3,207 research outputs found
Electron-positron energy deposition rate from neutrino pair annihilation on the rotation axis of neutron and quark stars
We investigate the deposition of energy due to the annihilations of neutrinos
and antineutrinos on the rotation axis of rotating neutron and quark stars,
respectively. The source of the neutrinos is assumed to be a neutrino-cooled
accretion disk around the compact object. Under the assumption of the
separability of the neutrino null geodesic equation of motion we obtain the
general relativistic expression of the energy deposition rate for arbitrary
stationary and axisymmetric space-times. The neutrino trajectories are obtained
by using a ray tracing algorithm, based on numerically solving the
Hamilton-Jacobi equation for neutrinos by reversing the proper time evolution.
We obtain the energy deposition rates for several classes of rotating neutron
stars, described by different equations of state of the neutron matter, and for
quark stars, described by the MIT bag model equation of state and in the CFL
(Color-Flavor-Locked) phase, respectively. The electron-positron energy
deposition rate on the rotation axis of rotating neutron and quark stars is
studied for two accretion disk models (isothermal disk and accretion disk in
thermodynamical equilibrium). Rotation and general relativistic effects modify
the total annihilation rate of the neutrino-antineutrino pairs on the rotation
axis of compact stellar, as measured by an observer at infinity. The
differences in the equations of state for neutron and quark matter also have
important effects on the spatial distribution of the energy deposition rate by
neutrino-antineutrino annihilation.Comment: 38 pages, 9 figures, accepted for publication in MNRA
The status of pentaquark spectroscopy on the lattice
The present work is a summary of the status of lattice pentaquark
calculations. After a pedagogic introduction to the basics of lattice hadron
spectroscopy we give a critical comparison of results presently available in
the literature. Special emphasis is put on presenting some of the possible
pitfalls of these calculations. In particular we discuss at length the choice
of the hadronic operators and the separation of genuine five-quark states from
meson-baryon scattering states.Comment: 13 pages LaTeX, 1 eps figur
Modeling the IDV emissions of the BL Lac Objects with a Langevin type stochastic differential equation
In this paper, we introduce a simplified model for explaining the
observations of the optical intraday variability (IDV) of the BL Lac Objects.
We assume that the source of the IDV are the stochastic oscillations of an
accretion disk around a supermassive black hole. The Stochastic Fluctuations on
the vertical direction of the accretion disk are described by using a Langevin
type equation with a damping term and a random, white noise type force.
Furthermore, the preliminary numerical simulation results are presented, which
are based on the numerical analysis of the Langevin stochastic differential
equation.Comment: 4 pages, 4 figures, accepted for publication in J. Astrophys. Ast
Pentaquark hadrons from lattice QCD
We study spin 1/2 isoscalar and isovector candidates in both parity channels
for the recently discovered \Theta^+(1540) pentaquark particle in quenched
lattice QCD. Our analysis takes into account all possible uncertainties, such
as statistical, finite size and quenching errors when performing the chiral and
continuum extrapolations and we have indications that our signal is separated
from scattering states. The lowest mass that we find in the I^P=0^- channel is
in complete agreement with the experimental value of the \Theta^+ mass. On the
other hand, the lowest mass state in the opposite parity I^P=0^+ channel is
much higher. Our findings suggests that the parity of the \Theta^+ is negative.Comment: 13 pages, 5 figures. Final version, appeared in JHE
Can accretion disk properties observationally distinguish black holes from naked singularities?
Naked singularities are hypothetical astrophysical objects, characterized by a gravitational singularity without an event horizon. Penrose has proposed a conjecture, according to which there exists a cosmic censor who forbids the occurrence of naked singularities. Distinguishing between astrophysical black holes and naked singularities is a major challenge for present day observational astronomy. In the context of stationary and axially symmetrical geometries, a possibility of differentiating naked singularities from black holes is through the comparative study of thin accretion disks properties around rotating naked singularities and Kerr-type black holes, respectively. In the present paper, we consider accretion disks around axially-symmetric rotating naked singularities, obtained as solutions of the field equations in the Einstein-massless scalar field theory. A first major difference between rotating naked singularities and Kerr black holes is in the frame dragging effect, the angular velocity of a rotating naked singularity being inversely proportional to its spin parameter. Because of the differences in the exterior geometry, the thermodynamic and electromagnetic properties of the disks (energy flux, temperature distribution and equilibrium radiation spectrum) are different for these two classes of compact objects, consequently giving clear observational signatures that could discriminate between black holes and naked singularities. For specific values of the spin parameter and of the scalar charge, the energy flux from the disk around a rotating naked singularity can exceed by several orders of magnitude the flux from the disk of a Kerr black hole. In addition to this, it is also shown that the conversion efficiency of the accreting mass into radiation by rotating naked singularities is always higher than the conversion efficiency for black holes, i.e., naked singularities provide a much more efficient mechanism for converting mass into radiation than black holes. Thus, these observational signatures may provide the necessary tools from clearly distinguishing rotating naked singularities from Kerr-type black holes. © 2010 The American Physical Society.published_or_final_versio
General Relativistic Ray-tracing Algorithm for the Determination of the Electron–positron Energy Deposition Rate from Neutrino Pair Annihilation around Rotating Neutron and Quark Stars
published_or_final_versio
Exact ground states for quasi 1D systems with hubbard interaction
Using a positive semidefinite operator technique we deduced exact ground states for a modified diamond chain described by a non-integrable Hubbard model with on-site repulsion. Our results are valid for arbitrary length of the chain and strength of the Hubbard interaction. For the analyzed quasi 1D chain structure we found that two flat bands are present in the bare band structure of the system, both for zero and for a fixed value of magnetic field. We obtained ground states of nonmagnetic and ferromagnetic insulator type and studied their physical properties.
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