1,752 research outputs found
Sea state monitoring using coastal GNSS-R
We report on a coastal experiment to study GPS L1 reflections. The campaign
was carried out at the Barcelona Port breaker and dedicated to the development
of sea-state retrieval algorithms. An experimental system built for this
purpose collected and processed GPS data to automatically generate a times
series of the interferometric complex field (ICF). The ICF was analyzed off
line and compared to a simple developed model that relates ICF coherence time
to the ratio of significant wave height (SWH) and mean wave period (MWP). The
analysis using this model showed good consistency between the ICF coherence
time and nearby oceanographic buoy data. Based on this result, preliminary
conclusions are drawn on the potential of coastal GNSS-R for sea state
monitoring using semi-empirical modeling to relate GNSS-R ICF coherence time to
SWH.Comment: All Starlab authors have contributed significantly; the Starlab
author list has been ordered randomly. Submitted to GR
Maximum Mass-Radius Ratios for Charged Compact General Relativistic Objects
Upper limits for the mass-radius ratio and total charge are derived for
stable charged general relativistic matter distributions. For charged compact
objects the mass-radius ratio exceeds the value 4/9 corresponding to neutral
stars. General restrictions for the redshift and total energy (including the
gravitational contribution) are also obtained.Comment: 6 pages, 2 figures, RevTex. To appear in Europhys. Let
The luminosity evolution over the EQuiTemporal Surfaces in the prompt emission of Gamma-Ray Bursts
Due to the ultrarelativistic velocity of the expanding "fireshell" (Lorentz
gamma factor \gamma \sim 10^2 - 10^3), photons emitted at the same time from
the fireshell surface do not reach the observer at the same arrival time. In
interpreting Gamma-Ray Bursts (GRBs) it is crucial to determine the properties
of the EQuiTemporal Surfaces (EQTSs): the locus of points which are source of
radiation reaching the observer at the same arrival time. In the current
literature this analysis is performed only in the latest phases of the
afterglow. Here we study the distribution of the GRB bolometric luminosity over
the EQTSs, with special attention to the prompt emission phase. We analyze as
well the temporal evolution of the EQTS apparent size in the sky. We use the
analytic solutions of the equations of motion of the fireshell and the
corresponding analytic expressions of the EQTSs which have been presented in
recent works and which are valid for both the fully radiative and the adiabatic
dynamics. We find the novel result that at the beginning of the prompt emission
the most luminous regions of the EQTSs are the ones closest to the line of
sight. On the contrary, in the late prompt emission and in the early afterglow
phases the most luminous EQTS regions are the ones closest to the boundary of
the visible region. This transition in the emitting region may lead to specific
observational signatures, i.e. an anomalous spectral evolution, in the rising
part or at the peak of the prompt emission. We find as well an expression for
the apparent radius of the EQTS in the sky, valid in both the fully radiative
and the adiabatic regimes. Such considerations are essential for the
theoretical interpretation of the prompt emission phase of GRBs.Comment: 11 pages, 5 figures, in the Proceedings of the 1st Galileo-Xu GuangQi
Meeting, October 26-30, 2009, Shangha
The Gravitational Polarization in General Relativity: Solution to Szekeres' Model of Quadrupole Polarization
A model for the static weak-field macroscopic medium is analyzed and the
equation for the macroscopic gravitational potential is derived. This is a
biharmonic equation which is a non-trivial generalization of the Poisson
equation of Newtonian gravity. In case of the strong gravitational quadrupole
polarization it essentially holds inside a macroscopic matter source. Outside
the source the gravitational potential fades away exponentially. The equation
is equivalent to a system of the Poisson equation and the nonhomogeneous
modified Helmholtz equations. The general solution to this system is obtained
by using Green's function method and it does not have a limit to Newtonian
gravity. In case of the insignificant gravitational quadrupole polarization the
equation for macroscopic gravitational potential becomes the Poisson equation
with the matter density renormalized by the factor including the value of the
quadrupole gravitational polarization of the source. The general solution to
this equation obtained by using Green's function method has a limit to
Newtonian gravity.Comment: 22 pages, no figures, to appear on Class. Quantum Gra
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
Coexistence of black holes and a long-range scalar field in cosmology
The exactly solvable scalar hairy black hole model (originated from the
modern high-energy theory) is proposed. It turns out that the existence of
black holes (BH) is strongly correlated to global scalar field, in a sense that
they mutually impose bounds upon their physical parameters like the BH mass
(lower bound) or the cosmological constant (upper bound). We consider the same
model also as a cosmological one and show that it agrees with recent
experimental data; additionally, it provides a unified quintessence-like
description of dark energy and dark matter.Comment: 4 pages, 4 figure
On the physical processes which lie at the bases of time variability of GRBs
The relative-space-time-transformation (RSTT) paradigm and the interpretation
of the burst-structure (IBS) paradigm are applied to probe the origin of the
time variability of GRBs. Again GRB 991216 is used as a prototypical case,
thanks to the precise data from the CGRO, RXTE and Chandra satellites. It is
found that with the exception of the relatively inconspicuous but
scientifically very important signal originating from the initial ``proper
gamma ray burst'' (P-GRB), all the other spikes and time variabilities can be
explained by the interaction of the accelerated-baryonic-matter pulse with
inhomogeneities in the interstellar matter. This can be demonstrated by using
the RSTT paradigm as well as the IBS paradigm, to trace a typical spike
observed in arrival time back to the corresponding one in the laboratory time.
Using these paradigms, the identification of the physical nature of the time
variablity of the GRBs can be made most convincingly. It is made explicit the
dependence of a) the intensities of the afterglow, b) the spikes amplitude and
c) the actual time structure on the Lorentz gamma factor of the
accelerated-baryonic-matter pulse. In principle it is possible to read off from
the spike structure the detailed density contrast of the interstellar medium in
the host galaxy, even at very high redshift.Comment: 11 pages, 5 figure
The EMBH model in GRB 991216 and GRB 980425
This is a summary of the two talks presented at the Rome GRB meeting by C.L.
Bianco and R. Ruffini. It is shown that by respecting the Relative Space-Time
Transformation (RSTT) paradigm and the Interpretation of the Burst Structure
(IBS) paradigm, important inferences are possible: a) in the new physics
occurring in the energy sources of GRBs, b) on the structure of the bursts and
c) on the composition of the interstellar matter surrounding the source.Comment: 8 pages, 3 figures, in the Proceedings of the "Third Rome Workshop on
Gamma-Ray Bursts in the Afterglow Era", 17-20 September 2002, M. Feroci, F.
Frontera, N. Masetti, L. Piro (editors
Evidence Against Astrophysical Dyadospheres
It is shown how pair production itself would almost certainly prevent the
astrophysical formation of macroscopic dyadospheres, hypothetical regions,
extending many electron Compton wavelengths in all directions, where the
electric field exceeds the critical value for microscopically rapid Schwinger
pair production. Pair production is a self-regulating process that would
discharge a growing electric field, in the example of a hypothetical collapsing
charged stellar core, before it reached 6% of the minimum dyadosphere value,
keeping the pair production rate more than 26 orders of magnitude below the
dyadosphere value, and keeping the efficiency below 0.0002 (M/M_sun)^{1/2}.Comment: 27 pages, LaTeX, shortened version of astro-ph/0605432 accepted 2006
Aug. 17 by The Astrophysical Journal, but also with some significant new
materia
Optical IP switching a solution to dynamic lightpath establishment in disaggregated network architectures
The landscape of the telecommunications environment is constantly evolving; in terms of architecture and increasing data-rate. Ensuring that routing decisions are taken at the lowest possible layer offers the possibility of greatest data throughput. We propose using wavelengths in a DWDM scheme as dedicated channels that bypass the routing lookup in a router. The future trend of telecommunications industry is, however, toward larger numbers of interlinked competing operator networks. This in turn means there is a lack of a unified control plane to allow current networks to dynamically provision optical paths. This paper will report on the concept of optical IP switching. This concept seeks to address optical control plane issues in disaggregated networks while providing a means to dynamically provision optical paths to cater for large data flows
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