625 research outputs found
GRB Spikes Could Resolve Stars
GRBs vary more rapidly than any other known cosmological phenomena. The lower
limits of this variability have not yet been explored. Improvements in
detectors would reveal or limit the actual rate of short GRBs. Were microsecond
"spike" GRBs to exist and be detectable, they would time-resolve stellar mass
objects throughout the universe by their gravitational microlensing effect.
Analyzing the time structure of sufficient numbers of GRB spikes would reveal
or limit , , and/or .Comment: 18 pages, 2 figures, in press: ApJ (Letters
Gravitational Waves from Sub-lunar Mass Primordial Black Hole Binaries - A New Probe of Extradimensions
In many braneworld models, gravity is largely modified at the electro-weak
scale ~ 1TeV. In such models, primordial black holes (PBHs) with lunar mass M ~
10^{-7}M_sun might have been produced when the temperature of the universe was
at ~ 1TeV. If a significant fraction of the dark halo of our galaxy consists of
these lunar mass PBHs, a huge number of BH binaries will exist in our
neighborhood. Third generation detectors such as EURO can detect gravitational
waves from these binaries, and can also determine their chirp mass. With a new
detector designed to be sensitive at high frequency bands greater than 1 kHz,
the existence of extradimensions could be confirmed.Comment: 4 pages, 1 figure, typos correcte
Gravitationally Lensed Gamma-Ray Bursts as Probes of Dark Compact Objects
If dark matter in the form of compact objects comprises a large fraction of
the mass of the universe, then gravitational lensing effects on gamma-ray
bursts are expected. We utilize BATSE and Ulysses data to search for lenses of
different mass ranges, which cause lensing in the milli, pico, and femto
regimes. Null results are used to set weak limits on the cosmological abundance
of compact objects in mass ranges from 10 to 10 . A
stronger limit is found for a much discussed universe dominated
by black holes of masses , which is ruled out at the
90% confidence level.Comment: 14 pages, 4 figures, fixed minor corrections. Accepted for
publication in ApJ(L
Optical and evaporative cooling of cesium atoms in the gravito-optical surface trap
We report on cooling of an atomic cesium gas closely above an evanescent-wave
atom mirror. At high densitities, optical cooling based on inelastic
reflections is found to be limited by a density-dependent excess temperature
and trap loss due to ultracold collisions involving repulsive molecular states.
Nevertheless, very good starting conditions for subsequent evaporative cooling
are obtained. Our first evaporation experiments show a temperature reduction
from 10muK down to 300nK along with a gain in phase-space density of almost two
orders of magnitude.Comment: 8 pages, 6 figures, submitted to Journal of Modern Optics, special
issue "Fundamentals of Quantum Optics V", edited by F. Ehlotzk
Limits on the cosmological abundance of supermassive compact objects from a millilensing search in gamma-ray burst data
A new search for the gravitational lens effects of a significant cosmological
density of supermassive compact objects (SCOs) on gamma-ray bursts has yielded
a null result. We inspected the timing data of 774 BATSE-triggered GRBs for
evidence of millilensing: repeated peaks similar in light-curve shape and
spectra. Our null detection leads us to conclude that, in all candidate
universes simulated, is favored for , while in some universes and mass ranges the density
limits are as much as 10 times lower. Therefore, a cosmologically significant
population of SCOs near globular cluster mass neither came out of the
primordial universe, nor condensed at recombination.Comment: 14 pages including 3 figures, appeared 2001 January 2
3D printed barium titanate/poly-(vinylidene fluoride) nano-hybrid with anisotropic dielectric properties
“3D-printed” anisotropy BTNFs/PVDF nanohybrids are successfully fabricated by the FDM technique which is attractive for developing novel functionalities in dielectric devices.</p
Human Gait Analysis in Neurodegenerative Diseases: a Review
This paper reviews the recent literature on technologies and methodologies for quantitative human gait analysis in the context of neurodegnerative diseases. The use of technological instruments can be of great support in both clinical diagnosis and severity assessment of these pathologies. In this paper, sensors, features and processing methodologies have been reviewed in order to provide a highly consistent work that explores the issues related to gait analysis. First, the phases of the human gait cycle are briefly explained, along with some non-normal gait patterns (gait abnormalities) typical of some neurodegenerative diseases. The work continues with a survey on the publicly available datasets principally used for comparing results. Then the paper reports the most common processing techniques for both feature selection and extraction and for classification and clustering. Finally, a conclusive discussion on current open problems and future directions is outlined
- …