9,220 research outputs found
Extraordinary focusing of sound above a soda can array without time reversal
Recently, Lemoult et al. [Phys. Rev. Lett. 107, 064301 (2011)] used time
reversal to focus sound above an array of soda cans into a spot much smaller
than the acoustic wavelength in air. In this study, we show that equally sharp
focusing can be achieved without time reversal, by arranging transducers around
a nearly circular array of soda cans. The size of the focal spot at the center
of the array is made progressively smaller as the frequency approaches the
Helmholtz resonance frequency of a can from below, and, near the resonance,
becomes smaller than the size of a single can. We show that the locally
resonant metamaterial formed by soda cans supports a guided wave at frequencies
below the Helmholtz resonance frequency. The small focal spot results from a
small wavelength of this guided wave near the resonance in combination with a
near field effect making the acoustic field concentrate at the opening of a
can. The focusing is achieved with propagating rather than evanescent waves. No
sub-diffraction-limited focusing is observed if the diffraction limit is
defined with respect to the wavelength of the guided mode in the metamaterial
medium rather than the wavelength of the bulk wave in air
What absorbs the early TeV photons of GRB 221009A?
The tera-electronvolt (TeV) light curve of gamma-ray burst (GRB) 221009A
shows an unprecedentedly rapid rise at the beginning epoch. This phenomenon
could be due to the strong absorption of photons and electrons within the
emitting region. As the external shock expands outwards and the radius
increases, the volume of matter also increases, leading to a gradual decrease
in the optical depth for TeV photons. We explore several possibilities for the
physical origin of this peculiar behavior. We calculate the optical depth for
TeV photons due to annihilation with lower energy photons in the external shock
and scattering by electrons produced via cascading of the TeV emission. Even
under aggressive assumptions, we find the optical depths for these processes
are orders of magnitude too small to explain the observed light curve. Other
sources of absorbers, such as electrons in the ejecta or external shock, also
do not yield sufficient optical depths. Therefore, the origin of the early
peculiar TeV light curve remains uncertain
The Radio Properties of Radio-Loud Narrow-Line Seyfert 1 Galaxies on Parsec Scales
We present the detection of compact radio structures of fourteen radio-loud
narrow line Seyfert 1 (NLS1) galaxies from Very Long Baseline Array
observations at 5 GHz, which were performed in 2013. While 50\% of the sources
of our sample show a compact core only, the remaining 50\% exhibit a core-jet
structure. The measured brightness temperatures of the cores range from
to K with a median value of K, indicating
that the radio emission is from non-thermal jets, and that, likely, most
sources are not strongly beamed, then implying a lower jet speed in these
radio-loud NLS1 galaxies. In combination with archival data taken at multiple
frequencies, we find that seven sources show flat or even inverted radio
spectra, while steep spectra are revealed in the remaining seven objects.
Although all these sources are very radio-loud with , their jet
properties are diverse, in terms of their milli-arcsecond (mas) scale (pc
scale) morphology and their overall radio spectral shape. The evidence for slow
jet speeds (i.e., less relativistic jets), in combination with the low
kinetic/radio power, may offer an explanation for the compact VLBA radio
structure in most sources. The mildly relativistic jets in these high accretion
rate systems are consistent with a scenario, where jets are accelerated from
the hot corona above the disk by the magnetic field and the radiation force of
the accretion disk. Alternatively, a low jet bulk velocity can be explained by
low spin in the Blandford-Znajek mechanism.Comment: 39 pages, 17 figures, ApJS accepte
NeuroImage 84 (2014) 657–671 Contents lists available at ScienceDirect
journal homepage: www.elsevier.com/locate/ynimg Spatial–temporal modelling of fMRI data through spatially regularize
Quantum delayed-choice experiment with a beam splitter in a quantum superposition
A quantum system can behave as a wave or as a particle, depending on the
experimental arrangement. When for example measuring a photon using a
Mach-Zehnder interferometer, the photon acts as a wave if the second
beam-splitter is inserted, but as a particle if this beam-splitter is omitted.
The decision of whether or not to insert this beam-splitter can be made after
the photon has entered the interferometer, as in Wheeler's famous
delayed-choice thought experiment. In recent quantum versions of this
experiment, this decision is controlled by a quantum ancilla, while the beam
splitter is itself still a classical object. Here we propose and realize a
variant of the quantum delayed-choice experiment. We configure a
superconducting quantum circuit as a Ramsey interferometer, where the element
that acts as the first beam-splitter can be put in a quantum superposition of
its active and inactive states, as verified by the negative values of its
Wigner function. We show that this enables the wave and particle aspects of the
system to be observed with a single setup, without involving an ancilla that is
not itself a part of the interferometer. We also study the transition of this
quantum beam-splitter from a quantum to a classical object due to decoherence,
as observed by monitoring the interferometer output.Comment: 9 pages, 7 figures, Accepted by Physical Review Letter
Possible dibaryons in the quark cluster model
In the framework of RGM, the binding energy of one channel
() and are studied in the
chiral SU(3) quark cluster model. It is shown that the binding energies of the
systems are a few tens of MeV. The behavior of the chiral field is also
investigated by comparing the results with those in the SU(2) and the extended
SU(2) chiral quark models. It is found that the symmetry property of the
system makes the contribution of the relative kinetic energy
operator between two clusters attractive. This is very beneficial for forming
the bound dibaryon. Meanwhile the chiral-quark field coupling also plays a very
important role on binding. The S-wave phase shifts and the corresponding
scattering lengths of the systems are also given.Comment: LeTex with 2 ps figure
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