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

Nanoarcsecond single-dish imaging of the Vela pulsar

We have measured the properties of the diffractive scintillation toward the Vela pulsar under the extremely strong scattering conditions encountered at 660 MHz. We obtain a decorrelation bandwidth of $\nu_d = 244 \pm 4$ Hz and diffractive decorrelation timescale of $t_{\rm diff} = 3.3\pm 0.3$ s. Our measurement of the modulation indices $m=0.87\pm 0.003\pm 0.05$ and $m=0.93\pm 0.03 \pm 0.05$ (one for each polarization stream), are at variance with the modulation index of the Vela pulsar obtained at 2.3 GHz by Gwinn et al. (1997) {\it if} the deviation from a modulation index of unity is ascribed to a source size effect.Comment: 4 pages, 1 figure, to appear in Proc. IAUC177 "Pulsar Astronomy - 2000 and Beyond" Eds. M. Kramer, N. Wex & R. Wielebinski (ASP Conf. Series; uses newpasp.sty

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&

Electric field representation of pulsar intensity spectra

Pulsar dynamic spectra exhibit high visibility fringes arising from interference between scattered radio waves. These fringes may be random or highly ordered patterns, depending on the nature of the scattering or refraction. Here we consider the possibility of decomposing pulsar dynamic spectra -- which are intensity measurements -- into their constituent scattered waves, i.e. electric field components. We describe an iterative method of achieving this decomposition and show how the algorithm performs on data from the pulsar B0834+06. The match between model and observations is good, although not formally acceptable as a representation of the data. Scattered wave components derived in this way are immediately useful for qualitative insights into the scattering geometry. With some further development this approach can be put to a variety of uses, including: imaging the scattering and refracting structures in the interstellar medium; interstellar interferometric imaging of pulsars at very high angular resolution; and mitigating pulse arrival time fluctuations due to interstellar scattering.Comment: 7 Pages, 2 Figures, revised version, accepted by MNRA