8,367 research outputs found
Disclosing connections between black holes and naked singularities: Horizon remnants, Killing throats and bottlenecks
We study the properties of black holes and naked singularities by considering
stationary observers and light surfaces in Kerr spacetimes. We reconsider the
notion of Killing horizons from a special perspective by exploring the entire
family of Kerr metrics. To this end, we introduce the concepts of extended
plane, Killing throats and bottlenecks for weak (slowly spinning) naked
singularities. Killing bottlenecks (or horizon remnants in analogy with the
corresponding definition of throats in black holes) are restrictions of the
Killing throats appearing in special classes of slowly spinning naked
singularities. Killing bottlenecks appear in association with the concept of
pre-horizon regime introduced in [1, 2]. In the extended plane of the Kerr
spacetime, we introduce particular sets, metric bundles, of metric tensors
which allow us to reinterpret the concept of horizon and to find connections
between black holes and naked singularities throughout the horizons. To
evaluate the effects of frame-dragging on the formation and structure of
Killing bottlenecks and horizons in the extended plane, we consider also the
Kerr-Newman and the Reissner-Norstrom spacetimes. We argue that these results
might be significant for the comprehension of processes that lead to the
formation and eventually destruction of Killing horizons.Comment: 33 pages, 32 multi-panels figures, 3 Table
Squeezing of toroidal accretion disks
Accretion disks around very compact objects such as very massive Black hole
can grow according to thick toroidal models. We face the problem of defining
how does change the thickness of a toroidal accretion disk spinning around a
Schwarzschild Black hole under the influence of a toroidal magnetic field and
by varying the fluid angular momentum. We consider both an hydrodynamic and a
magnetohydrodynamic disk based on the Polish doughnut thick model. We show that
the torus thickness remains basically unaffected but tends to increase or
decrease slightly depending on the balance of the magnetic, gravitational and
centrifugal effects which the disk is subjected to.Comment: 6 pages, 17 figures, to appear in EP
Observers in Kerr spacetimes: the ergoregion on the equatorial plane
We perform a detailed analysis of the properties of stationary observers
located on the equatorial plane of the ergosphere in a Kerr spacetime,
including light-surfaces. This study highlights crucial differences between
black hole and the super-spinner sources. In the case of Kerr naked
singularities, the results allow us to distinguish between "weak" and "strong"
singularities, corresponding to spin values close to or distant from the
limiting case of extreme black holes, respectively. We derive important
limiting angular frequencies for naked singularities. We especially study very
weak singularities as resulting from the spin variation of black holes. We also
explore the main properties of zero angular momentum observers for different
classes of black hole and naked singularity spacetimes.Comment: 20 pages, 13 multi-panels figures, 2 table
The ergoregion in the Kerr spacetime: properties of the equatorial circular motion
We investigate in detail the circular motion of test particles on the
equatorial plane of the ergoregion in the Kerr spacetime. We consider all the
regions where circular motion is allowed, and we analyze the stability
properties and the energy and angular momentum of the test particles. We show
that the structure of the stability regions has definite features that make it
possible to distinguish between black holes and naked singularities. The naked
singularity case presents a very structured non-connected set of regions of
orbital stability, where the presence of counterrotating particles and zero
angular momentum particles for a specific class of naked singularities is
interpreted as due to the presence of a repulsive field generated by the
central source of gravity. In particular, we analyze the effects of the
dynamical structure of the ergoregion (the union of the orbital regions for
different attractor spins) on the behavior of accretion disks around the
central source. The properties of the circular motion turn out to be so
distinctive that they allow the introduction of a complete classification of
Kerr spacetimes, each class of which is characterized by different physical
effects that could be of especial relevance in observational Astrophysics. We
also identify some special black hole spacetimes where these effects could be
relevant.Comment: 19 pages, 9 figure multi-panels; 3 Tables. This and a slightly
modified version with the addition of new references and some new discussion.
To appear in EPJ
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