1,625 research outputs found
The Gaussian Plasma Lens in Astrophysics. Refraction
We consider the geometrical optics for refraction of a distant radio source
by an interstellar plasma lens, with application to a lens with a Gaussian
electron column density profile. The refractive properties of the lens are
specified completely by a dimensionless parameter, alpha, which is a function
of the wavelength of observation, the lens' electron column density, the
lens-observer distance, and the transverse diameter of the lens. Relative
motion of the observer and lens produces modulations in the source's light
curve. Plasma lenses are diverging so the light curve displays a minimum, when
the lens is on-axis, surrounded by enhancements above the unlensed flux
density. Lensing can also produce caustics, multiple imaging, and angular
position wander of the background source. If caustics are formed, the
separation of the outer caustics can constrain alpha, while the separation of
the inner caustics can constrain the size of the lens. We apply our analysis to
0954+654, a source for which we can identify caustics in its light curve, and
1741-038, for which polarization observations were obtained during and after
the scattering event. We find general agreement between modelled and observed
light curves at 2.25 GHz, but poor agreement at 8.1 GHz. The discrepancies may
result from a combination of lens substructure or anisotropic shape, a lens
that only grazes the source, or unresolved source substructure. Our analysis
places the following constraints on the lenses: Toward 0954+654 (1741-038) the
lens was 0.38 AU (0.065 AU) in diameter, with a peak column density of 0.24 pc
cm^{-3} (1E-4 pc cm^{-3}) and an electron density of 1E5 cm^{-3} (300 cm^{-3}).
The angular wander caused by the lens was 250 mas (0.4 mas) at 2.25 GHz. For
1741-038, we place an upper limit of 100 mG on the lens' magnetic field.Comment: 26 pages, LaTeX2e using AASTeX macro aaspp4, 11 PostScript figures;
to be published in Ap
Проблемы ипотечного кредитования в России на современном этапе
По мере развития в России банковского сектора большую популярность приобрели ипотечные кредиты. Данный вопрос является актуальным в наши дни, так как многие взрослые люди хоть раз задумывались о возможностях использования данного банковского продукта. У ипотеки, как и у других форм кредитования, есть свои плюсы и минусы, хотя люди, задумываясь о приобретении собственного жилья, все же останавливаются на выборе ипотечного кредитования
Total evaporation estimates from a Renosterveld and dryland wheat/fallow surface at the Voëlvlei Nature Reserve (South Africa)
Accurate quantification of the water balance, in particular evapotranspiration, is fundamental in managing water resources, especially in semi-arid areas. The objective of this study was to compare evaporation from endemic vegetation – Renosterveld – and a dryland wheat/fallow cropping system. The study was carried out in the mid-reaches of the Berg River catchment (South Africa), characterised by dryland salinity. Measurements of total evaporation from these 2 land uses were carried out with large aperture scintillometers during window periods from 2005 to 2007. Total evaporation was measured to be higher in Renosterveld than in wheat during the rainy winter season. In the dry summer season, total evaporation from Renosterveld was limited by soil water supply, and vegetation was under water stress. Spatial variability of total evaporation from both wheat/fallow land and Renosterveld was estimated using the Surface Energy Balance Algorithm for Land (SEBAL) model for 3 climatically different years. The scintillometer measurements were used to determine basal crop coefficients for long-term (20 years) simulations with the HYDRUS-1D model to assess temporal variability in total evaporation. Long-term simulations indicated that well-established, deep-rooted Renosterveld uses 39% more water than the shallowrooted wheat/fallow system. A change in land use from Renosterveld to dryland annual crops could therefore affect the soil water balance, cause shallow saline groundwater tables and degradation of soil and water resources.Keywords: evapotranspiration; large aperture scintillometer; Renosterveld; soil water balance; Surface Energy Balance Algorithm for Land (SEBAL); wheat/fallow syste
Freeze gelated porous membranes for periodontal tissue regeneration
Guided tissue regeneration (GTR) membranes have been used for the management of destructive forms of periodontal disease as a means of aiding regeneration of lost supporting tissues, including the alveolar bone, cementum, gingiva and periodontal ligaments (PDL). Currently available GTR membranes are either non-biodegradable, requiring a second surgery for removal, or biodegradable. The mechanical and biofunctional limitations of currently available membranes result in a limited and unpredictable treatment outcome in terms of periodontal tissue regeneration. In this study, porous membranes of chitosan (CH) were fabricated with or without hydroxyapatite (HA) using the simple technique of freeze gelation (FG) via two different solvents systems, acetic acid (ACa) or ascorbic acid (ASa). The aim was to prepare porous membranes to be used for GTR to improve periodontal regeneration. FG membranes were characterized for ultra-structural morphology, physiochemical properties, water uptake, degradation, mechanical properties, and biocompatibility with mature and progenitor osteogenic cells. Fourier transform infrared (FTIR) spectroscopy confirmed the presence of hydroxyapatite and its interaction with chitosan. μCT analysis showed membranes had 85–77% porosity. Mechanical properties and degradation rate were affected by solvent type and the presence of hydroxyapatite. Culture of human osteosarcoma cells (MG63) and human embryonic stem cell-derived mesenchymal progenitors (hES-MPs) showed that all membranes supported cell proliferation and long term matrix deposition was supported by HA incorporated membranes. These CH and HA composite membranes show their potential use for GTR applications in periodontal lesions and in addition FG membranes could be further tuned to achieve characteristics desirable of a GTR membrane for periodontal regeneration
Angular Broadening of Intraday Variable AGN. II. Interstellar and Intergalactic Scattering
We analyze a sample of 58 multi-wavelength, Very Long Baseline Array
observations of active galactic nuclei (AGN) to determine their scattering
properties. Approximately 75% of the sample consists of AGN that exhibit
centimeter-wavelength intraday variability (interstellar scintillation) while
the other 25% do not show intraday variability. We find that interstellar
scattering is measurable for most of these AGN, and the typical broadening
diameter is 2 mas at 1 GHz. We find that the scintillating AGN are typically at
lower Galactic latitudes than the non-scintillating AGN, consistent with the
scenario that intraday variability is a propagation effect from the Galactic
interstellar medium. The magnitude of the inferred interstellar broadening
measured toward the scintillating AGN, when scaled to higher frequencies, is
comparable to the diameters inferred from analyses of the light curves for the
more well-known intraday variable sources. However, we find no difference in
the amount of scattering measured toward the scintillating versus
non-scintillating AGN. A consistent picture is one in which the scintillation
results from localized regions ("clumps") distributed throughout the Galactic
disk, but which individually make little contribution to the angular
broadening. Of the 58 AGN observed, 37 (64%) have measured redshifts. At best,
a marginal trend is found for scintillating (non-scintillating) AGN to have
smaller (larger) angular diameters at higher redshifts. We also use our
observations to try to constrain the possibility of intergalactic scattering.
While broadly consistent with the scenario of a highly turbulent intergalactic
medium, our observations do not place significant constraints on its
properties.Comment: 13 pages, 4 figures; AASTeX format; ApJ in pres
- …