Stationary scalar clouds, quasinormal ringing, and observational signatures of (near-)extremal rotating Kalb–Ramond black holes: from the superradiant threshold to EHT bounds
- 1Cyprus International University
- 2Eastern Mediterranean University
Abstract
We study a massive scalar field on the rotating Kalb–Ramond black hole of Kumar et al. [7], near extremality in (a,γ ,λ ) ( a , γ , λ ) . The Klein–Gordon–Fock equation separates, with Λ rad=Λ ang+a2ω 2−2amω Λ rad = Λ ang + a 2 ω 2 - 2 a m ω . At extremality Δ Δ and K ( r ) share a zero, keeping K2/Δ K 2 / Δ regular and reducing the radial problem to Whittaker form on the Kerr line γ =0 γ = 0 and the Kerr–Newman line λ =1 λ = 1 . Stationary resonances saturate ω =m Ω H ω = m Ω H with μ cloud>m Ω H μ cloud > m Ω H , overtones accumulating from above. In the genuine Kalb–Ramond interior 0<λ <1 0 < λ < 1 no such reduction exists; we solve the exact radial equation numerically, validated against the closed forms to eleven digits. Off extremality, a=aext(1−δ 2) a = a ext ( 1 - δ 2 ) , the clouds join the co-rotating zero-damped branch ω n=m Ω H−i κ (n+1/2+β ) ω n = m Ω H - i κ ( n + 1 2 + β ) , governed by the near-horizon exponent β β and distinct from the photon-sphere family. Greybody factors and superradiance follow from direct integration, recovering the classical ℓ =m=1 ℓ = m = 1 Kerr result. Strong-field lensing follows in the Bozza–Tsukamoto limit, the shadow from Carter separation. Intersecting the Event Horizon Telescope window of Zubair et al. [8] with our cloud-existence region bounds (γ ,λ ) ( γ , λ ) more tightly than either alone.
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BibTeX
@article{aydemir2026stationary,
title = {{Stationary scalar clouds, quasinormal ringing, and observational signatures of (near-)extremal rotating Kalb–Ramond black holes: from the superradiant threshold to EHT bounds}},
author = {Gülnihal Aydemir and İzzet Sakallı},
journal = {The European Physical Journal C},
year = {2026},
month = oct,
doi = {10.1140/epjc/s10052-026-16352-1},
url = {https://link.springer.com/article/10.1140/epjc/s10052-026-16352-1},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Stationary scalar clouds, quasinormal ringing, and observational signatures of (near-)extremal rotating Kalb–Ramond black holes: from the superradiant threshold to EHT bounds
AU - Gülnihal Aydemir
AU - İzzet Sakallı
PY - 2026
DA - 2026/10/23
JO - The European Physical Journal C
DO - 10.1140/epjc/s10052-026-16352-1
UR - https://link.springer.com/article/10.1140/epjc/s10052-026-16352-1
AB - We study a massive scalar field on the rotating Kalb–Ramond black hole of Kumar et al. [7], near extremality in (a,γ ,λ ) ( a , γ , λ ) . The Klein–Gordon–Fock equation separates, with Λ _rad=Λ _ang+a^2ω ^2−2amω Λ rad = Λ ang + a 2 ω 2 - 2 a m ω . At extremality Δ Δ and K ( r ) share a zero, keeping K^2/Δ K 2 / Δ regular and reducing the radial problem to Whittaker form on the Kerr line γ =0 γ = 0 and the Kerr–Newman line λ =1 λ = 1 . Stationary resonances saturate ω =m Ω _H ω = m Ω H with μ _cloud>m Ω _H μ cloud > m Ω H , overtones accumulating from above. In the genuine Kalb–Ramond interior 0<λ <1 0 < λ < 1 no such reduction exists; we solve the exact radial equation numerically, validated against the closed forms to eleven digits. Off extremality, a=a_ext(1−δ ^2) a = a ext ( 1 - δ 2 ) , the clouds join the co-rotating zero-damped branch ω _n=m Ω _H−i κ (n+1/2+β ) ω n = m Ω H - i κ ( n + 1 2 + β ) , governed by the near-horizon exponent β β and distinct from the photon-sphere family. Greybody factors and superradiance follow from direct integration, recovering the classical ℓ =m=1 ℓ = m = 1 Kerr result. Strong-field lensing follows in the Bozza–Tsukamoto limit, the shadow from Carter separation. Intersecting the Event Horizon Telescope window of Zubair et al. [8] with our cloud-existence region bounds (γ ,λ ) ( γ , λ ) more tightly than either alone.
SN - 1434-6044
SN - 1434-6052
ER -