1,880 research outputs found
The initial value problem for linearized gravitational perturbations of the Schwarzschild naked singularity
The coupled equations for the scalar modes of the linearized Einstein
equations around Schwarzschild's spacetime were reduced by Zerilli to a 1+1
wave equation with a potential , on a field . For smooth metric
perturbations is singular at , the
mode harmonic number, and has a second order pole at . This is
irrelevant to the black hole exterior stability problem, where , and
, but it introduces a non trivial problem in the naked singular case
where , and the singularity appears in the relevant range of
. We solve this problem by developing a new approach to the evolution of the
even mode, based on a {\em new gauge invariant function}, -related
to by an intertwiner operator- that is a regular function of the
metric perturbation {\em for any value of }. This allows to address the
issue of evolution of gravitational perturbations in this non globally
hyperbolic background, and to complete the proof of the linear instability of
the Schwarzschild naked singularity, by showing that a previously found
unstable mode is excitable by generic initial data. This is further illustrated
by numerically solving the linearized equations for suitably chosen initial
data.Comment: typos corrected, references adde
A path to radio-loudness through gas-poor galaxy mergers and the role of retrograde accretion
In this proceeding we explore a pathway to radio-loudness under the
hypothesis that retrograde accretion onto giant spinning black holes leads to
the launch of powerful jets, as seen in radio loud QSOs and recently in
LAT/Fermi and BAT/Swift Blazars. Counter-rotation of the accretion disc
relative to the BH spin is here associated to gas-poor galaxy mergers
progenitors of giant (missing-light) ellipticals. The occurrence of retrograde
accretion enters as unifying element that may account for the
radio-loudness/galaxy morphology dichotomy observed in AGN.Comment: To appear in the proceedings of the conference "Accretion and
Ejection in AGN: A global view, June 22-26 2009 - Como, Italy
On the orientation and magnitude of the black hole spin in galactic nuclei
Massive black holes in galactic nuclei vary their mass M and spin vector J
due to accretion. In this study we relax, for the first time, the assumption
that accretion can be either chaotic, i.e. when the accretion episodes are
randomly and isotropically oriented, or coherent, i.e. when they occur all in a
preferred plane. Instead, we consider different degrees of anisotropy in the
fueling, never confining to accretion events on a fixed direction. We follow
the black hole growth evolving contemporarily mass, spin modulus a and spin
direction. We discover the occurrence of two regimes. An early phase (M <~ 10
million solar masses) in which rapid alignment of the black hole spin direction
to the disk angular momentum in each single episode leads to erratic changes in
the black hole spin orientation and at the same time to large spins (a ~ 0.8).
A second phase starts when the black hole mass increases above >~ 10 million
solar masses and the accretion disks carry less mass and angular momentum
relatively to the hole. In the absence of a preferential direction the black
holes tend to spin-down in this phase. However, when a modest degree of
anisotropy in the fueling process (still far from being coherent) is present,
the black hole spin can increase up to a ~ 1 for very massive black holes (M >~
100 million solar masses), and its direction is stable over the many accretion
cycles. We discuss the implications that our results have in the realm of the
observations of black hole spin and jet orientations.Comment: 14 pages, 7 figures, accepted for publication in Ap
Probing the formation history of the nuclear star cluster at the Galactic Centre with millisecond pulsars
The origin of the Nuclear Star Cluster in the centre of our Galaxy is still
unknown. One possibility is that it formed after the disruption of stellar
clusters that spiralled into the Galactic Centre due to dynamical friction. We
trace the formation of the Nuclear Star Cluster around the central black hole,
using state-of-the-art N-body simulations, and follow the dynamics of the
neutron stars born in the clusters. We then estimate the number of Millisecond
Pulsars (MSPs) that are released in the Nuclear Star Cluster, during its
formation. The assembly and tidal dismemberment of globular clusters lead to a
population of MSPs distributed over a radius of about 20 pc, with a peak near 3
pc. No clustering is found on the sub-parsec scale. We simulate the
detectability of this population with future radio telescopes like the MeerKAT
radio telescope and SKA1, and find that about of order ten MSPs can be observed
over this large volume, with a paucity of MSPs within the central parsec. This
helps discriminating this scenario from the in-situ formation model for the
Nuclear Star Cluster that would predict an over abundance of MSPs closer to the
black hole. We then discuss the potential contribution of our MSP population to
the gamma-ray excess at the Galactic Centre.Comment: 11 pages, 8 figures, accepted for publication in MNRA
Growing massive black holes through super-critical accretion of stellar-mass seeds
The rapid assembly of the massive black holes that power the luminous quasars
observed at remains a puzzle. Various direct collapse models have
been proposed to head-start black hole growth from initial seeds with masses
, which can then reach a billion solar mass while
accreting at the Eddington limit. Here we propose an alternative scenario based
on radiatively inefficient super-critical accretion of stellar-mass holes
embedded in the gaseous circum-nuclear discs (CNDs) expected to exist in the
cores of high redshift galaxies. Our sub-pc resolution hydrodynamical
simulations show that stellar-mass holes orbiting within the central 100 pc of
the CND bind to very high density gas clumps that arise from the fragmentation
of the surrounding gas. Owing to the large reservoir of dense cold gas
available, a stellar-mass black hole allowed to grow at super-Eddington rates
according to the "slim disc" solution can increase its mass by 3 orders of
magnitudes within a few million years. These findings are supported by
simulations run with two different hydro codes, RAMSES based on the Adaptive
Mesh Refinement technique and GIZMO based on a new Lagrangian Godunov-type
method, and with similar, but not identical, sub-grid recipes for star
formation, supernova feedback, black hole accretion and feedback. The low
radiative efficiency of super-critical accretion flows are instrumental to the
rapid mass growth of our black holes, as they imply modest radiative heating of
the surrounding nuclear environment.Comment: 12 pages, 8 figures, 2 tables. Accepted for publication in MNRA
Gravitational instability of Einstein-Gauss-Bonnet black holes under tensor mode perturbations
We analyze the tensor mode perturbations of static, spherically symmetric
solutions of the Einstein equations with a quadratic Gauss-Bonnet term in
dimension . We show that the evolution equations for this type of
perturbations can be cast in a Regge-Wheeler-Zerilli form, and obtain the exact
potential for the corresponding Schr\"odinger-like stability equation. As an
immediate application we prove that for and , the sign
choice for the Gauss-Bonnet coefficient suggested by string theory, all
positive mass black holes of this type are stable. In the exceptional case , we find a range of parameters where positive mass asymptotically flat
black holes, with regular horizon, are unstable. This feature is found also in
general for .Comment: 7 pages, 1 figure, minor corrections, references adde
Hyperk\"ahler torsion structures invariant by nilpotent Lie groups
We study HKT structures on nilpotent Lie groups and on associated
nilmanifolds. We exhibit three weak HKT structures on which are
homogeneous with respect to extensions of Heisenberg type Lie groups. The
corresponding hypercomplex structures are of a special kind, called abelian. We
prove that on any 2-step nilpotent Lie group all invariant HKT structures arise
from abelian hypercomplex structures. Furthermore, we use a correspondence
between abelian hypercomplex structures and subspaces of to
produce continuous families of compact and noncompact of manifolds carrying non
isometric HKT structures. Finally, geometrical properties of invariant HKT
structures on 2-step nilpotent Lie groups are obtained.Comment: LateX, 12 page
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