2,833 research outputs found
TeV Neutrinos from Successful and Choked Gamma-Ray Bursts
Core collapse of massive stars resulting in a relativistic fireball jet which
breaks through the stellar envelope is a widely discussed scenario for
gamma-ray burst production. For very extended or slow rotating stars, the
fireball may be unable to break through the envelope. Both penetrating and
choked jets will produce, by photo-meson interactions of accelerated protons, a
burst of neutrinos with energies in excess of 5 TeV while propagating in the
envelope. The predicted flux, from both penetrating and chocked fireballs,
should be easily detectable by planned cubic kilometer neutrino telescopes.Comment: Phys.Rev.Letters, in press, final version accepted 8/31/01 (orig.
3/17/01
Galactic Anisotropy as Signature of ``Top-Down'' Mechanisms of Ultra-High Energy Cosmic Rays
We show that ``top-down'' mechanisms of Ultra-High Energy Cosmic Rays which
involve heavy relic particle-like objects predict Galactic anisotropy of
highest energy cosmic rays at the level of minimum . This anisotropy
is large enough to be either observed or ruled out in the next generation of
experiments.Comment: 8 pages, 1 figure, LaTeX. Final version appeared in Pisma Zh. Eksp.
Teor. Fi
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
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
No Radio Afterglow from the Gamma-Ray Burst of February 28, 1997
We present radio observations of the gamma-ray burster GRB 970228 made with
the Very Large Array (VLA) and the Owens Valley Radio Observatory (OVRO)
spanning a range of postburst timescales from one to 300 days. A search for a
time-variable radio source was conducted covering an area which included a
fading X-ray source and an optical transient, both of which are thought to be
the long wavelength counterparts to the gamma-ray burst. At the position of the
optical transient sensitive limits between 10 uJy and 1 mJy can be placed on
the absence of a radio counterpart to GRB 970228 between 1.4 and 240 GHz. We
apply a simple formulation of a fireball model which has been used with some
success to reproduce the behavior of the optical and X-ray light curves. Using
this model we conclude that the radio non-detections are consistent with the
peak flux density of the afterglow lying between 20-40 uJy and it requires that
the optical flux peaked between 4 and 16 hours after the burst.Comment: ApJ Let (submitted
Statistics of clustering of ultra-high energy cosmic rays and the number of their sources
Observation of clustering of ultra-high energy cosmic rays (UHECR) suggests
that they are emitted by compact sources. Assuming small deflection of UHECR
during the propagation, the statistical analysis of clustering allows to
estimate the spatial density of the sources, h, including those which have not
yet been observed directly. When applied to astrophysical models involving
extra-galactic sources, the estimate based on 14 events with energy E>10^{20}
eV gives h ~ 6 X 10^{-3} Mps^{-3}. With increasing statistics, this estimate
may lead to exclusion of the models which associate the production of UHECR
with exceptional galaxies such as AGN, powerful radio-galaxies, dead quasars,
and models based on gamma ray bursts.Comment: The version accepted for publication in Phys. Rev. Lett. Notations
changed to conventional ones. The estimate of the effective GZK radius
replaced by the result of numerical simulatio
Food Recognition using Fusion of Classifiers based on CNNs
With the arrival of convolutional neural networks, the complex problem of
food recognition has experienced an important improvement in recent years. The
best results have been obtained using methods based on very deep convolutional
neural networks, which show that the deeper the model,the better the
classification accuracy will be obtain. However, very deep neural networks may
suffer from the overfitting problem. In this paper, we propose a combination of
multiple classifiers based on different convolutional models that complement
each other and thus, achieve an improvement in performance. The evaluation of
our approach is done on two public datasets: Food-101 as a dataset with a wide
variety of fine-grained dishes, and Food-11 as a dataset of high-level food
categories, where our approach outperforms the independent CNN models
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
Implications of the -ray Polarization of GRB 021206
We compare two possible scenarios for the producing of high level of
polarization within the prompt emission of a GRB: synchrotron emission from a
relativistic jet with a uniform (in space and time) magnetic field and
synchrotron emission from a jet with a random magnetic field in the plane of
the shock. Somewhat surprisingly we find that both scenarios can produce a
comparable level of polarization (% for the uniform field and % for a random field). Uniform time independent field most naturally
arises by expansion of the field from the compact object. It requires a
G field at the source and a transport of the field as . It {\it does not} imply Poynting flux domination of the energy of the
wind. There is a serious difficulty however, within this scenario, accounting
for particle acceleration (which requires random magnetic fields) both for
Poynting flux and non-Poynting flux domination. Significant polarization can
also arise from a random field provided that the observer is located within
orientation from a narrow () jet. While most
jets are wider, the jet of GRB 021206 from which strong polarization was
recently observed, was most likely very narrow. GRB 021206 is among the
strongest bursts ever. Adopting the energy-angle relation we find an estimated
angle of rad or even smaller. Thus, for this particular burst the
required geometry is not unusual. We conclude that the RHESSI observations
suggest that the prompt emission results from synchrotron radiation. However,
in view of the comparable levels of polarizations predicted by both the random
field and the homogeneous field scenarios these observations are insufficient
to rule out or confirm either one.Comment: 14 pages, 4 figure
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