2,753 research outputs found
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
Light rings on stationary axisymmetric spacetimes: blind to the topology and able to coexist
It has been established that Black Hole (BH) spacetimes obeying some general
set of assumptions always possess, at least, one light ring (per rotation
sense) [arXiv:2003.06445]. This theorem was originally established for
asymptotically flat, stationary, axial symmetric, 1+3 dimensional circular
spacetimes harbouring a non-extremal and topologically spherical Killing
horizon. Following the mantra that a theorem is only as strong as its
assumptions in this work we extend this theorem to non topologically spherical
(toroidal) BHs and to spacetimes harbouring more than one BH. As in
[arXiv:2003.06445], we show that each BH still contributes with, at least, one
LR (per rotation sense).Comment: 17 pages, 9 figure
Null and timelike circular orbits from equivalent 2D metrics
The motion of particles on spherical 1 + 3 dimensional spacetimes can, under
some assumptions, be described by the curves on a two-dimensional manifold,
the optical and Jacobi manifolds for null and timelike curves, respectively.
In this paper we resort to auxiliary two-dimensional metrics to study circular geodesics of generic static, spherically symmetric, and asymptotically flat
1 + 3 dimensional spacetimes, whose functions are at least C
2
smooth. This
is done by studying the Gaussian curvature of the bidimensional equivalent
manifold as well as the geodesic curvature of circular paths on these. This
study considers both null and timelike circular geodesics. The study of null
geodesics through the optical manifold retrieves the known result of the number of light rings on the spacetime outside a black hole and on spacetimes with
horizonless compact objects. With an equivalent procedure we can formulate a
similar theorem on the number of marginally stable timelike circular orbits of
a given spacetime satisfying the previously mentioned assumptions.publishe
The imitation game reloaded: effective shadows of dynamically robust spinning Proca stars
We analyse the lensing images by dynamically robust rotating (mini-)Proca
stars surrounded by thin accretion disks. Due to their peculiar geodesic
structure we show that these images exhibit striking similarities with the ones
of BHs, for appropriately chosen disk intensity profile, when imposing a
GRMHD-motivated emission cut off. Additionally, and unlike the non-rotating
case, these similarities prevail even when considering equatorial observations.
This example illustrates how a horizonless compact object without light rings,
with a plausible formation mechanism and dynamically robust, could mimic
detailed features of black hole imagiology.Comment: 23 pages, 11 figures + appendi
Lensing and shadow of a black hole surrounded by a heavy accretion disk
We consider a static, axially symmetric spacetime describing the
superposition of a Schwarzschild black hole (BH) with a thin and heavy
accretion disk. The BH-disk configuration is a solution of the Einstein field
equations within the Weyl class. The disk is sourced by a distributional
energy-momentum tensor and it is located at the equatorial plane. It can be
interpreted as two streams of counter-rotating particles, yielding a total
vanishing angular momentum. The phenomenology of the composed system depends on
two parameters: the fraction of the total mass in the disk, , and the
location of the inner edge of the disk, . We start by determining the
sub-region of the space of parameters wherein the solution is physical, by
requiring the velocity of the disk particles to be sub-luminal and real. Then,
we study the null geodesic flow by performing backwards ray-tracing under two
scenarios. In the first scenario the composed system is illuminated by the disk
and in the second scenario the composed system is illuminated by a far-away
celestial sphere. Both cases show that, as grows, the shadow becomes more
prolate. Additionally, the first scenario makes clear that as grows, for
fixed , the geometrically thin disk appears optically enlarged, i.e.,
thicker, when observed from the equatorial plane. This is to due to light rays
that are bent towards the disk, when backwards ray traced. In the second
scenario, these light rays can cross the disk (which is assumed to be
transparent) and may oscillate up to a few times before reaching the far away
celestial sphere. Consequently, an almost equatorial observer sees different
patches of the sky near the equatorial plane, as a chaotic "mirage". As
one recovers the standard test, i.e., negligible mass, disk
appearance.Comment: 24 pages, 15 figure
Spontaneously scalarized Kerr black holes in extended scalar-tensor-Gauss-Bonnet gravity
We construct asymptotically flat, spinning, regular on and outside an event horizon, scalarized black holes (SBHs) in extended scalar-tensor-Gauss-Bonnet models. They reduce to Kerr BHs when the scalar field vanishes. For an illustrative choice of nonminimal coupling, we scan the domain of existence. For each value of spin, SBHs exist in an interval between two critical masses, with the lowest one vanishing in the static limit. Non-uniqueness with Kerr BHs of equal global charges is observed; the SBHs are entropically favoured. This suggests that SBHs form dynamically from the spontaneous scalarization of Kerr BHs, which are prone to a scalar-triggered tachyonic instability, below the largest critical mass. Phenomenologically, the introduction of BH spin damps the maximal observable difference between comparable scalarized and vacuum BHs. In the static limit, (perturbatively stable) SBHs can store over 20% of the spacetime energy outside the event horizon; in comparison with Schwarzschild BHs, their geodesic frequency at the ISCO can differ by a factor of 2.5 and deviations in the shadow areal radius may top 40%. As the BH spin grows, low mass SBHs are excluded, and the maximal relative differences decrease, becoming of the order of a few percent for dimensionless spin j≳0.5. This reveals a spin selection effect: non-GR effects are only significant for low spin. We discuss if and how the recently measured shadow size of the M87 supermassive BH constrains the length scale of the Gauss-Bonnet coupling.publishe
The imitation game: Proca stars that can mimic the Schwarzschild shadow
Can a dynamically robust bosonic star (BS) produce an (effective) shadow that
mimics that of a black hole (BH)? The BH shadow is linked to the existence of
light rings (LRs). For free bosonic fields, yielding mini-BSs, it is known that
these stars can become ultra-compact - i.e., possess LRs - but only for
perturbatively unstable solutions. We show this remains the case even when
different self-interactions are considered. However, an effective shadow can
arise in a different way: if BSs reproduce the existence of an innermost stable
circular orbit (ISCO) for timelike geodesics (located at for
a Schwarzschild BH of mass M), the accretion flow morphology around BHs is
mimicked and an effective shadow arises in an astrophysical environment. Even
though spherical BSs may accommodate stable timelike circular orbits all the
way down to their centre, we show the angular velocity along such orbits may
have a maximum away from the origin, at ; this scale was recently
observed to mimic the BH's ISCO in some scenarios of accretion flow. Then: (i)
for free scalar fields or with quartic self-interactions,
only for perturbatively unstable BSs; (ii) for higher scalar self-interactions,
e.g. axionic, is possible for perturbatively stable BSs, but
no solution with was found in the parameter space explored;
(iii) but for free vector fields, yielding Proca stars (PSs), perturbatively
stable solutions with exist, and indeed for
a particular solution. Thus, dynamically robust spherical PSs can mimic the
shadow of a (near-)equilibrium Schwarzschild BH with the same M, in an
astrophysical environment, despite the absence of a LR, at least under some
observation conditions, as we confirm by comparing the lensing of such PSs and
Schwarzschild BHs.Comment: Abstract abridged due to arXiv length limit; 22 pages, 9 figure
Shadows of charged rotating black holes: Kerr-Newman versus Kerr-Sen
Celebrating the centennial of its first experimental test, the theory of
General Relativity (GR) has successfully and consistently passed all subsequent
tests with flying colours. It is expected, however, that at certain scales new
physics, in particular in the form of quantum corrections, will emerge,
changing some of the predictions of GR, which is a classical theory. In this
respect, black holes (BHs) are natural configurations to explore the quantum
effects on strong gravitational fields. BH solutions in the low-energy
effective field theory description of the heterotic string theory, which is one
of the leading candidates to describe quantum gravity, have been the focus of
many studies in the last three decades. The recent interest in strong
gravitational lensing by BHs, in the wake of the Event Horizon Telescope
observations, suggests comparing the BH lensing in both GR and heterotic string
theory, in order to assess the phenomenological differences between these
models. In this work, we investigate the differences in the shadows of two
charged BH solutions with rotation: one arising in the context of GR, namely
the Kerr-Newman solution, and the other within the context of low-energy
heterotic string theory, the Kerr-Sen solution. We show and interpret, in
particular, that the stringy BH always has a larger shadow, for the same
physical parameters and observation conditions.Comment: 11 pages, 11 figures, Contribution to Selected Papers of the Fifth
Amazonian Symposium on Physics, v2: references added, typos corrected,
matches published versio
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