126 research outputs found
Cosmological energy in a thermo-horizon and the first law
We consider a cosmological horizon, named thermo-horizon, to which are
associated a temperature and an entropy of Bekenstein-Hawking and which obeys
the first law for an energy flow calculated through the corresponding limit
surface. We point out a contradiction between the first law and the definition
of the total energy contained inside the horizon. This contradiction is removed
when the first law is replaced by a Gibbs' equation for a vacuum-like component
associated to the event horizon
Kerr Geodesics Following the Axis of Symmetry
We present here the general expressions for the acceleration of massive test
particles along the symmetry axis of the Kerr metric, and then study the main
properties of this acceleration in different regions of the spacetime. In
particular, we show that there exists a region near the black hole in which the
gravitational field is repulsive. We provide possible physical interpretations
about the role of this effect in terms of the different conserved parameters.
The studies of these geodesics are important not only to understand better the
structure of the Kerr spacetime but also to its use as a possible mechanism for
the production of extragalactic jets. Our results are obtained with the help of
expressing the geodesics of the Kerr spacetime in terms of the Weyl
coordinates.Comment: revtex4, no figures and tables. Gen. Relativ. Grav. 48 (2016) 6
Gravitational Model of High Energy Particles in a Collimated Jet
Observations suggest that relativistic particles play a fundamental role in
the dynamics of jets emerging from active galactic nuclei as well as in their
interaction with the intracluster medium. However, no general consensus exists
concerning the acceleration mechanism of those high energy particles. A
gravitational acceleration mechanism is here proposed, in which particles
leaving precise regions within the ergosphere of a rotating supermassive black
hole produce a highly collimated flow. These particles follow unbound geodesics
which are asymptotically parallel to the spin axis of the black hole and are
characterized by the energy , the Carter constant and zero
angular momentum of the component . If environmental effects are
neglected, the present model predicts at distances of about 140 kpc from the
ergosphere the presence of electrons with energies around 9.4 GeV. The present
mechanism can also accelerate protons up to the highest energies observed in
cosmic rays by the present experiments.Comment: 27 pages and 5 figures. Accepted for publication in Astrophysical
Journal. arXiv admin note: text overlap with arXiv:1011.654
Composition and the thermoelectric performance of β-Zn_4Sb_3
β-Zn_4Sb_3 is a promising thermoelectric material due to the abundance of zinc and antimony and reports of high efficiency in bulk samples. This work establishes the high temperature properties of β-Zn_4Sb_3 across the phase stability window. By controlling the stoichiometry, the Hall carrier concentration can be tuned from 6–9 × 10^(19) cm^(−3) without requiring extrinsic dopants. The trend in Seebeck coefficient on carrier concentration is rationalized with a single, parabolic band model. Extremely low lattice thermal conductivity (0.4–0.6 W m^(−1) K^(−1)) coupled with a moderate effective mass (1.2 m_e) and mobility leads to a large figure of merit (zT of 0.8 by 550 K). The single parabolic band model is used to obtain the carrier concentration dependence of the figure of merit and an optimum carrier concentration near 5 × 10^(19) cm^(−3) is predicted
Kerr Geodesics, the Penrose Process and Jet Collimation by a Black Hole
We re-examine the possibility that astrophysical jet collimation may arise
from the geometry of rotating black holes and the presence of high-energy
particles resulting from a Penrose process, without the help of magnetic
fields. Our analysis uses the Weyl coordinates, which are revealed better
adapted to the desired shape of the jets. We numerically integrate the
2D-geodesics equations. We give a detailed study of these geodesics and give
several numerical examples. Among them are a set of perfectly collimated
geodesics with asymptotes parallel to the axis, with
only depending on the ratios and
, where and are the parameters of the Kerr black hole,
the particle energy and the Carter's constant.Comment: Accepted by Astronomy and Astrophysics. AA style with 3 EPS figures.
Content amended after AA's refereeing. Discussion of geodesics also corrected
and expanded earlier. Conclusions amended accordingl
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