375 research outputs found
Effects of the ISM on Detection of Low-frequency Gravitational Waves
Time variable delays due to radio wave propagation in the ionized
interstellar medium are a substantial source of error in pulsar timing array
efforts. We describe the physical origin of these effects, discussing
dispersive and scattering effects separately. Where possible, we give estimates
of the magnitude of timing errors produced by these effects and their scaling
with radio frequency. Although there is general understanding of the
interstellar medium propagation errors to be expected with pulsar timing array
observations, detailed comparison between theory and practice is still in its
infancy, particularly with regard to scattering effects.Comment: 18 pages, 9 figures. Accepted by Classical and Quantum Gravity for
Focus Issue on Pulsar Timing Array
Summary of the nondestructive evaluation session
Advanced material systems and designs require improved nondestructive evaluation (NDE) techniques for quality control, failure analysis and properties monitoring. Some advanced NDE techniques such as infrared scanning systems, laser holographic applications and the use of the Mossbauer effect are outlined. Applications of these techniques are discussed for evaluating the new space shuttle external insulation materials, aircraft structures and engine components, and precursors to crack initiation and growth
Characterization of Open Circuit Voltage and Capacity as a Function of Time
It is noted from the data presented that Li/BCX cells lose approximately 8% of their rated capacity in the first 2 months of storage. After this period of time, little difference is noted in the average realized capacity; however, a significant increase in the range is observed. Over the same period of time the OCV falls at a rate of 0.02V per 1,000 hours. After a period of 8,000 hours the OCV appears stabilize at a value of approximately 3.75V. This may be related to changes in Cl2 concentration due to self-discharge or other reactions. These data indicate that no correlation exists between the reduced open circuit voltage and the realized capacity
Improvements in safety testing of lithium cells
A systematic approach was developed for evaluating the basic safety parameters of high power lithium soluble cathode cells. This approach consists of performing a series of tests on each cell model during the design, prototype and production phases. Abusive testing is performed in a facility where maximum protection is given to test personnel
Diffractive and refractive timescales at 4.8 GHz in PSR B0329+54
We present the results of flux density monitoring of PSR B0329+54 at the
frequency of 4.8 GHz using the 32-meter TCfA radiotelescope. The observations
were conducted between 2002 and 2005. The main goal of the project was to find
interstellar scintillation (ISS) parameters for the pulsar at the frequency at
which it was never studied in detail. To achieve this the 20 observing sessions
consisted of 3-minute integrations which on average lasted 24 hours.
Flux density time series obtained for each session were analysed using
structure functions. For some of the individual sessions as well as for the
general average structure function we were able to identify two distinctive
timescales present, the timescales of diffractive and refractive
scintillations. To the best of our knowledge, this is the first case when both
scintillation timescales, t_DISS=42.7 minutes and t_RISS=305 minutes, were
observed simultaneously in a uniform data set and estimated using the same
method.
The obtained values of the ISS parameters combined with the data found in the
literature allowed us to study the frequency dependence of these parameters
over a wide range of observing frequencies, which is crucial for understanding
the ISM turbulence. We found that the Kolmogorov spectrum is not best suited
for describing the density fluctuations of the ISM, and a power-law spectrum
with beta =4 seems to fit better with our results. We were also able to
estimate the transition frequency (transition from strong to weak scintillation
regimes) as 10.1 GHz, much higher than was previously predicted. We were also
able to estimate the strength of scattering parameter u=2.67$ and the Fresnel
scale as 6.7x10^8 meters.Comment: 11 pages, 4 figures, accepted for publication in: Astronomy and
Astrophysic
Special nondestructive techniques for evaluating space shuttle surface insulation
NDT techniques have been developed for performing in-process evaluations for material variability and for process control. Several of these techniques show considerable promise for evaluating the reusable surface insulation during the operational phase of the shuttle. Considered are radiographic dosimetry, sonic velocity and modulus sounding, infrared coating evaluation, and beta backscatter monitoring of coating thickness
Ellipticity and Deviations from Orthogonality in the Polarization Modes of PSR B0329+54
We report on an analysis of the polarization of single pulses of PSR B0329+54
at 328 MHz. We find that the distribution of polarization orientations in the
central component diverges strongly from the standard picture of orthogonal
polarization modes (OPMs), making a remarkable partial annulus on the Poincare
sphere. A second, tightly clustered region of density appears in the opposite
hemisphere, at a point antipodal to the centre of the annulus. We argue that
this can be understood in terms of birefringent alterations in the relative
phase of two elliptically polarized propagation modes in the pulsar
magnetosphere (i.e. generalised Faraday rotation). The ellipticity of the modes
implies a significant charge density in the plasma, while the presence of both
senses of circular polarization, and the fact that only one mode shows the
effect, supports the view that refracted ordinary-mode rays are involved in the
production of the annulus. At other pulse longitudes the polarization
(including the circular component) is broadly consistent with an origin in
elliptical OPMs, shown here quantitatively for the first time, however
considerable non-orthogonal contributions serve to broaden the orientation
distribution in an isotropic manner.Comment: 13 pages, 5 figures, to appear in A&
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