2,686 research outputs found
Geoacoustic Inversion of Ship Radiated Noise in Shallow Water Using Data From a Single Hydrophone
The Centre for Maritime Research and Experimentation conducted a geoacoustic inverse experiment in the Mediterranean Sea in the summer of 2012. Among the objectives was to employ an autono- mous underwater vehicle to collect acoustic data to invert for properties of the seafloor. Inversion results for the compression wave speed in the bottom and the source spectrum of the R/V Alliance during a close approach to the bottom moored vehicle are presented. The estimated wave speed was 1529 m/s (r ¼ 10). The source spectrum of the Alliance was estimated across more than six octaves of frequency
SN1991bg-like supernovae are associated with old stellar populations
SN1991bg-like supernovae are a distinct subclass of thermonuclear supernovae
(SNe Ia). Their spectral and photometric peculiarities indicate their
progenitors and explosion mechanism differ from `normal' SNe Ia. One method of
determining information about supernova progenitors we cannot directly observe
is to observe the stellar population adjacent to the apparent supernova
explosion site to infer the distribution of stellar population ages and
metallicities. We obtain integral field observations and analyse the spectra
extracted from regions of projected radius about the
apparent SN explosion site for 11 91bg-like SNe in both early- and late-type
galaxies. We utilize full-spectrum spectral fitting to determine the ages and
metallicities of the stellar population within the aperture. We find that the
majority of the stellar populations that hosted 91bg-like supernovae have
little recent star formation. The ages of the stellar populations suggest that
that 91bg-like SN progenitors explode after delay times of ,
much longer than the typical delay time of normal SNe Ia, which peaks at .Comment: 12 pages, 3 figures, 3 tables, submitted to Publications of the
Astronomical Society of Australi
Like-charge attraction through hydrodynamic interaction
We demonstrate that the attractive interaction measured between like-charged
colloidal spheres near a wall can be accounted for by a nonequilibrium
hydrodynamic effect. We present both analytical results and Brownian dynamics
simulations which quantitatively capture the one-wall experiments of Larsen and
Grier (Nature 385, p. 230, 1997).Comment: 10 pages, 4 figure
Hydrodynamic Coupling of Two Brownian Spheres to a Planar Surface
We describe direct imaging measurements of the collective and relative
diffusion of two colloidal spheres near a flat plate. The bounding surface
modifies the spheres' dynamics, even at separations of tens of radii. This
behavior is captured by a stokeslet analysis of fluid flow driven by the
spheres' and wall's no-slip boundary conditions. In particular, this analysis
reveals surprising asymmetry in the normal modes for pair diffusion near a flat
surface.Comment: 4 pages, 4 figure
S-band Polarization All Sky Survey (S-PASS): survey description and maps
We present the S-Band Polarization All Sky Survey (S-PASS), a survey of
polarized radio emission over the southern sky at Dec~ taken with
the Parkes radio telescope at 2.3~GHz. The main aim was to observe at a
frequency high enough to avoid strong depolarization at intermediate Galactic
latitudes (still present at 1.4 GHz) to study Galactic magnetism, but low
enough to retain ample Signal-to-Noise ratio (S/N) at high latitudes for
extragalactic and cosmological science. We developed a new scanning strategy
based on long azimuth scans, and a corresponding map-making procedure to make
recovery of the overall mean signal of Stokes and possible, a
long-standing problem with polarization observations. We describe the scanning
strategy, map-making procedure, and validation tests. The overall mean signal
is recovered with a precision better than 0.5\%. The maps have a mean
sensitivity of 0.81 mK on beam--size scales and show clear polarized signals,
typically to within a few degrees of the Galactic plane, with ample S/N
everywhere (the typical signal in low emission regions is 13 mK, and 98.6\% of
the pixels have S/N ). The largest depolarization areas are in the inner
Galaxy, associated with the Sagittarius Arm. We have also computed a Rotation
Measure map combining S-PASS with archival data from the WMAP and Planck
experiments. A Stokes map has been generated, with a sensitivity limited to
the confusion level of 9 mK.Comment: Accepted for publication on MNRAS. Maps are available for download at
the website indicated in the manuscrip
Diffuse Galactic antimatter from faint thermonuclear supernovae in old stellar populations
Our Galaxy hosts the annihilation of a few low-energy
positrons every second. Radioactive isotopes capable of supplying such
positrons are synthesised in stars, stellar remnants, and supernovae. For
decades, however, there has been no positive identification of a main stellar
positron source leading to suggestions that many positrons originate from
exotic sources like the Galaxy's central super-massive black hole or dark
matter annihilation. %, but such sources would not explain the
recently-detected positron signal from the extended Galactic disk. Here we show
that a single type of transient source, deriving from stellar populations of
age 3-6 Gyr and yielding ~0.03 of the positron emitter Ti, can
simultaneously explain the strength and morphology of the Galactic positron
annihilation signal and the solar system abundance of the Ti decay
product Ca. This transient is likely the merger of two low-mass white
dwarfs, observed in external galaxies as the sub-luminous, thermonuclear
supernova known as SN1991bg-like.Comment: 28 pages main text with 4 figures in preprint style; 26 pages of
Supplementary Informatio
Faraday Tomography of the North Polar Spur: Constraints on the distance to the Spur and on the Magnetic Field of the Galaxy
We present radio continuum and polarization images of the North Polar Spur
(NPS) from the Global Magneto-Ionic Medium Survey (GMIMS) conducted with the
Dominion Radio Astrophysical Observatory 26-m Telescope. We fit polarization
angle versus wavelength squared over 2048 frequency channels from 1280 to 1750
MHz to obtain a Faraday Rotation Measure (RM) map of the NPS. Combining this RM
map with a published Faraday depth map of the entire Galaxy in this direction,
we derive the Faraday depth introduced by the NPS and the Galactic interstellar
medium (ISM) in front of and behind the NPS. The Faraday depth contributed by
the NPS is close to zero, indicating that the NPS is an emitting only feature.
The Faraday depth caused by the ISM in front of the NPS is consistent with zero
at b>50 degree, implying that this part of the NPS is local at a distance of
approximately several hundred parsecs. The Faraday depth contributed by the ISM
behind the NPS gradually increases with Galactic latitude up to b=44 degree,
and decreases at higher Galactic latitudes. This implies that either the part
of the NPS at b<44 degree is distant or the NPS is local but there is a sign
change of the large-scale magnetic field. If the NPS is local, there is then no
evidence for a large-scale anti-symmetry pattern in the Faraday depth of the
Milky Way. The Faraday depth introduced by the ISM behind the NPS at latitudes
b>50 degree can be explained by including a coherent vertical magnetic field.Comment: 9 pages, 8 figures, accepted for publication in ApJ. Some figures
have been degraded to reduce sizes, for a high resolution version, see
http://physics.usyd.edu.au/~xhsun/ms_nps.pd
A Detailed Study of the Radio--FIR Correlation in NGC6946 with Herschel-PACS/SPIRE from KINGFISH
We derive the distribution of the synchrotron spectral index across NGC6946
and investigate the correlation between the radio continuum (synchrotron) and
far-infrared (FIR) emission using the KINGFISH Herschel PACS and SPIRE data.
The radio--FIR correlation is studied as a function of star formation rate,
magnetic field strength, radiation field strength, and the total gas surface
brightness. The synchrotron emission follows both star-forming regions and the
so-called magnetic arms present in the inter-arm regions. The synchrotron
spectral index is steepest along the magnetic arms (), while
it is flat in places of giant H{\sc ii} regions and in the center of the galaxy
(). The map of provides an observational
evidence for aging and energy loss of cosmic ray electrons propagating in the
disk of the galaxy. Variations in the synchrotron--FIR correlation across the
galaxy are shown to be a function of both star formation and magnetic fields.
We find that the synchrotron emission correlates better with cold rather than
with warm dust emission, when the interstellar radiation field is the main
heating source of dust. The synchrotron--FIR correlation suggests a coupling
between the magnetic field and the gas density. NGC6946 shows a power-law
behavior between the total (turbulent) magnetic field strength B and the star
formation rate surface density with an index of
0.14\,(0.16)0.01. This indicates an efficient production of the turbulent
magnetic field with the increasing gas turbulence expected in actively star
forming regions. The scale-by-scale analysis of the synchrotron--FIR
correlation indicates that the ISM affects the propagation of old/diffused
cosmic ray electrons, resulting in a diffusion coefficient of \,cm\,s for 2.2\,GeV CREs.Comment: 23 pages, 13 figures, accepted for publication in Astronomy &
Astrophysics Journa
Phase behaviour of a model of colloidal particles with a fluctuating internal state
Colloidal particles are not simple rigid particles, in general an isolated
particle is a system with many degrees of freedom in its own right, e.g., the
counterions around a charged colloidal particle.The behaviour of model
colloidal particles, with a simple phenomenological model to account for these
degrees of freedom, is studied. It is found that the interaction between the
particles is not pairwise additive. It is even possible that the interaction
between a triplet of particles is attractive while the pair interaction is
repulsive. When this is so the liquid phase is either stable only in a small
region of the phase diagram or absent altogether.Comment: 12 pages including 4 figure
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