Near-horizon BF bounds for the $t^{+}$ KK tower on extremal AdS${}_5$ black holes

Abstract

We study the charged Breitenlohner--Freedman (BF) bound for all horizon harmonics of the type IIB $t^{+}$ Kaluza--Klein tower on extremal charged AdS${}_5$ black holes with equal rotations. The scalar equation follows from maximal five-dimensional gauged supergravity at level zero and the linearized exceptional-field-theory equations at higher levels. For aligned charge and rotation, a quadratic form in charge and spin changes signature on the curve of supersymmetric (BPS) black holes, where every mode satisfies the bound. Above the curve, where ${\Omega }_H>1$, every black hole has a violating mode, although no finite truncation in level or spin detects violation at every such black hole. Below the curve, where ${\Phi }_H>1$, the BF-passing set is confined to a thin strip and controlled by charge and spin discreteness. At large radius and fixed rotation, a level-four mode with spin proportional to the radius violates the bound even where all U(2) singlets satisfy it. These results concern the near-horizon BF bound of one tower in two-derivative supergravity; full radial stability remains open.

arXiv comment44 pages, 5 figures