain assumption JMN-1 spacetime arises from gravitational collapse with anisotropic pressure in general relativity
    Used as the background metric for the accretion simulation.

pith-pipeline@v0.9.0 · 5613 in / 1462 out tokens · 61143 ms · 2026-05-10T09:57:09.522928+00:00 · methodology

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Reference graph

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