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KN Killing generators, surface gravity, area and potential

Statement

Let M>0 and M2≥a2+q2>0 in the selected ingoing KN model. For either rH=r±=M±M2−a2−q2 set RH=rH2+a2, ΣH=rH2+a2cos⁡2θ, and χH=∂v+ΩHΨ,ΩH=aRH, where Ψ=X∂Y−Y∂X is the smooth rotation field on S2. The surface r=rH is a null hypersurface and χH is its future null normal and Killing generator. Under the specified normalization d(g(χH,χH))=−2κHχH♭ in Causal solutions, Killing horizons, escape and metric extensions, κH=Δ′(rH)2RH=rH−rother2RH,Area⁡(SH)=4πRH,−Ain(χH)=qrHRH. At the double root κH=0. These are local geometric calculations on the specified model; they assert no maximality or uniqueness.

Facts & Assumptions

Given: The exact ingoing KN metric and potential with M>0 and M2≥a2+q2>0.

[F1]

Regular ingoing KN metric, field and sphere axes: the product metric and potential are smooth on Rv×(0,∞)r×S2, with Σ=r2+a2cos⁡2θ>0, and the future timelike field is N=∂v−(1+f02)∂r/2. On a polar chart, gvv=−Δ+a2sin⁡2θΣ,gvψ=−a(R−Δ)sin⁡2θΣ,gψψ=(R2−a2Δsin⁡2θ)sin⁡2θΣ, gvr=1, grψ=−asin⁡2θ, and gθθ=Σ; Ain=−qrΣ−1(dv−asin⁡2θ dψ).

[F2]

Killing fields and conserved currents, and on-shell conservation of the Hilbert stress-energy tensor: a smooth field with zero metric Lie derivative is Killing; along an affinely parameterized geodesic, its metric pairing with the tangent is constant.

[F3]

Classification of future and past causal vectors by a timelike field: a nonzero null vector is future-directed when its pairing with the selected future timelike field is negative.

[F4]

Explicit rational and real power regimes: each nonnegative real has a unique nonnegative square root, with 0=0.

[L1]

Causal solutions, Killing horizons, escape and metric extensions, Definition (b)-(c): the null conormal criterion, nonempty dense regular Killing-generator portion, ambient signed normalization d(g(χ,χ))=−2κHχ♭, and intrinsic positive induced area density apply to the specified horizon and section.

[A1]

Ψ is the global smooth generator of rotations of the round sphere; on polar charts it equals ∂ψ. The section SH is the sphere at fixed v,rH. Parameters and radial coordinates have length units; angles are dimensionless, and ΩH,κH have inverse-length units.

Proof

1.1F1F2F4A1algebra

The quadratic factors as Δ=(r−r+)(r−r−), with D=M2−a2−q2≥0. Its nonnegative root s=D satisfies s<M: if s≥M, then s2≥M2, contrary to s2=M2−(a2+q2)<M2. Hence both roots M±s are positive. The metric coefficients are independent of v and invariant under rotations, so [F2] makes ∂v, Ψ, and their constant linear combination χH Killing.

2.1F1F3L1step 1.1A1algebra

In a polar chart, substitute ΩH=a/RH into the metric at r=rH. The result is χH♭=ΣHRH dr. The multiplier is positive. Since dr(χH)=0, this identity gives g(χH,χH)=0 and shows χH is tangent and normal to the level r=rH. It also gives g−1(dr,dr)=(RH/ΣH)2g(χH,χH)=0, so the level is null. Moreover dr(N)=−(1+f02)/2<0, whence g(χH,N)=(ΣH/RH)dr(N)<0; [F3] makes the generator future. The generator has v component one everywhere, so its nonzero-generator part is the whole horizon, nonempty and dense. This verifies the Killing-horizon conditions in [L1]. The formula is smooth at the axes because it is an identity of global tensors agreeing on the dense polar charts.

3.1F1L1step 2.1algebra

Direct contraction on a polar chart gives g(χH,χH)=−ΔΣ(1−asin⁡2θ ΩH)2+sin⁡2θΣ(RΩH−a)2. At rH the second squared factor vanishes, as does its first radial derivative. Also 1−a2sin⁡2θ/RH=ΣH/RH. Therefore d(g(χH,χH))=−Δ′(rH)ΣHRH2 dron r=rH. Comparing with [L1] and step 2.1 yields κH=Δ′(rH)/(2RH). Differentiating the factorization of Δ gives Δ′(rH)=rH−rother, including zero at the double root.

4.1F1L1step 3.1algebra∎

The metric induced on the fixed-v,rH sphere is ΣH dθ2+RH2ΣHsin⁡2θ dψ2. Its area density is RHsin⁡θ, so integration over 0≤θ≤π, 0≤ψ<2π gives 4πRH. Contracting the potential in [F1] gives −Ain(χH)=qrHΣH(1−a2sin⁡2θRH)=qrHRH, which is independent of θ.

Remarks

The current vocabulary supplies the exact Killing-horizon, signed surface-gravity and area definitions. The coordinate calculations above verify those interfaces for the displayed smooth metric, including both roots and the double-root limit.

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