17,188 research outputs found
Interstellar Scintillation of the Polarized Flux Density in Quasar, PKS 0405-385
The remarkable rapid variations in radio flux density and polarization of the
quasar PKS 0405-385 observed in 1996 are subject to a correlation analysis,
from which characteristic time scales and amplitudes are derived. The
variations are interpreted as interstellar scintillations. The cm wavelength
observations are in the weak scintillation regime for which models for the
various auto- and cross-correlations of the Stokes parameters are derived and
fitted to the observations. These are well modelled by interstellar
scintillation (ISS) of a 30 by 22 micro-as source, with about 180 degree
rotation of the polarization angle along its long dimension. This success in
explaining the remarkable intra-day variations (IDV)in polarization confirms
that ISS gives rise to the IDV in this quasar. However, the fit requires the
scintillations to be occurring much closer to the Earth than expected according
to the standard model for the ionized interstellar medium (IISM). Scattering at
distances in the range 3-30 parsec are required to explain the observations.
The associated source model has a peak brightness temperature near 2.0
10^{13}K, which is about twenty-five times smaller than previously derived for
this source. This reduces the implied Doppler factor in the relativistic jet,
presumed responsible to 10-20, high but just compatible with cm wavelength VLBI
estimates for the Doppler factors in Active Galactic Nuclei (AGNs).Comment: 43 pages 15 figures, accepted for ApJ Dec 200
High-speed, high-frequency ultrasound, \u3ci\u3ein utero\u3c/i\u3e vector-flow imaging of mouse embryos
Real-time imaging of the embryonic murine cardiovascular system is challenging due to the small size of the mouse embryo and rapid heart rate. High-frequency, linear-array ultrasound systems designed for small-animal imaging provide high-frame-rate and Doppler modes but are limited in regards to the field of view that can be imaged at fine-temporal and -spatial resolution. Here, a plane-wave imaging method was used to obtain high-speed image data from in utero mouse embryos and multi-angle, vector-flow algorithms were applied to the data to provide information on blood flow patterns in major organs. An 18-MHz linear array was used to acquire plane-wave data at absolute frame rates ≥10 kHz using a set of fixed transmission angles. After beamforming, vector-flow processing and image compounding, effective frame rates were on the order of 2 kHz. Data were acquired from the embryonic liver, heart and umbilical cord. Vector-flow results clearly revealed the complex nature of blood-flow patterns in the embryo with fine-temporal and -spatial resolution
Interstellar Scintillation Observations of 146 Extragalactic Radio Sources
From 1979--1996 the Green Bank Interferometer was used by the Naval Research
Laboratory to monitor the flux density from 146 compact radio sources at
frequencies near 2 and 8 GHz. We filter the ``light curves'' to separate
intrinsic variations on times of a year or more from more rapid interstellar
scintilation (ISS) on times of 5--50 d. Whereas the intrinsic variation at 2
GHz is similar to that at 8 GHz (though diminished in amplitude), the ISS
variation is much stronger at 2 than at 8 GHz. We characterize the ISS
variation by an rms amplitude and a timescale and examine the statistics of
these parameters for the 121 sources with significant ISS at 2 GHz. We model
the scintillations using the NE2001 Galactic electron model assuming the
sources are brightness-limited.
We find the observed rms amplitude to be in general agreement with the model,
provided that the compact components of the sources have about 50% of their
flux density in a component with maximum brightness temperatures
--K. Thus our results are consistent with cm-wavelength VLBI
studies of compact AGNs, in that the maximum brightness temperatures found are
consistent with the inverse synchrotron limit at K, boosted
in jet configurations by Doppler factors up to about 20. The average of the
observed 2 GHz ISS timescales is in reasonable agreement with the model at
Galactic latitudes above about 10\de. At lower latitudes the observed
timescales are too fast, suggesting that the transverse plasma velocity
increases more than expected beyond about 1 kpc.Comment: 32 pages, 16 figures. Submitted to Ap
Observational appearance of rapidly rotating neutron stars: X-ray bursts, cooling tail method, and radius determination
Neutron stars (NSs) in low-mass X-ray binaries rotate at frequencies high
enough to significantly deviate from sphericity ( 200--600 Hz). We
investigate the effects of rapid rotation on the observational appearance of a
NS. We propose analytical formulae relating gravitational mass and equatorial
radius of the rapidly rotating NS to the mass and radius of a
non-rotating NS of the same baryonic mass using accurate fully relativistic
computations. We compute spectra from an oblate rotating NS observed at
different inclination angles using the modified oblate Schwarzschild (MOS)
approximation, where light bending is computed in Schwarzschild metric, but
frame dragging and quadrupole moment of a NS are approximately accounted for in
the photon redshift calculations. We generalize the cooling tail method to the
case of a rapidly rotating NS to obtain the most probable values of and
of the corresponding non-rotating NS with the same baryonic mass. We
approximate the local spectra from the NS surface by a diluted blackbody using
previously computed NS atmosphere models. We show that the NS radius could be
overestimated by 3--3.5 km for face-on stars of km rotating at
700 Hz if the version of the cooling tail method for a non-rotating
NS is used. We apply the method to an X-ray burst observed from the NS rotating
at 532 Hz in SAX J1810.82609. The resulting radius of the
non-rotating NS (assuming ) becomes km if it is
viewed at inclination i=60 deg and km for a face-on view, which
are smaller by 0.6 and 1.2 km than the radius obtained using standard cooling
tail method ignoring rotation. The corresponding equatorial radii of these
rapidly rotating NSs are 12.3 km (for i=60 deg) and 11.6\,km
(for i=0 deg).Comment: 17 pages, 16 figures, accepted for publication in Astronomy and
Astrophysic
The GNSS-R Eddy Experiment II: L-band and Optical Speculometry for Directional Sea-Roughness Retrieval from Low Altitude Aircraft
We report on the retrieval of directional sea-roughness (the full directional
mean square slope, including MSS, direction and isotropy) through inversion of
Global Navigation Satellite System Reflections (GNSS-R) and SOlar REflectance
Speculometry (SORES)data collected during an experimental flight at 1000 m. The
emphasis is on the utilization of the entire Delay-Doppler Map (for GNSS-R) or
Tilt Azimuth Map (for SORES) in order to infer these directional parameters.
Obtained estimations are analyzed and compared to Jason-1 measurements and the
ECMWF numerical weather model.Comment: Proceedings from the 2003 Workshop on Oceanography with GNSS
Reflections, Barcelona, Spain, 200
Time resolved tracking of a sound scatterer in a turbulent flow: non-stationary signal analysis and applications
It is known that ultrasound techniques yield non-intrusive measurements of
hydrodynamic flows. For example, the study of the echoes produced by a large
number of particle insonified by pulsed wavetrains has led to a now standard
velocimetry technique. In this paper, we propose to extend the method to the
continuous tracking of one single particle embedded in a complex flow. This
gives a Lagrangian measurement of the fluid motion, which is of importance in
mixing and turbulence studies. The method relies on the ability to resolve in
time the Doppler shift of the sound scattered by the continuously insonfied
particle.
For this signal processing problem two classes of approaches are used:
time-frequency analysis and parametric high resolution methods. In the first
class we consider the spectrogram and reassigned spectrogram, and we apply it
to detect the motion of a small bead settling in a fluid at rest. In more
non-stationary turbulent flows where methods in the second class are more
robust, we have adapted an Approximated Maximum Likelihood technique coupled
with a generalized Kalman filter.Comment: 16 pages 9 figure
High-frequency ultrasonic speckle velocimetry in sheared complex fluids
High-frequency ultrasonic pulses at 36 MHz are used to measure velocity
profiles in a complex fluid sheared in the Couette geometry. Our technique is
based on time-domain cross-correlation of ultrasonic speckle signals
backscattered by the moving medium. Post-processing of acoustic data allows us
to record a velocity profile in 0.02--2 s with a spatial resolution of 40
m over 1 mm. After a careful calibration using a Newtonian suspension, the
technique is applied to a sheared lyotropic lamellar phase seeded with
polystyrene spheres of diameter 3--10 m. Time-averaged velocity profiles
reveal the existence of inhomogeneous flows, with both wall slip and shear
bands, in the vicinity of a shear-induced ``layering'' transition. Slow
transient regimes and/or temporal fluctuations can also be resolved and exhibit
complex spatio-temporal flow behaviors with sometimes more than two shear
bands.Comment: 15 pages, 18 figures, submitted to Eur. Phys. J. A
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