283 research outputs found
Self-similar accretion in thin disks around near-extremal black holes
Near-maximally spinning black holes display conformal symmetry in their
near-horizon region, which is therefore the locus of critical phenomena. In
this paper, we revisit the Novikov-Thorne accretion thin disk model and find a
new self-similar radiation-dominated solution in the extremely high spin
regime. Motivated by the self-consistency of the model, we require that matter
flows at the sound speed at the innermost stable circular orbit (ISCO). We
observe that, when the disk pressure is dominated by radiation at the ISCO,
which occurs for the best-fitting Novikov-Thorne model of GRS 1915+105, the
Shakura-Sunyaev viscosity parameter can be expressed in terms of the spin, mass
accretion rate and radiative efficiency. We quantitatively describe how the
exact thin disk solution approaches the self-similar solution in the vicinity
of the ISCO and for increasing spins.Comment: 13 pages, 6 figures; v2 matches published version in MNRAS; v3: typos
fixed, results unchange
Gravitational multipole moments from Noether charges
We define the mass and current multipole moments for an arbitrary theory of
gravity in terms of canonical Noether charges associated with specific residual
transformations in canonical harmonic gauge, which we call multipole
symmetries. We show that our definition exactly matches Thorne's mass and
current multipole moments in Einstein gravity, which are defined in terms of
metric components. For radiative configurations, the total multipole charges --
including the contributions from the source and the radiation -- are given by
surface charges at spatial infinity, while the source multipole moments are
naturally identified by surface integrals in the near-zone or, alternatively,
from a regularization of the Noether charges at null infinity. The conservation
of total multipole charges is used to derive the variation of source multipole
moments in the near-zone in terms of the flux of multipole charges at null
infinity.Comment: v1: 22 pages + 13 pages of appendices, 1 figure; v2: published
version in JHE
Mass of Kerr-Newman Black Holes in an external magnetic field
The explicit solution for a Kerr-Newman black hole immersed in an external
magnetic field, sometimes called the Melvin-Kerr-Newman black hole, has been
derived by Ernst and Wild in 1976. In this paper, we clarify the first law and
Smarr formula for black holes in a magnetic field. We then define the unique
mass which is integrable and reduces to the Kerr-Newman mass in the absence of
magnetic field. This defines the thermodynamic potentials of the black hole.
Quite strikingly, the mass coincides with the standard Christodoulou-Ruffini
mass of a black hole as a function of the entropy, angular momentum and
electric charge.Comment: 21 pages; v2 matches published versio
Boundary conditions for spacelike and timelike warped AdS_3 spaces in topologically massive gravity
We propose a set of consistent boundary conditions containing the spacelike
warped black holes solutions of Topologically Massive Gravity. We prove that
the corresponding asymptotic charges whose algebra consists in a Virasoro
algebra and a current algebra are finite, integrable and conserved. A similar
analysis is performed for the timelike warped AdS_3 spaces which contain a
family of regular solitons. The energy of the boundary Virasoro excitations is
positive while the current algebra leads to negative (for the spacelike warped
case) and positive (for the timelike warped case) energy boundary excitations.
We discuss the relationship with the Brown-Henneaux boundary conditions.Comment: 16 pages, ESI proceedings, v2: typos corrected, published versio
Symplectic and Killing Symmetries of AdS Gravity: Holographic vs Boundary Gravitons
The set of solutions to the AdS Einstein gravity with Brown-Henneaux
boundary conditions is known to be a family of metrics labeled by two arbitrary
periodic functions, respectively left and right-moving. It turns out that there
exists an appropriate presymplectic form which vanishes on-shell. This promotes
this set of metrics to a phase space in which the Brown-Henneaux asymptotic
symmetries become symplectic symmetries in the bulk of spacetime. Moreover, any
element in the phase space admits two global Killing vectors. We show that the
conserved charges associated with these Killing vectors commute with the
Virasoro symplectic symmetry algebra, extending the Virasoro symmetry algebra
with two generators. We discuss that any element in the phase space
falls into the coadjoint orbits of the Virasoro algebras and that each orbit is
labeled by the Killing charges. Upon setting the right-moving function
to zero and restricting the choice of orbits, one can take a near-horizon
decoupling limit which preserves a chiral half of the symplectic symmetries.
Here we show two distinct but equivalent ways in which the chiral Virasoro
symplectic symmetries in the near-horizon geometry can be obtained as a limit
of the bulk symplectic symmetries.Comment: 39 pages, v2: a reference added, the version to appear in JHE
Extremal Rotating Black Holes in the Near-Horizon Limit: Phase Space and Symmetry Algebra
We construct the NHEG phase space, the classical phase space of Near-Horizon
Extremal Geometries with fixed angular momenta and entropy, and with the
largest symmetry algebra. We focus on vacuum solutions to dimensional
Einstein gravity. Each element in the phase space is a geometry with
isometries which has vanishing and constant charges. We construct an on-shell vanishing symplectic
structure, which leads to an infinite set of symplectic symmetries. In four
spacetime dimensions, the phase space is unique and the symmetry algebra
consists of the familiar Virasoro algebra, while in dimensions the
symmetry algebra, the NHEG algebra, contains infinitely many Virasoro
subalgebras. The nontrivial central term of the algebra is proportional to the
black hole entropy. This phase space and in particular its symmetries might
serve as a basis for a semiclassical description of extremal rotating black
hole microstates.Comment: Published in PLB, 5 page
Wiggling Throat of Extremal Black Holes
We construct the classical phase space of geometries in the near-horizon
region of vacuum extremal black holes as announced in [arXiv:1503.07861].
Motivated by the uniqueness theorems for such solutions and for perturbations
around them, we build a family of metrics depending upon a single periodic
function defined on the torus spanned by the isometry directions. We
show that this set of metrics is equipped with a consistent symplectic
structure and hence defines a phase space. The phase space forms a
representation of an infinite dimensional algebra of so-called symplectic
symmetries. The symmetry algebra is an extension of the Virasoro algebra whose
central extension is the black hole entropy. We motivate the choice of
diffeomorphisms leading to the phase space and explicitly derive the symplectic
structure, the algebra of symplectic symmetries and the corresponding conserved
charges. We also discuss a formulation of these charges with a Liouville type
stress-tensor on the torus defined by the isometries and outline
possible future directions.Comment: 56 pages, 3 figure
Inner Mechanics of 3d Black Holes
We investigate properties of the inner horizons of certain black holes in
higher-derivative three-dimensional gravity theories. We focus on BTZ and
Spacelike Warped Anti-de Sitter black holes, as well as on asymptotically
Warped de-Sitter solutions exhibiting both a cosmological and a black hole
horizon. We verify that a First Law is satisfied at the Inner horizon, in
agreement with the proposal of \cite{Castro:2012av}. We then show that, in
Topologically Massive Gravity, the product of the areas of the inner and outer
horizons fails to be independent on the mass, and trace this to the
diffeomorphism anomaly of the theory.Comment: 5 page
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