3,379 research outputs found
Observation of infinite-range intensity correlations above, at and below the 3D Anderson localization transition
We investigate long-range intensity correlations on both sides of the
Anderson transition of classical waves in a three-dimensional (3D) disordered
material. Our ultrasonic experiments are designed to unambiguously detect a
recently predicted infinite-range C0 contribution, due to local density of
states fluctuations near the source. We find that these C0 correlations, in
addition to C2 and C3 contributions, are significantly enhanced near mobility
edges. Separate measurements of the inverse participation ratio reveal a link
between C0 and the anomalous dimension \Delta_2, implying that C0 may also be
used to explore the critical regime of the Anderson transition.Comment: 13 pages, 11 figures (main text plus supplemental information).
Updated version includes an improved introductory paragraph, minor text
revisions, a revised title and additional supplemental information on the
experimental detail
Mesoscopic phase statistics of diffuse ultrasound in dynamic matter
Temporal fluctuations in the phase of waves transmitted through a dynamic,
strongly scattering, mesoscopic sample are investigated using ultrasonic waves,
and compared with theoretical predictions based on circular Gaussian
statistics. The fundamental role of phase in Diffusing Acoustic Wave
Spectroscopy is revealed, and phase statistics are also shown to provide a
sensitive and accurate way to probe scatterer motions at both short and long
time scales.Comment: 4 pages, 4 figures, submitted to Physical Review Letter
Viking navigation
A comprehensive description of the navigation of the Viking spacecraft throughout their flight from Earth launch to Mars landing is given. The flight path design, actual inflight control, and postflight reconstruction are discussed in detail. The preflight analyses upon which the operational strategies and performance predictions were based are discussed. The inflight results are then discussed and compared with the preflight predictions and, finally, the results of any postflight analyses are presented
FACT - Threshold prediction for higher duty cycle and improved scheduling
The First G-APD Cherenkov telescope (FACT) is the first telescope using
silicon photon detectors (G-APD aka. SiPM). The use of Silicon devices promise
a higher photon detection efficiency, more robustness and higher precision than
photo-multiplier tubes. Being operated during different light-conditions, the
threshold settings of a Cherenkov telescope have to be adapted to feature the
lowest possible threshold but also an efficient suppression of triggers from
night-sky background photons. Usually this threshold is set either by
experience or a mini-ratescan. Since the measured current through the sensors
is directly correlated with the noise level, the current can be used to set the
best threshold at any time. Due to the correlation between the physical
threshold and the final energy threshold, the current can also be used as a
measure for the energy threshold of any observation. This presentation
introduces a method which uses the properties of the moon and the source
position to predict the currents and the corresponding energy threshold for
every upcoming observation allowing to adapt the observation schedule
accordingly
Continuum and CO/HCO+ Emission from the Disk Around the T Tauri Star LkCa 15
We present OVRO Millimeter Array lambda = 3.4 - 1.2 mm dust continuum and
spectral line observations of the accretion disk encircling the T Tauri star
LkCa 15. The 1.2 mm dust continuum emission is resolved, and gives a minimum
diameter of 190 AU and an inclination angle of 57+/-5 degrees. There is a
noticeable, but at present poorly constrained, decrease in the continuum
spectral slope with frequency that may result from the coupled processes of
grain growth and dust settling. Imaging of the fairly intense emission from the
lowest rotational transitions of CO, 13CO and HCO+ reveals a rotating disk and
emission extends to 750 AU and the characteristic radius of the disk is
determined to be around 425 AU (HWHM) based on model fits to the CO velocity
field. The disk mass derived from the CO isotopologues with ``typical'' dense
cloud abundances is still nearly two orders of magnitude less than that
inferred from the dust emission, which is probably due to extensive molecular
depletion in the cold, dense disk midplane. N2H+ 1-0 emission has also been
detected which, along with HCO+, sets a lower limit to the fractional
ionization of 10^{-8} in the near-surface regions of protoplanetary disks. This
first detection of N2H+ in circumstellar disks has also made possible a
determination of the N2/CO ratio (~2) that is at least an order of magnitude
larger than those in the envelopes of young stellar objects and dense clouds.
The large N2/CO ratio indicates that our observations probe disk layers in
which CO is depleted but some N2 remains in the gas phase. Such differential
depletion can lead to large variations in the fractional ionization with height
in the outer reaches of circumstellar disks, and may help to explain the
relative nitrogen deficiency observed in comets.Comment: Submitted to ApJ, 28 pages, 7 figure
FACT - Long-term stability and observations during strong Moon light
The First G-APD Cherenkov Telescope (FACT) is the first Cherenkov telescope
equipped with a camera made of silicon photon detectors (G-APD aka. SiPM).
Since October 2011, it is regularly taking data on the Canary Island of La
Palma. G-APDs are ideal detectors for Cherenkov telescopes as they are robust
and stable. Furthermore, the insensitivity of G-APDs towards strong ambient
light allows to conduct observations during bright Moon and twilight. This gain
in observation time is essential for the long-term monitoring of bright TeV
blazars. During the commissioning phase, hundreds of hours of data (including
data from the the Crab Nebula) were taken in order to understand the
performance and sensitivity of the instrument. The data cover a wide range of
observation conditions including different weather conditions, different zenith
angles and different light conditions (ranging from dark night to direct full
Moon). We use a new parmetrisation of the Moon light background to enhance our
scheduling and to monitor the atmosphere. With the data from 1.5 years, the
long-term stability and the performance of the camera during Moon light is
studied and compared to that achieved with photomultiplier tubes so far.Comment: 3 pages, 3 figures, FACT Contribution to the 33rd International
Cosmic Ray Conference (ICRC), Rio de Janeir
FACT - How stable are the silicon photon detectors?
The First G-APD Cherenkov telescope (FACT) is the first telescope using
silicon photon detectors (G-APD aka. SiPM). The use of Silicon devices promise
a higher photon detection efficiency, more robustness and higher precision than
photo-multiplier tubes. Since the properties of G-APDs depend on auxiliary
parameters like temperature, a feedback system adapting the applied voltage
accordingly is mandatory.
In this presentation, the feedback system, developed and in operation for
FACT, is presented. Using the extraction of a single photon-equivalent (pe)
spectrum as a reference, it can be proven that the sensors can be operated with
very high precision. The extraction of the single-pe, its spectrum up to
10\,pe, its properties and their precision, as well as their long-term behavior
during operation are discussed. As a by product a single pulse template is
obtained. It is shown that with the presented method, an additional external
calibration device can be omitted. The presented method is essential for the
application of G-APDs in future projects in Cherenkov astronomy and is supposed
to result in a more stable and precise operation than possible with
photo-multiplier tubes
Transverse confinement of ultrasound through the Anderson transition in 3D mesoglasses
We report an in-depth investigation of the Anderson localization transition
for classical waves in three dimensions (3D). Experimentally, we observe clear
signatures of Anderson localization by measuring the transverse confinement of
transmitted ultrasound through slab-shaped mesoglass samples. We compare our
experimental data with predictions of the self-consistent theory of Anderson
localization for an open medium with the same geometry as our samples. This
model describes the transverse confinement of classical waves as a function of
the localization (correlation) length, (), and is fitted to our
experimental data to quantify the transverse spreading/confinement of
ultrasound all of the way through the transition between diffusion and
localization. Hence we are able to precisely identify the location of the
mobility edges at which the Anderson transitions occur.Comment: 16 pages, 11 figure
FACT - Long-term Monitoring of Bright TeV-Blazars
Since October 2011, the First G-APD Cherenkov Telescope (FACT) is operated
successfully on the Canary Island of La Palma. Apart from the proof of
principle for the use of G-APDs in Cherenkov telescopes, the major goal of the
project is the dedicated long-term monitoring of a small sample of bright TeV
blazars. The unique properties of G-APDs permit stable observations also during
strong moon light. Thus a superior sampling density is provided on time scales
at which the blazar variability amplitudes are expected to be largest, as
exemplified by the spectacular variations of Mrk 501 observed in June 2012.
While still in commissioning, FACT monitored bright blazars like Mrk 421 and
Mrk 501 during the past 1.5 years so far. Preliminary results including the Mrk
501 flare from June 2012 will be presented.Comment: 4 pages, 4 figures, presented at the 33rd ICRC (2013
A Millimeter-Wave Galactic Plane Survey With The BICEP Polarimeter
In addition to its potential to probe the Inflationary cosmological paradigm,
millimeter-wave polarimetry is a powerful tool for studying the Milky Way
galaxy's composition and magnetic field structure. Towards this end, presented
here are Stokes I, Q, and U maps of the Galactic plane from the millimeter-wave
polarimeter BICEP covering the Galactic longitude range 260 - 340 degrees in
three atmospheric transmission windows centered on 100, 150, and 220 GHz. The
maps sample an optical depth 1 < AV < 30, and are consistent with previous
characterizations of the Galactic millimeter-wave frequency spectrum and the
large-scale magnetic field structure permeating the interstellar medium.
Polarized emission is detected over the entire region within two degrees of the
Galactic plane and indicates that the large-scale magnetic field is oriented
parallel to the plane of the Galaxy. An observed trend of decreasing
polarization fraction with increasing total intensity rules out the simplest
model of a constant Galactic magnetic field throughout the Galaxy. Including
WMAP data in the analysis, the degree-scale frequency spectrum of Galactic
polarization fraction is plotted between 23 and 220 GHz for the first time. A
generally increasing trend of polarization fraction with electromagnetic
frequency is found, which varies from 0.5%-1.5%at frequencies below 50 GHz to
2.5%-3.5%above 90 GHz. The BICEP and WMAP data are fit to a two-component
(synchrotron and dust) model showing that the higher frequency BICEP data are
necessary to tightly constrain the amplitude and spectral index of Galactic
dust. Furthermore, the dust amplitude predicted by this two-component fit is
consistent with model predictions of dust emission in the BICEP bands
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