182,871 research outputs found
Shapes and Positions of Black Hole Shadows in Accretion Disks and Spin Parameters of Black Holes
Can we determine a spin parameter of a black hole by observation of a black
hole shadow in an accretion disk? In order to answer this question, we make a
qualitative analysis and a quantitative analysis of a shape and a position of a
black hole shadow casted by a rotating black hole on an optically thick
accretion disk and its dependence on an angular momentum of a black hole. We
have found black hole shadows with a quite similar size and a shape for largely
different black hole spin parameters and a same black hole mass. Thus, it is
practically difficult to determine a spin parameter of a black hole from a size
and a shape of a black hole shadow in an accretion disk. We newly introduce a
bisector axis of a black hole shadow named a shadow axis. For a rotating black
hole a shape and a position of a black hole shadow are not symmetric with
respect to a rotation axis of a black hole shadow. So, in this case the minimum
interval between a mass center of a black hole and a shadow axis is finite. An
extent of this minimum interval is roughly proportional to a spin parameter of
a black hole for a fixed inclination angle between a rotation axis of a black
hole and a direction of an observer. In order to measure a spin parameter of a
black hole, if a shadow axis is determined observationally, it is crucially
important to determine a position of a mass center of a black hole in a region
of a black hole shadow.Comment: 13 pages, 6 figures, accepted for publication in Ap
Canonical entropy of three-dimensional BTZ black hole
Recently, Hawking radiation of the black hole has been studied using the
tunnel effect method. It is found that the radiation spectrum of the black hole
is not a strictly pure thermal spectrum. How does the departure from pure
thermal spectrum affect the entropy? This is a very interesting problem. In
this paper, we calculate the partition function by energy spectrum obtained
from tunnel effect. Using the partition function, we compute the black hole
entropy and derive the expression of the black hole entropy after considering
the radiation. And we derive the entropy of charged black hole. In our
calculation, we consider not only the correction to the black hole entropy due
to fluctuation of energy but also the effect of the change of the black hole
charges on entropy. There is no other hypothesis. Our result is more
reasonable.According to the fact that the black hole entropy is not divergent,
we obtain the lower limit of Banados-Teitelboim-Zanelli black hole energy. That
is, the least energy of Banados-Teitelboim-Zanelli black hole, which satisfies
the stationary condition in thermodynamics.Comment: 10 page
Regular black hole in three dimensions
We find a new black hole in three dimensional anti-de Sitter space by
introducing an anisotropic perfect fluid inspired by the noncommutative black
hole. This is a regular black hole with two horizons. We compare thermodynamics
of this black hole with that of non-rotating BTZ black hole. The first-law of
thermodynamics is not compatible with the Bekenstein-Hawking entropy.Comment: 15 pages, 16 figures, 3D noncommutative black hole included as Sec 4,
a version to appear in EPJ
Black Hole Thermodynamics without a Black Hole?
In the present paper we consider, using our earlier results, the process of
quantum gravitational collapse and argue that there exists the final quantum
state when the collapse stops. This state, which can be called the ``no-memory
state'', reminds the final ``no-hair state'' of the classical gravitational
collapse. Translating the ``no-memory state'' into classical language we
construct the classical analogue of quantum black hole and show that such a
model has a topological temperature which equals exactly the Hawking's
temperature. Assuming for the entropy the Bekenstein-Hawking value we develop
the local thermodynamics for our model and show that the entropy is naturally
quantized with the equidistant spectrum S + gamma_0*N. Our model allows, in
principle, to calculate the value of gamma_0. In the simplest case, considered
here, we obtain gamma_0 = ln(2).Comment: 20 pages, it will be submitted to Phys.Lett.
Rotating black hole in Rastall theory
Rotating black hole solutions in theories of modified gravity are important
as they offer an arena to test these theories through astrophysical
observation. The non-rotating black hole can be hardly tested since the black
hole spin is very important in any astrophysical process. We present rotating
counterpart of a recently obtained spherically symmetric exact black hole
solution surrounded by perfect fluid in the context of Rastall theory, viz,
rotating Rastall black hole that generalize the Kerr-Newman black hole
solution. In turn, we analyze the specific cases of the Kerr-Newman black holes
surrounded by matter like dust and quintessence fields. Interestingly, for a
set of parameters and a chosen surrounding field, there exists a critical
rotation parameter (), which corresponds to an extremal black hole
with degenerate horizons, while for , it describes a non-extremal
black hole with Cauchy and event horizons, and no black hole for with
value is also influenced by these parameters. We also discuss the
thermodynamical quantities associated with rotating Rastall black hole, and
analyze the particle motion with the behavior of effective potential.Comment: 26 pages, 8 figures. Matched with the published versio
A Toy Model for Blandford-Znajek Mechanism
A toy model for the Blandford-Znajek mechanism is investigated: a Kerr black
hole with a toroidal electric current residing in a thin disk around the black
hole. The toroidal electric current generates a poloidal magnetic field
threading the black hole and disk. Due to the interaction of the magnetic field
with remote charged particles, the rotation of the black hole and disk induces
an electromotive force, which can power an astrophysical load at remote
distance. The power of the black hole and disk is calculated. It is found that,
for a wide range of parameters specifying the rotation of the black hole and
the distribution of the electric current in the disk, the power of the disk
exceeds the power of the black hole. The torque provided by the black hole and
disk is also calculated. The torque of the disk is comparable to the torque of
the black hole. As the disk loses its angular momentum, the mass of the disk
gradually drifts towards the black hole and gets accreted. Ultimately the power
comes from the gravitational binding energy between the disk and the black
hole, as in the standard theory of accretion disk, instead of the rotational
energy of the black hole. This suggests that the Blandford-Znajek mechanism may
be less efficient in extracting energy from a rotating black hole with a thin
disk. The limitations of our simple model and possible improvements deserved
for future work are also discussed.Comment: 16 pages, 4 figures. Accepted for publication in Physical Review
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