1,523 research outputs found
On the core-halo distribution of dark matter in galaxies
We investigate the distribution of dark matter in galaxies by solving the
equations of equilibrium of a self-gravitating system of massive fermions
(`inos') at selected temperatures and degeneracy parameters within general
relativity. Our most general solutions show, as a function of the radius, a
segregation of three physical regimes: 1) an inner core of almost constant
density governed by degenerate quantum statistics; 2) an intermediate region
with a sharply decreasing density distribution followed by an extended plateau,
implying quantum corrections; 3) an asymptotic, classical
Boltzmann regime fulfilling, as an eigenvalue problem, a fixed value of the
flat rotation curves. This eigenvalue problem determines, for each value of the
central degeneracy parameter, the mass of the ino as well as the radius and
mass of the inner quantum core. Consequences of this alternative approach to
the central and halo regions of galaxies, ranging from dwarf to big spirals,
for SgrA*, as well as for the existing estimates of the ino mass, are outlined.Comment: 8 pages, 5 figures. Accepted for publication by MNRA
Strong-field gravitational-wave emission in Schwarzschild and Kerr geometries: some general considerations
We show how the concurrent implementation of the exact solutions of the
Einstein equations, of the equations of motion of the test particles, and of
the relativistic estimate of the emission of gravitational waves from test
particles, can establish a priori constraints on the possible phenomena
occurring in Nature. Two examples of test particles starting at infinite
distance or from finite distance in a circular orbit around a Kerr black hole
are considered: the first leads to a well defined gravitational wave burst the
second to a smooth merging into the black hole. This analysis is necessary for
the study of the waveforms in merging binary systems.Comment: Resubmitted to PRD after Referee repor
Novel constraints on fermionic dark matter from galactic observables I: The Milky Way
We have recently introduced a new model for the distribution of dark matter
(DM) in galaxies based on a self-gravitating system of massive fermions at
finite temperatures, the Ruffini-Arg\"uelles-Rueda (RAR) model. We show that
this model, for fermion masses in the keV range, explains the DM halo of the
Galaxy and predicts the existence of a denser quantum core at the center. We
demonstrate here that the introduction of a cutoff in the fermion phase-space
distribution, necessary to account for the finite Galaxy size, defines a new
solution with a central core which represents an alternative to the black hole
(BH) scenario for SgrA*. For a fermion mass in the range --
~keV, the DM halo distribution is in agreement with the Milky Way rotation
curve data, while harbors a dense quantum core of about
within the S2-star pericenter.Comment: 11 pages, 5 figures. Published in Physics of the Dark Univers
Circular motion of neutral test particles in Reissner-Nordstr\"om spacetime
We investigate the motion of neutral test particles in the gravitational
field of a mass with charge described by the Reissner-Nordstr\"om (RN)
spacetime. We focus on the study of circular stable and unstable orbits around
configurations describing either black holes or naked singularities. We show
that at the classical radius, defined as , there exist orbits with zero
angular momentum due to the presence of repulsive gravity. The analysis of the
stability of circular orbits indicates that black holes are characterized by a
continuous region of stability. In the case of naked singularities, the region
of stability can split into two non-connected regions inside which test
particles move along stable circular orbits.Comment: 23 pages, 22 figures. To be published Phys. Rev.
A Model for Short Gamma-Ray Bursts: Heated Neutron Stars in Close Binary Systems
In this paper we present a model for the short (< second) population of
gamma-ray bursts (GRBs). In this model heated neutron stars in a close binary
system near their last stable orbit emit neutrinos at large luminosities (~
10^53 ergs/sec). A fraction of these neutrinos will annihilate to form an
electron-positron pair plasma wind which will, in turn, expand and recombine to
photons which make the gamma-ray burst. We study neutrino annihilation and show
that a substantial fraction (~ 50%) of energy deposited comes from inter-star
neutrinos, where each member of the neutrino pair originates from each neutron
star. Thus, in addition to the annihilation of neutrinos blowing off of a
single star, we have a new source of baryon free energy that is deposited
between the stars. To model the pair plasma wind between stars, we do
three-dimensional relativistic numerical hydrodynamic calculations.
Preliminary results are also presented of new, fully general relativistic
calculations of gravitationally attracting stars falling from infinity with no
angular momentum. These simulations exhibit a compression effect.Comment: 3 pages, 3 postscript figs (2 color), to appear in "Gamma-Ray Burst
and Afterglow Astronomy 2001", Woods Hole; 5-9 Nov, 200
The role of self-interacting right-handed neutrinos in galactic structure
It has been shown previously that the DM in galactic halos can be explained
by a self-gravitating system of massive keV fermions (`inos') in thermodynamic
equilibrium, and predicted the existence of a denser quantum core of inos
towards the center of galaxies. In this article we show that the inclusion of
self-interactions among the inos, modeled within a relativistic
mean-field-theory approach, allows the quantum core to become massive and
compact enough to explain the dynamics of the S-cluster stars closest to the
Milky Way's galactic center. The application of this model to other galaxies
such as large elliptical harboring massive central dark objects of is also investigated. We identify these interacting inos with sterile
right-handed neutrinos pertaining to minimal extensions of the Standard Model,
and calculate the corresponding total cross-section within an
electroweak-like formalism to be compared with other observationally inferred
cross-section estimates. The coincidence of an ino mass range of few tens of
keV derived here only from the galactic structure, with the range obtained
independently from other astrophysical and cosmological constraints, points
towards an important role of the right-handed neutrinos in the cosmic
structure.Comment: 33 pages, 9 figures, version to appear in JCA
- …