1,051 research outputs found
Introducing the black hole
The quasi-stellar object, the pulsar, the neutron star have all come onto the scene of physics within the space of a few years. Is the next entrant destined to be the black hole? If so, it is difficult to think of any development that could be of greater significance. A black hole, whether of âordinary sizeâ (approximately one solar mass, 1 Mâ) or much larger (around 10^6 Mâ to 10^10 Mâ, as proposed in the nuclei of some galaxies), provides our âlaboratory modelâ for the gravitational collapse, predicted by Einstein's theory, of the universe itself
SGRs and AXPs as massive fast rotating highly magnetized white dwarfs: the case of SGR 0418+5729
We describe one of the so-called low magnetic field magnetars SGR 0418+5729,
as a massive fast rotating highly magnetized white dwarf following Malheiro et.
al. 2012. We give bounds for the mass, radius, moment of inertia, and magnetic
field for these sources, by requesting the stability of realistic general
relativistic uniformly rotating configurations. Based on these parameters, we
improve the theoretical prediction of the lower limit of the spin-down rate of
SGR 0418+5729. In addition, we compute the electron cyclotron frequencies
corresponding to the predicted surface magnetic fields.Comment: 6 pages, 1 figure, 1 table. The Thirteenth Marcel Grossmann Meeting:
On Recent Developments in Theoretical and Experimental General Relativity,
Astrophysics and Relativistic Field Theories - Proceedings of the MG13
Meeting on General Relativity (in 3 Volumes). Edited by Rosquist Kjell et. a
Exact versus approximate beaming formulas in Gamma-Ray Burst afterglows
We present the exact analytic expressions to compute, assuming the emitted
Gamma-Ray Burst (GRB) radiation is not spherically symmetric but is confined
into a narrow jet, the value of the detector arrival time at which we start to
"see" the sides of the jet, both in the fully radiative and adiabatic regimes.
We obtain this result using our exact analytic expressions for the EQuiTemporal
Surfaces (EQTSs) in GRB afterglows. We re-examine the validity of three
different approximate formulas currently adopted for the adiabatic regime in
the GRB literature. We also present an empirical fit of the numerical solutions
of the exact equations, compared and contrasted with the three above
approximate formulas. The extent of the differences is such as to require a
reassessment on the existence and entity of beaming in the cases considered in
the current literature, as well as on its consequences on the GRB energetics.Comment: 4 pages, 4 figures, to appear on ApJ Let
On the self-consistent general relativistic equilibrium equations of neutron stars
We address the existence of globally neutral neutron star configurations in
contrast with the traditional ones constructed by imposing local neutrality.
The equilibrium equations describing this system are the Einstein-Maxwell
equations which must be solved self-consistently with the general relativistic
Thomas-Fermi equation and -equilibrium condition. To illustrate the
application of this novel approach we adopt the Baym, Bethe, and Pethick (1971)
strong interaction model of the baryonic matter in the core and of the
white-dwarf-like material of the crust. We illustrate the crucial role played
by the boundary conditions satisfied by the leptonic component of the matter at
the interface between the core and the crust. For every central density an
entire new family of equilibrium configurations exists for selected values of
the Fermi energy of the electrons at the surface of the core. Each such
configuration fulfills global charge neutrality and is characterized by a
non-trivial electrodynamical structure. The electric field extends over a thin
shell of thickness between the core and the crust and
becomes largely overcritical in the limit of decreasing values of the crust
mass
Space Time Foam: a ground state candidate for Quantum Gravity
A model of space-time foam, made by wormholes is considered. The Casimir
energy leading to such a model is computed by means of the phase shift method
which is in agreement with the variational approach used in Refs.[9-14]. The
collection of Schwarzschild and Reissner-Nordstr\"{o}m wormholes are separately
considered to represent the foam. The Casimir energy shows that the
Reissner-Nordstr\"{o}m wormholes cannot be used to represent the foam.Comment: 6 pages.RevTeX with package epsf and two eps figures. To be submitted
to the proceedings of the 4th Workshop of `Mysteries, Puzzles And Paradoxes
In Quantum Mechanics' Gargnano (Italy), 27 August-1 September 200
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