3,066 research outputs found
Mechanics of couple-stress fluid coatings
The formal development of a theory of viscoelastic surface fluids with bending resistance - their kinematics, dynamics, and rheology are discussed. It is relevant to the mechanics of fluid drops and jets coated by a thin layer of immiscible fluid with rather general rheology. This approach unifies the hydrodynamics of two-dimensional fluids with the mechanics of an elastic shell in the spirit of a Cosserat continuum. There are three distinct facets to the formulation of surface continuum mechanics. Outlined are the important ideas and results associated with each: the kinematics of evolving surface geometries, the conservation laws governing the mechanics of surface continua, and the rheological equations of state governing the surface stress and moment tensors
Angular size and emission time scales of relativistic fireballs
The detection of delayed X-ray, optical and radio emission, ``afterglow,''
associated with gamma-ray bursts (GRBs) is consistent with models, where the
bursts are produced by relativistic expanding blast waves, driven by expanding
fireballs at cosmological distances. In particular, the time scales over which
radiation is observed at different wave bands agree with model predictions. It
had recently been claimed that the commonly used relation between observation
time t and blast wave radius r, t=r/2\gamma^{2}c where \gamma(r) is the fluid
Lorentz factor, should be replaced with t=r/16\gamma^{2}c due to blast wave
deceleration. Applying the suggested deceleration modification would make it
difficult to reconcile observed time scales with model predictions. It would
also imply an apparent source size which is too large to allow attributing
observed radio variability to diffractive scintillation. We present a detailed
analysis of the implications of the relativistic hydrodynamics of expanding
blast waves to the observed afterglow. We find that modifications due to shock
deceleration are small, therefore allowing for both the observed afterglow time
scales and for diffractive scintillation. We show that at time t the fireball
appears on the sky as a narrow ring of radius h=r/\gamma and width 0.1h, where
r and t are related by t=r/2\gamma^{2}c.Comment: Submitted to ApJL (11 pages, LaTeX
Gamma-Ray Burst Afterglow: Polarization and Analytic Light Curves
GRB afterglow polarization is discussed. We find an observable, up to 10%,
polarization, if the magnetic field coherence length grows at about the speed
of light after the field is generated at the shock front. Detection of a
polarized afterglow would show that collisionless ultrarelativistic shocks can
generate strong large scale magnetic fields and confirm the synchrotron
afterglow model. Non-detection, at a 1% level, would imply that either the
synchrotron emission model is incorrect, or that strong magnetic fields, after
they are generated in the shock, somehow manage to stay un-dissipated at
``microscopic'', skin depth, scales. Analytic lightcurves of synchrotron
emission from an ultrarelativistic self-similar blast wave are obtained for an
arbitrary electron distribution function, taking into account the effects of
synchrotron cooling. The peak synchrotron flux and the flux at frequencies much
smaller than the peak frequency are insensitive to the details of the electron
distribution function; hence their observational determination would provide
strong constraints on blast wave parameters.Comment: 19 pages, submitted to Ap
Radiative Efficiencies of Continuously Powered Blast Waves
We use general arguments to show that a continuously powered radiative blast
wave can behave self similarly if the energy injection and radiation mechanisms
are self similar. In that case, the power-law indices of the blast wave
evolution are set by only one of the two constituent physical mechanisms. If
the luminosity of the energy source drops fast enough, the radiation mechanisms
set the power-law indices, otherwise, they are set by the behavior of the
energy source itself. We obtain self similar solutions for the Newtonian and
the ultra-relativistic limits. Both limits behave self similarly if we assume
that the central source supplies energy in the form of a hot wind, and that the
radiative mechanism is the semi-radiative mechanism of Cohen, Piran & Sari
(1998). We calculate the instantaneous radiative efficiencies for both limits
and find that a relativistic blast wave has a higher efficiency than a
Newtonian one. The instantaneous radiative efficiency depends strongly on the
hydrodynamics and cannot be approximated by an estimate of local microscopic
radiative efficiencies, since a fraction of the injected energy is deposited in
shocked matter. These solutions can be used to calculate Gamma Ray Bursts
afterglows, for cases in which the energy is not supplied instantaneously.Comment: 28 LaTeX pages, including 9 figures and 3 table
Extra galactic sources of high energy neutrinos
The main goal of the construction of large volume, high energy neutrino
telescopes is the detection of extra-Galactic neutrino sources. The existence
of such sources is implied by observations of ultra-high energy, >10^{19} eV,
cosmic-rays (UHECRs), the origin of which is a mystery. The observed UHECR flux
sets an upper bound to the extra-Galactic high energy neutrino intensity, which
implies that the detector size required to detect the signal in the energy
range of 1 TeV to 1 PeV is >=1 giga-ton, and much larger at higher energy.
Optical Cerenkov neutrino detectors, currently being constructed under ice and
water, are expected to achieve 1 giga-ton effective volume for 1 TeV to 1 PeV
neutrinos. Coherent radio Cerenkov detectors (and possibly large air-shower
detectors) will provide the >> 1 giga-ton effective volume required for
detection at ~10^{19} eV. Detection of high energy neutrinos associated with
electromagnetically identified sources will allow to identify the sources of
UHECRs, will provide a unique probe of the sources, which may allow to resolve
open questions related to the underlying physics of models describing these
powerful accelerators, and will provide information on fundamental neutrino
properties.Comment: 8 pages, 4 figures; Summary of talk presented at the Nobel Symposium
129: Neutrino Physics, Sweden 200
The Gradient Expansion for the Free-Energy of a Clean Superconductor
We describe a novel method for obtaining the gradient expansion for the free
energy of a clean BCS superconductor. We present explicit results up to fourth
order in the gradients of the order parameter.Comment: 33 pages, Late
Temperature dependence of the nitrogen-vacancy magnetic resonance in diamond
The temperature dependence of the magnetic resonance spectra of
nitrogen-vacancy (NV-) ensembles in the range of 280-330 K was studied. Four
samples prepared under different conditions were studied with NV-
concentrations ranging from 10 ppb to 15 ppm. For all of these samples, the
axial zero-field splitting (ZFS) parameter, D, was found to vary significantly
with temperature, T, as dD/dT = -74.2(7) kHz/K. The transverse ZFS parameter,
E, was non-zero (between 4 and 11 MHz) in all samples, and exhibited a
temperature dependence of dE/(EdT) = -1.4(3) x 10^(-4) K^(-1). The results
might be accounted for by considering local thermal expansion. The observation
of the temperature dependence of the ZFS parameters presents a significant
challenge for room-temperature diamond magnetometers and may ultimately limit
their bandwidth and sensitivity.Comment: 5 pages, 2 figures, 1 tabl
High Energy Neutrinos from Astrophysical Sources: An Upper Bound
We show that cosmic-ray observations set a model-independent upper bound to
the flux of high-energy, > 10^14 eV, neutrinos produced by photo-meson (or p-p)
interactions in sources of size not much larger than the proton photo-meson (or
pp) mean-free-path. The bound applies, in particular, to neutrino production by
either AGN jets or GRBs. This upper limit is two orders of magnitude below the
flux predicted in some popular AGN jet models, but is consistent with our
predictions from GRB models. We discuss the implications of these results for
future km^2 high-energy neutrino detectors.Comment: Added discussion showing bound cannot be evaded by invoking magnetic
fields. Accepted Phys Rev
GeV Photons from Ultra High Energy Cosmic Rays accelerated in Gamma Ray Bursts
Gamma-ray bursts are produced by the dissipation of the kinetic energy of a
highly relativistic fireball, via the formation of a collisionless shock. When
this happens, Ultra High Energy Cosmic Rays up to 10^20 eV are produced. I show
in this paper that these particles produce, via synchrotron emission as they
cross the acceleration region, photons up to 300 GeV which carry away a small,
~0.01, but non-negligible fraction of the total burst energy. I show that, when
the shock occurs with the interstellar medium, the optical depth to
photon-photon scattering, which might cause energy degradation of the photons,
is small. The burst thusly produced would be detected at Earth simultaneoulsy
with the parent gamma-ray burst, although its duration may differ significantly
from that of the lower energy photons. The expected fluences, ~10^{-5}-10^{-6}
erg/cm^2 are well within the range of planned detectors. A new explanation for
the exceptional burst GRB 940217 is discussed.Comment: Accepted for publication in The Physical Review Letters. 4 pages,
RevTeX needed, no figure
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