Near-extremal black holes at late times, backreacted

Black holes display universal behavior near extremality. One such feature is the late-time blowup of derivatives of linearized perturbations across the horizon. For generic initial data, this instability is regulated by backreaction, and the final state is a near-extremal black hole. The aim of this paper is to study the late time behavior of such black holes analytically using the weakly broken conformal symmetry of their near-horizon region. In particular, gravitational backreaction is accounted for within the Jackiw-Teitelboim model for near-horizon, near-extremal dynamics coupled to bulk matter.Comment: v2: published versio

Fast plunges into Kerr black holes

Most extreme-mass-ratio-inspirals of small compact objects into supermassive black holes end with a fast plunge from an eccentric last stable orbit. For rapidly rotating black holes such fast plunges may be studied in the context of the Kerr/CFT correspondence because they occur in the near-horizon region where dynamics are governed by the infinite dimensional conformal symmetry. In this paper we use conformal transformations to analytically solve for the radiation emitted from fast plunges into near-extreme Kerr black holes. We find perfect agreement between the gravity and CFT computations.Comment: 16 pages, 2 figure

Exact Gravitational Wave Signatures from Colliding Extreme Black Holes

The low-energy dynamics of any system admitting a continuum of static configurations is approximated by slow motion in moduli (configuration) space. Here, following Ferrell and Eardley, this moduli space approximation is utilized to study collisions of two maximally charged Reissner--Nordstr{\"o}m black holes of arbitrary masses, and to compute analytically the gravitational radiation generated by their scattering or coalescence. The motion remains slow even though the fields are strong, and the leading radiation is quadrupolar. A simple expression for the gravitational waveform is derived and compared at early and late times to expectations.Comment: 6 page

Signatures of particle collisions near extreme black holes

Finite-energy particles in free fall can collide with diverging center-of-mass energy near rapidly rotating black holes. What are the most salient observational signatures of this remarkable geometric effect? Here we revisit the problem from the standpoint of the near-horizon extreme Kerr geometry, where these collisions naturally take place. It is shown that the ingoing particle kinematics admits a simple, universal form. Given a scattering cross section, determination of emission properties is reduced to evaluation of particular integrals on the sky of a near-horizon orbiting particle. We subsequently apply this scheme to the example of single-photon bremsstrahlung, substantiating past results which indicate that ejected particles are observable, but their energies are bounded by the rest masses of the colliding particles. Our framework is readily applicable for any scattering process.Comment: 11 pages, 4 figure

Extreme lensing induces spectro-temporal correlations in black-hole signals

Rapid progress in electromagnetic black hole observation presents a theoretical challenge: how can the universal signatures of extreme gravitational lensing be distilled from stochastic astrophysical signals? With this motivation, the two-point correlation function of specific intensity fluctuations across image positions, times, and frequencies is here considered. The contribution of strongly deflected light rays, those which make up the photon ring, is analytically computed for a Kerr black hole illuminated by a simple geometric-statistical emission model. We subsequently integrate over the image to yield a spectro-temporal correlation function which is relevant for unresolved sources. Finally, some observational aspects are discussed and a preliminary assessment of detectability with current and upcoming missions is provided