Point source

About: Point source is a research topic. Over the lifetime, 5077 publications have been published within this topic receiving 94091 citations.

The angular distribution of characteristic x radiation and its origin within a solid target

Abstract: Angular distributions of characteristic x-ray intensity have been measured for targets of widely different atomic number with incident electron energies from 0-50 kev. For the light elements, where characteristic radiation is produced mainly by direct electron impact, the experimental results are well represented by a universal set of curves. A point source model of x-ray production explains the main features of these curves. In heavy elements fluorescence generated by the continuous x-ray spectrum is important. Theoretical distributions of continuous fluorescence have been derived by numerical integration, and they agree with the corresponding observed distributions within the limits of experimental error.

High-frequency radio properties of sources in the Fermi-LAT 1-year Point Source Catalogue

Abstract: The high-frequency radio sky, like the gamma-ray sky surveyed by the Fermi satellite, is dominated by flat spectrum radio quasars and BL Lac objects at bright flux levels. To investigate the relationship between radio and gamma-ray emission in extragalactic sources, we have cross-matched the Australia Telescope 20 GHz survey catalog (AT20G) with the Fermi-LAT 1 year Point Source Catalog (1FGL). The 6.0 sr of sky covered by both catalogs ({\delta} 1.\circ 5) contains 5890 AT20G radio sources and 604 1FGL gamma-ray sources. The AT20G source positions are accurate to within ~1 arcsec and, after excluding known Galactic sources, 43% of Fermi 1FGL sources have an AT20G source within the 95% Fermi confidence ellipse. Monte Carlo tests imply that at least 95% of these matches are genuine associations. Only five gamma-ray sources (1% of the Fermi catalog) have more than one AT20G counterpart in the Fermi error box. The AT20G matches also generally support the active galactic nucleus (AGN) associations in the First LAT AGN Catalog. We find a trend of increasing gamma-ray flux density with 20 GHz radio flux density. The Fermi detection rate of AT20G sources is close to 100% for the brightest 20 GHz sources, decreasing to 20% at 1 Jy, and to roughly 1% at 100 mJy. Eight of the matched AT20G sources have no association listed in 1FGL and are presented here as potential gamma-ray AGNs for the first time.We also identify an alternative AGN counterpart to one 1FGL source. The percentage of Fermi sources with AT20G detections decreases toward the Galactic plane, suggesting that the 1FGL catalog contains at least 50 Galactic gamma-ray sources in the southern hemisphere that are yet to be identified.

Signatures of galactic magnetic lensing upon ultra high energy cosmic rays

Abstract: We analyse several implications of lensing by the regular component of the galactic magnetic field upon the observed properties of ultra high energy cosmic rays. Magnetic fields deflect cosmic ray trajectories, causing flux (de)magnification, formation of multiple images of a single source, and time delays. We derive the energy dependence of these effects near the caustics at which the flux amplification of a point source diverges. We show that the large magnification of images around caustics leads to an amplification bias, which can make them dominate the flux in some energy ranges. We argue that clustering in the arrival directions of UHECRs of comparable energy may be due to magnetic lensing around caustics. We show that magnetic lensing can also significantly alter the observed composition of cosmic rays at the highest energies. We also show that the time delay between events from a single image may monotonically decrease with decreasing energy in the neighborhood of a caustic, opposite to its behaviour in normal regions. Lensing effects in the magnetic field model considered are significant for cosmic rays with ratio between energy and electric charge E/Z between 1018 and 5 × 1019 eV approximately. Similar effects may also occur for higher E/Z values if the galactic magnetic field is stronger or more extended than what is assumed here, or in the magnetic field of source galaxies, or even in intergalactic fields.

Turbulent gravitational convection from a point source in a non-uniformly stratified environment

Abstract: We examine the turbulent gravitational convection which develops above a point source of buoyant fluid in a stably stratified environment in which the buoyancy frequency varies with height according to N 2 = N 2 s ( z / z s ) β . This generalizes the classical model of turbulent buoyant plumes rising through uniform and uniformly stratified environments originally developed by Morton et al . (1956). By analogy, the height of rise of a plume with initial buoyancy flux F s has the form H p = A p e p −1/2 F s 1/4 N s −3/4 h p (λ, β) where e p is the entrainment constant for plume motion, A p is an O (1) constant, and the non-dimensional plume height, h p is a function of &λ= A p e p −1/2 F s 1/4 N s −3/4 / z s and β. In the case β>0, the stratification becomes progressively stronger with height, and so plumes are always confined within a finite distance above the origin. Furthermore, the non-dimensional height of rise h decreases with λ. In contrast, in the case β<0, the stratification becomes progressively weaker with height, and so the non-dimensional plume height increases monotonically with λ. For slowly decaying stratification, β>−8/3, the motion is confined within a finite distance above the source. However, for each value of β with β<−8/3, there is a critical value λ c (β) such that for λ c a plume is confined to a region near the source while for λ[ges ]λ c the motion is unbounded. In the unbounded case, the motion asymptotes to the solution for a buoyant plume rising through a uniform environment, with asymptotic buoyancy flux F ∞ (λ) F s . We show that in the limiting case λ=λ c , dividing bounded and unbounded motion, as z →∞ the plume asymptotes to a new similarity solution of the second kind which describes the motion of a plume in a non-uniformly stratified environment. These similarity solutions are unstable in the sense that small perturbations to the initial conditions result in very different behaviour far from the source. Analogous results for an instantaneous release of buoyant fluid from a point source, which forms a thermal, are also presented. The model is applied to describe the motion of plumes and thermals in the upper ocean and in naturally ventilated buildings since in both cases the stratification is typically non-uniform.

Localization of Luminescent Inhomogeneities in Turbid Media with Spatially Resolved Measurements of cw Diffuse Luminescence Emittance.

Abstract: We present a steady-state method for localizing a source of luminescence (i.e., fluorescence or phosphorescence) buried in a semi-infinite turbid medium with unknown optical properties. A diffusion theory expression describing the emittance of an isotropic point source is fit to spatially resolved surface measurements of the diffuse emittance from the luminescent source. The technique reports the location of the center of a 6.0-mm-diameter, fluorophore-containing spherical bulb embedded in a liquid phantom with an accuracy of 1.0 mm or better for source depths as great as 40.0 mm. Monte Carlo data are analyzed to investigate the range and the possible sources of error in the reconstructed source depth.