83 research outputs found
Scattering of spinning bodies by a radiation field in Schwarzschild spacetime
We extend the analysis of Poynting-Robertson effect, i.e., the deviation from geodesic motion of test particles due to scattering by a superposed radiation field to the Schwarzschild background, to the case of spinning bodies. The extra contribution of the deviation due to spin can be relevant for astrophysical systems like the binary pulsar system PSR J0737-3039 orbiting Sgr A∗, but not for the Earth-Sun system
Strains and Jets in Black Hole Fields
We study the behaviour of an initially spherical bunch of particles emitted
along trajectories parallel to the symmetry axis of a Kerr black hole. We show
that, under suitable conditions, curvature and inertial strains compete to
generate jet-like structures.Comment: To appear in the Proceedings of the Spanish Relativity Meeting 2007
held in Tenerife (Spain) 3 Figure
Dark energy from cosmological fluids obeying a Shan-Chen nonideal equation of state
We consider a Friedmann-Robertson-Walker universe with a fluid source obeying a nonideal equation of state with ‘‘asymptotic freedom,’’ namely ideal gas behavior (pressure changes directly proportional to density changes) both at low and high density regimes, following a fluid dynamical model due to Shan and Chen. It is shown that, starting from an ordinary energy density component, such fluids naturally evolve towards a universe with a substantial ‘‘dark energy’’ component at the present time, with no need of invoking any cosmological constant. Moreover, we introduce a quantitative indicator of darkness abundance, which provides a consistent picture of the actual matter-energy content of the Universe
Wedging spacetime principal null directions
Taking wedge products of the distinct principal null directions
associated with the eigen-bivectors of the Weyl tensor associated with the
Petrov classification, when linearly independent, one is able to express them
in terms of the eigenvalues governing this decomposition. We study here
algebraic and differential properties of such -forms by completing previous
geometrical results concerning type I spacetimes and extending that analysis to
algebraically special spacetimes with at least 2 distinct principal null
directions. A number of vacuum and nonvacuum spacetimes are examined to
illustrate the general treatment.Comment: 24 pages, no figure
The galactic center black hole as a possible retro-lens for the S2 orbiting star
Holz & Wheeler (\cite{hw}) have recently proposed that a Schwarzschild black
hole may act as a retro-lens which, if illuminated by a powerful light source,
deflects light ray paths to large bending angles and a series of luminous arcs
(or rings in the case of aligned objects) centered on the black hole may form.
Obviously, the most convenient geometry to get retro-lensing images would be
that of a very bright star close to a massive black hole, say the putative
M black hole at the galactic center. Recent
observations of the galactic center region in the -band have revealed the
presence of a very bright main sequence star (labelled S2) with mass
M orbiting at close distance (130-1900 AU) from Sgr A. The
relatively vicinity of S2 to the central massive black hole may offer a unique
laboratory to test the formation of retro-lensing images. The next generation
of space-based telescopes in the -band (like NGST) may have high enough
limiting magnitude necessary to observe such retro-lensing images.Comment: 4 pages, 2 Postscript figures, accepted for pubblications on
Astronomy and Astrophysic
Emission vs Fermi coordinates: applications to relativistic positioning systems
A 4-dimensional relativistic positioning system for a general spacetime is
constructed by using the so called "emission coordinates". The results apply in
a small region around the world line of an accelerated observer carrying a
Fermi triad, as described by the Fermi metric. In the case of a Schwarzschild
spacetime modeling the gravitational field around the Earth and an observer at
rest at a fixed spacetime point, these coordinates realize a relativistic
positioning system alternative to the current GPS system. The latter is indeed
essentially conceived as Newtonian, so that it necessarily needs taking into
account at least the most important relativistic effects through Post-Newtonian
corrections to work properly. Previous results concerning emission coordinates
in flat spacetime are thus extended to this more general situation.
Furthermore, the mapping between spacetime coordinates and emission coordinates
is completely determined by means of the world function, which in the case of a
Fermi metric can be explicitly obtained.Comment: 12 pages iop style, 2 eps figures, to appear on Classical and Quantum
Gravity, 200
pair creation by vacuum polarization around electromagnetic black holes
The concept of "dyadotorus" was recently introduced to identify in the
Kerr-Newman geometry the region where vacuum polarization processes may occur,
leading to the creation of pairs. This concept generalizes the
original concept of "dyadosphere" initially introduced for
Reissner-Nordstr\"{o}m geometries. The topology of the axially symmetric
dyadotorus is studied for selected values of the electric field and its
electromagnetic energy is estimated by using three different methods all giving
the same result. It is shown by a specific example the difference between a
dyadotorus and a dyadosphere. The comparison is made for a Kerr-Newman black
hole with the same total mass energy and the same charge to mass ratio of a
Reissner-Nordstr\"{o}m black hole. It turns out that the Kerr-Newman black hole
leads to larger values of the electromagnetic field and energy when compared to
the electric field and energy of the Reissner-Nordstr\"{o}m one. The
significance of these theoretical results for the realistic description of the
process of gravitational collapse leading to black hole formation as well as
the energy source of gamma ray bursts are also discussed.Comment: Revised version with minor corrections; to appear on Phys. Rev.
On the equilibrium of a charged massive particle in the field of a Reissner-Nordstr\"om black hole
The multiyear problem of a two-body system consisting of a
Reissner-Nordstr\"om black hole and a charged massive particle at rest is here
solved by an exact perturbative solution of the full Einstein-Maxwell system of
equations. The expressions of the metric and of the electromagnetic field,
including the effects of the electromagnetically induced gravitational
perturbation and of the gravitationally induced electromagnetic perturbation,
are presented in closed analytic formulas.Comment: 9 pages, els macro
Spin precession in the Schwarzschild spacetime: circular orbits
We study the behavior of nonzero rest mass spinning test particles moving
along circular orbits in the Schwarzschild spacetime in the case in which the
components of the spin tensor are allowed to vary along the orbit, generalizing
some previous work.Comment: To appear on Classical and Quantum Gravity, 200
Unified Treatment of Heterodyne Detection: the Shapiro-Wagner and Caves Frameworks
A comparative study is performed on two heterodyne systems of photon
detectors expressed in terms of a signal annihilation operator and an image
band creation operator called Shapiro-Wagner and Caves' frame, respectively.
This approach is based on the introduction of a convenient operator
which allows a unified formulation of both cases. For the Shapiro-Wagner
scheme, where , quantum phase and amplitude
are exactly defined in the context of relative number state (RNS)
representation, while a procedure is devised to handle suitably and in a
consistent way Caves' framework, characterized by , within the approximate simultaneous measurements of
noncommuting variables. In such a case RNS phase and amplitude make sense only
approximately.Comment: 25 pages. Just very minor editorial cosmetic change
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