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Point source
About: Point source is a research topic. Over the lifetime, 5077 publications have been published within this topic receiving 94091 citations.
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TL;DR: In this article, the authors study the effect of interference on the concentration statistics and in particular on the variance of the concentration fluctuations, and propose a model based on the well-known fluctuating plume model, by which they can extend their simulation results to atmospheric situations.
34 citations
TL;DR: In this article, the magnetic field geometry and the transport processes of the cosmic rays of the edge-on spiral starburst galaxy NGC 4666 from CHANG-ES radio continuum data in two frequencies; 6 GHz (C-band) and 1.5 GHz (L-band).
Abstract: We analyze the magnetic field geometry and the transport processes of the cosmic rays of the edge-on spiral starburst galaxy NGC 4666 from CHANG-ES radio continuum data in two frequencies; 6 GHz (C-band) and 1.5 GHz (L-band). Supplementary X-ray data are used to investigate the hot gas in NGC 4666. We determine the radio scale heights of total power emission at both frequencies for this galaxy. We show the magnetic field orientations derived from the polarization data. Using rotation measure (RM) synthesis we further study the behavior of the RM values along the disk in C-band to investigate the large-scale magnetic-field pattern. We use the revised equipartition formula to calculate a map of the magnetic field strength. Furthermore, we model the processes of cosmic-ray transport into the halo with the 1D SPINNAKER model. The extended radio halo of NGC 4666 is box-shaped and is probably produced by the previously observed supernova-driven superwind. This is supported by our finding of an advective cosmic-ray transport such as that expected for a galactic wind. The scaleheight analysis revealed an asymmetric halo above and below the disk as well as between the two sides of the major axis. A central point source as well as a bubble structure is seen in the radio data for the first time. Our X-ray data show a box-shaped hot halo around NGC 4666 and furthermore confirm the AGN nature of the central source. NGC 4666 has a large-scale X-shaped magnetic field in the halo, as has been observed in other edge-on galaxies. The analysis furthermore revealed that the disk of NGC 4666 shows hints of field reversals along its radius, which is the first detection of this phenomenon in an external galaxy.
34 citations
TL;DR: In this article, a model based on the interaction-by-exchange-with-the-conditional-mean (IECM) approach is formulated to calculate concentration fluctuation statistics for a line source and a point source in inhomogeneous and non-Gaussian turbulence in the convective boundary layer.
Abstract: The micromixing technique, widely used in engineering calculations of mixing and chemical reaction, is extended to atmospheric boundary-layer flows. In particular, a model based on the interaction-by-exchange-with-the-conditional-mean (IECM) micromixing approach is formulated to calculate concentration fluctuation statistics for a line source and a point source in inhomogeneous and non-Gaussian turbulence in the convective boundary layer. The mixing time scale is parameterised as a linear function of time with the intercept value determined by the source size at small times. Good agreement with laboratory data for the intensity of concentration fluctuations is obtained with a value of 0.9 for the coefficient of the linear term in the time-scale parameterisation for a line source, and a value of 0.6 for a point source. Calculation of higher-order moments of the concentration field for a line source shows that non-Gaussian effects persist into the vertically well-mixed region. The cumulative distribution function predicted by the model for a point source agrees reasonably well with laboratory data, especially in the far field. In the limit of zero mixing time scale, the model reduces to a meandering plume model, thus enabling the concentration variance to be partitioned into meandering and relative components. The meandering component is shown to be more persistent for a point source than for a line source.
33 citations
TL;DR: In this paper, the ultraluminous X-ray source in NGC 6946 associated with the optical nebula MF 16 was detected in the far UV using the Hubble Space Telescope.
Abstract: We present Hubble Space Telescope observations in the far UV of the ultraluminous X-ray source in NGC 6946 associated with the optical nebula MF 16. Both a point-like source coincident with the X-ray source and the surrounding nebula are detected in the FUV. The point source has a flux of 5 x 10{sup -16} erg s{sup -1} cm{sup -2} A{sup -1}, and the nebula has a flux of 1.6 x 10{sup -15} erg s{sup -1} cm{sup -2} A{sup -1}, quoted at 1533 A and assuming an extinction of A{sub V} = 1.54. Thus, MF 16 appears to host the first directly detected ultraluminous UV source. The flux of the point-like source is consistent with a blackbody with T{approx} 30,000 K, possibly from a massive companion star, but this spectrum does not create sufficient ionizing radiation to produce the nebular He II flux, and a second, hotter emission component would be required. A multicolor disk blackbody spectrum truncated with an outer disk temperature of {approx}16,000 K provides an adequate fit to the FUV, B, V, I, and He II fluxes and can produce the needed ionizing radiation. Additional observations are required to determine the physical nature of the source.
33 citations
TL;DR: In this paper, the authors derived quantum lower bounds on the error of locating point sources in free space, taking full account of the quantum, non-paraxial, and vectoral nature of photons.
Abstract: Motivated by the importance of optical microscopes to science and engineering, scientists have pondered for centuries how to improve their resolution and the existence of fundamental resolution limits. In recent years, a new class of microscopes that overcome a long-held belief about the resolution have revolutionized biological imaging. Termed "superresolution" microscopy, these techniques work by accurately locating optical point sources from far field. To investigate the fundamental localization limits, here I derive quantum lower bounds on the error of locating point sources in free space, taking full account of the quantum, nonparaxial, and vectoral nature of photons. These bounds are valid for any measurement technique, as long as it obeys quantum mechanics, and serve as general no-go theorems for the resolution of microscopes. To arrive at analytic results, I focus mainly on the cases of one and two classical monochromatic sources with an initial vacuum optical state. For one source, a lower bound on the root-mean-square position estimation error is on the order of $\lambda_0/\sqrt{N}$, where $\lambda_0$ is the free-space wavelength and $N$ is the average number of radiated photons. For two sources, owing to the statistical effect of nuisance parameters, the error bound diverges when their radiated fields overlap significantly. The use of squeezed light to enhance further the accuracy of locating one classical point source and the localization limits for partially coherent sources and single-photon sources are also discussed. The presented theory establishes a rigorous quantum statiscal inference framework for the study of superresolution microscopy and points to the possibility of using quantum techniques for true resolution enhancement.
33 citations