152 research outputs found

### Wiggly tails: a gravitational wave signature of massive fields around black holes

Massive fields can exist in long-lived configurations around black holes. We
examine how the gravitational wave signal of a perturbed black hole is affected
by such `dirtiness' within linear theory. As a concrete example, we consider
the gravitational radiation emitted by the infall of a massive scalar field
into a Schwarzschild black hole. Whereas part of the scalar field is
absorbed/scattered by the black hole and triggers gravitational wave emission,
another part lingers in long-lived quasi-bound states. Solving numerically the
Teukolsky master equation for gravitational perturbations coupled to the
massive Klein-Gordon equation, we find a characteristic gravitational wave
signal, composed by a quasi-normal ringing followed by a late time tail. In
contrast to `clean' black holes, however, the late time tail contains small
amplitude wiggles with the frequency of the dominating quasi-bound state.
Additionally, an observer dependent beating pattern may also be seen. These
features were already observed in fully non-linear studies; our analysis shows
they are present at linear level, and, since it reduces to a 1+1 dimensional
numerical problem, allows for cleaner numerical data. Moreover, we discuss the
power law of the tail and that it only becomes universal sufficiently far away
from the `dirty' black hole. The wiggly tails, by constrast, are a generic
feature that may be used as a smoking gun for the presence of massive fields
around black holes, either as a linear cloud or as fully non-linear hair.Comment: 6 pages, 4 figure

### On the interaction between two Kerr black holes

The double-Kerr solution is generated using both a Backlund transformation
and the Belinskii-Zakharov inverse-scattering technique. We build a dictionary
between the parametrisations naturally obtained in the two methods and show
their equivalence. We then focus on the asymptotically flat double-Kerr system
obeying the axis condition which is Z_2^\phi invariant; for this system there
is an exact formula for the force between the two black holes, in terms of
their physical quantities and the coordinate distance. We then show that 1) the
angular velocity of the two black holes decreases from the usual Kerr value at
infinite distance to zero in the touching limit; 2) the extremal limit of the
two black holes is given by |J|=cM^2, where c depends on the distance and
varies from one to infinity as the distance decreases; 3) for sufficiently
large angular momentum the temperature of the black holes attains a maximum at
a certain finite coordinate distance. All of these results are interpreted in
terms of the dragging effects of the system.Comment: 19 pages, 4 figures. v2: changed statement about thermodynamical
equilibrium in section 3; minor changes; added references. v3: added
references to previous relevant work; removed one equation (see note added);
other minor corrections; final version to be published in JHE

### Spherical electro-vacuum black holes with resonant, scalar $Q$-hair

The asymptotically flat, spherical, electro-vacuum black holes (BHs) are
shown to support static, spherical configurations of a gauged,
self-interacting, scalar field, minimally coupled to the geometry. Considering
a $Q$-ball type potential for the scalar field, we dub these configurations
$Q$-clouds, in the test field approximation. The clouds exist under a resonance
condition, at the threshold of (charged) superradiance. This is similar to the
stationary clouds supported by Kerr BHs, which exist for a synchronisation
condition, at the threshold of (rotational) superradiance. In contrast with the
rotating case, however, $Q$-clouds require the scalar field to be massive and
self-interacting; no similar clouds exist for massive but free scalar fields.
First, considering a decoupling limit, we construct $Q$-clouds around
Schwarzschild and Reissner-Nordstr\"om BHs, showing there is always a mass gap.
Then, we make the $Q$-clouds backreact, and construct fully non-linear
solutions of the Einstein-Maxwell-gauged scalar system describing spherical,
charged BHs with resonant, scalar $Q$-hair. Amongst other properties, we
observe there is non-uniqueness of charged BHs in this model and the $Q$-hairy
BHs can be entropically preferred over Reissner-Nordstr\"om, for the same
charge to mass ratio; some $Q$-hairy BH solutions can be overcharged. We also
discuss how some well known no-hair theorems in the literature, applying to
electro-vacuum plus minimally coupled scalar fields, are circumvented by this
new type of BHs.Comment: 18 pages, 5 figures; v2. typos corrected, matches published versio

### Stationary scalar and vector clouds around Kerr-Newman black holes

Massive bosons in the vicinity of Kerr-Newman black holes can form pure bound
states when their phase angular velocity fulills the synchronisation condition,
i.e. at the threshold of superradiance. The presence of these stationary clouds
at the linear level is intimately linked to the existence of Kerr black holes
with synchronised hair at the non-linear level. These configurations are very
similar to the atomic orbitals of the electron in a hydrogen atom. They can be
labeled by four quantum numbers: $n$, the number of nodes in the radial
direction; $\ell$, the orbital angular momentum; $j$, the total angular
momentum; and $m_j$, the azimuthal total angular momentum. These synchronised
configurations are solely allowed for particular values of the black hole's
mass, angular momentum and electric charge. Such quantization results in an
existence surface in the three-dimensional parameter space of Kerr-Newman black
holes. The phenomenology of stationary scalar clouds has been widely addressed
over the last years. However, there is a gap in the literature concerning their
vector cousins. Following the separability of the Proca equation in
Kerr(-Newman) spacetime, this work explores and compares scalar and vector
stationary clouds around Kerr and Kerr-Newman black holes, extending previous
research.Comment: 17 pages, 6 figures. Contribution to Selected Papers of the Fifth
Amazonian Symposium on Physics (accepted in IJMPD

### Shadows and strong gravitational lensing: a brief review

For ultra compact objects (UCOs), Light Rings (LRs) and Fundamental Photon
Orbits (FPOs) play a pivotal role in the theoretical analysis of strong
gravitational lensing effects, and of BH shadows in particular. In this short
review, specific models are considered to illustrate how FPOs can be useful in
order to understand some non-trivial gravitational lensing effects. This paper
aims at briefly overviewing the theoretical foundations of these effects,
touching also some of the related phenomenology, both in General Relativity
(GR) and alternative theories of gravity, hopefully providing some intuition
and new insights for the underlying physics, which might be critical when
testing the Kerr black hole hypothesis.Comment: 32 pages, 9 figures; Review paper in the General Relativity and
Gravitation (GRG) Topical Collection "Testing the Kerr spacetime with
gravitational-wave and electromagnetic observations" (Guest Editor: Emanuele
Berti); v2: Typo corrected and two references adde

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