REVIEW 3 major objections 6 minor 94 references
Homogeneous and inhomogeneous plasma push black-hole shadow size in opposite directions; realistic dark-matter halos barely matter.
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 · grok-4.5
2026-07-14 22:52 UTC pith:KCZN5E3T
load-bearing objection Solid Kerr-plasma calculation on an Einasto-halo metric; the homogeneous enlargement is mostly the known refractive Bardeen factor, and realistic DM is negligible. the 3 major comments →
Plasma effects on gravitational lensing and shadow observables of a Kerr-like black hole in a dark matter halo
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
For a Kerr-like black hole sitting in an Einasto dark-matter halo, increasing plasma density increases both the shadow radius and its deformation when the plasma is homogeneous, but decreases both quantities when the plasma is inhomogeneous. Astrophysically realistic dark-matter densities produce no appreciable shift in the photon trajectories, while the energy-emission rate tracks the same opposite plasma trends.
What carries the argument
The separable plasma-frequency ansatz ω_p² = (f_r(r) + f_θ(θ))/Σ, which keeps the Hamilton–Jacobi equation separable and yields closed-form impact parameters and asymptotic Bardeen celestial coordinates (including a refractive factor 1/√(1−ω_c²/ω₀²) for the homogeneous profile).
Load-bearing premise
The plasma density must be chosen so that the ray equations separate into independent radial and angular pieces; real magnetized, turbulent plasma need not obey that restriction.
What would settle it
A multi-frequency EHT or next-generation shadow measurement that shows the shadow radius either growing or shrinking with frequency in a way that cannot be fit by either of the two plasma models (or that requires dark-matter densities far above the galactic values used here).
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript studies null geodesics and black-hole shadow observables for a Kerr-like metric embedded in an Einasto dark-matter halo, in the presence of non-magnetized pressureless plasma. Two separable plasma profiles (homogeneous and inhomogeneous) are adopted so that the Hamilton–Jacobi equation separates. The authors compute critical photon orbits, deflection angles, Bardeen celestial coordinates (including the homogeneous-plasma refractive factor arising from Δ(r) f_r(r) ∼ r⁴), shadow radius R_s, deformation δ_s, ellipticity K_s, and a heuristic energy-emission rate, and compare geometric critical-curve sizes with EHT-inferred intervals for M87* and Sgr A*. The main reported trends are that realistic Einasto densities leave photon trajectories essentially unchanged, while increasing plasma strength enlarges (shrinks) the shadow for homogeneous (inhomogeneous) plasma, with corresponding effects on emission rate and on allowed plasma-density windows.
Significance. The work is a careful, incremental parameter study in a well-established phenomenological niche (plasma + environmental halo + Kerr-like shadows). Its strengths are (i) explicit recovery of the homogeneous-plasma refractive normalization of the asymptotic Bardeen coordinates, (ii) systematic numerical scans of R_s, δ_s, K_s and emission rate across spin, inclination and plasma strength, and (iii) concrete, falsifiable upper bounds on ω_c²/ω₀² from EHT size intervals for the homogeneous profile. The negative result that astrophysically reasonable Einasto densities do not move the critical curve is useful. The paper does not introduce a new metric or a non-separable plasma model, so its significance is primarily as a controlled comparison of two standard plasma profiles on a previously studied halo spacetime, with direct EHT benchmarks.
major comments (3)
- §IV A, Eqs. (20a)–(20b): the closed-form expressions for the critical impact parameters η and χ contain no explicit dependence on f̃_r or f̃_r′, yet the defining conditions R(r_p)=0 and R′(r_p)=0 (Eqs. 18–19) do. Moreover Eq. (20b) is algebraically garbled (identical summands “8 a r_p² Δ + 8 a r_p² Δ”). Because every subsequent shadow, deformation and emission-rate plot is built from these critical parameters, the manuscript must either (i) supply the correct plasma-dependent analytic solution of the R=R′=0 system or (ii) state clearly that η,χ are obtained purely numerically and remove the incorrect closed forms. As written, the central pipeline is not reproducible from the text.
- §V A, Eqs. (29a)–(29b) and Figs. 6, 8f, 9c: the dominant growth of R_s with homogeneous-plasma strength is the overall multiplicative factor 1/√(1−ω_c²/ω₀²). Because this factor multiplies both α and β uniformly, pure refractive rescaling leaves shape measures (δ_s, K_s) invariant. Any reported increase of deformation with ω_c²/ω₀² must therefore come from the plasma-induced change of the photon-region boundary (r_p±, η(r_p), χ(r_p)). The text should quantitatively separate the two contributions (e.g., by plotting the un-normalized critical curve in the (η,χ) plane alongside the Bardeen image) so that the claim “homogeneous plasma increases deformation” is not misread as a pure normalization artifact.
- §VI, Eqs. (34)–(38): the energy-emission rate is constructed from the geometric shadow area σ_lim=π R_s² and the vacuum surface gravity of the Kerr-like horizons. Plasma modifies photon propagation and, in a consistent treatment, the grey-body factors and the notion of asymptotic energy; the manuscript never discusses whether T_H or the high-frequency absorption cross-section remain valid once ω_p≠0. At minimum the section should be labeled as a purely geometric proxy and the ordering of emission curves should be tied explicitly to the ordering of R_s already shown in Fig. 8, rather than presented as an independent physical prediction.
minor comments (6)
- Notation for plasma strength is inconsistent: the text uses ω_c²/ω₀² throughout the figures, yet Eqs. (29a,b) write √(1−ω_c²). Please adopt a single dimensionless ratio everywhere.
- PACS codes 11.25.Hf and 04.60.-m are inappropriate for a classical GR/plasma-optics paper; replace with standard codes such as 04.70.-s, 95.30.Sf, 98.62.Sb.
- Fig. 1 caption and surrounding text: the statement that ∂r_h/∂a → ∞ at extremality is true for the usual Kerr square-root branch point, but the figure’s “derivative going to infinity” is hard to read; a zoomed inset or a plot of (∂r_h/∂a)⁻¹ would help.
- Eq. (4) conversion of ρ_e, r_e into geometric units is given, but the numerical values actually used for Sgr A* versus M87* in §§VII–VIII should be collected in a single table for reproducibility.
- Several references appear with incomplete bibliographic data (e.g., Perlick & Tsupko entries missing journal/year in the list). Please clean the bibliography.
- Abstract and §V: “asymptotically stable photon sphere” should be “unstable spherical photon orbits” (the critical curve is unstable).
Circularity Check
No circularity: geodesic-derived shadows and EHT-constrained plasma bounds are independent of fitted inputs or self-referential definitions.
full rationale
The paper constructs a Kerr-like metric with an Einasto halo (Eqs. 1–3, taken from external literature [73]), imposes the standard separability condition on the plasma frequency (Eq. 10, from Perlick–Tsupko), and solves the resulting Hamilton–Jacobi equation for null geodesics. Shadow observables (Rs, δs, Ks) and the energy-emission rate follow directly from the critical-curve asymptotics (Eqs. 29–33, 36). The homogeneous-plasma refractive factor 1/√(1−ωc^{2}/ω0^{2}) is an explicit leading-order consequence of Δ(r)fr(r)∼r^{4}(1−ωc^{2}/ω0^{2}) when fr∝r^{2}; the paper states this openly rather than presenting it as an independent geometric prediction. Comparison with EHT areal-radius intervals for M87* and Sgr A* (external observational inputs) yields upper bounds on ωc^{2}/ω0^{2}; those bounds are not fitted parameters re-labeled as predictions. No self-citation is load-bearing, no uniqueness theorem is imported from the authors, and no quantity is defined in terms of the result it is claimed to predict. The derivation chain is therefore self-contained against external benchmarks.
Axiom & Free-Parameter Ledger
free parameters (3)
- ω_c²/ω_0² (plasma strength)
- Einasto halo parameters (ρ_e, r_e, α)
- black-hole spin a and observer inclination θ_0
axioms (4)
- domain assumption Plasma frequency must be of the separable form ω_p² = (f_r(r)+f_θ(θ))/Σ for the Hamilton–Jacobi equation to separate.
- domain assumption Plasma is non-magnetized and pressureless; no back-reaction on the metric.
- domain assumption The Kerr-like metric with Einasto halo (Eqs. 2–3) correctly describes the spacetime.
- standard math Geometric-optics limit and refractive-index formulation remain valid near the photon sphere.
read the original abstract
Plasma surrounding a black hole modifies light propagation and can alter the observed shadow, potentially affecting the interpretation of Event Horizon Telescope data. We study the effects of dark matter and nonmagnetized pressureless plasma on the shadow of a Kerr-like black hole by analyzing null geodesics in both homogeneous and inhomogeneous plasma distributions. For the homogeneous plasma profile, the asymptotic Bardeen coordinates acquire a refractive normalization factor arising from the leading-order coupling between the metric function $\Delta(r)$ and the radial plasma function $f_r(r)\propto r^2$. We show that increasing the black hole spin generally enlarges the shadow radius and increases its deformation, while moving the observer away from the equatorial plane decreases both quantities. For the parameter ranges considered, astrophysically reasonable dark matter densities in this model do not produce appreciable changes in the photon trajectories. Plasma effects, however, are significant: increasing the plasma density increases the shadow radius and deformation for homogeneous plasma, but decreases them for inhomogeneous plasma. The energy emission rate likewise depends strongly on the plasma model, with homogeneous plasma producing a substantially larger rate as the plasma strength increases. As an illustrative benchmark, we compare the resulting geometric critical-curve sizes with EHT-inferred shadow-size intervals for M87* and Sgr A*.
Figures
Reference graph
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discussion (0)
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