Point source

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

General plume dispersion model (GPDM) for point source emission

Abstract: Gaussian-based dispersion models are widely used to estimate local pollution levels. The accuracy of such models depends on stability classification schemes as well as plume rise equations. A general plume dispersion model (GPDM) for a point source emission, based on Gaussian plume dispersion equation, was developed. The program complex was developed using Java and Visual basic tools. It has the flexibility of using five kinds of stability classification schemes, i.e., Lapse Rate, Pasquill–Gifford (PG), Turner, σ–θ and Richardson number. It also has the option of using two types of plume rise formulations – Briggs and Holland’s. The model, applicable for both rural and urban roughness conditions, uses meteorological and emission data as its input parameters, and calculates concentrations of pollutant at the center of each cell in a predefined grid area with respect to the given source location. Its performance was tested by comparing with 4-h average field data of continuous releases of SO2 from Dadri thermal power plant (Uttar Pradesh, India). Results showed that the Turner scheme used with Holland’s equation gives the best outcome having a degree of agreement (d) of 0.522.

Love waves due to a point source in an isotropic elastic medium under initial stress

Abstract: The propagation of Love type waves due to a point source in an isotropic inhomogeneous medium under initial stress has been investigated in this paper. Firstly, the propagation of Love waves in a homogeneous crust overlying an inhomogeneous substratum, both under compressive initial stress, due to the presence of a point source in the interface has been considered. Finally, the case in which the upper layer is inhomogeneous and the lower medium is homogeneous has been discussed. The Green's function technique developed by Ghosh (1963) has been used to solve the problem.

A method for correcting aspect solution errors in rosat hri observations of compact sources

Abstract: X-ray point sources observed with the ROSAT High Resolution Imager (HRI) often appear elongated over scales of approximately 5 sec-10 sec from the image core. This elongation has been attributed to errors in the attitude correction as the satellite is wobbled during the observations, and affects sources with both soft and hard X-ray spectra. In this paper, I report the results of an attempt to rid a high signal-to-noise observation of the soft X-ray point source HZ 43 of its characteristic elongation. I divided the observation into 181 separate images, each containing photons from only a small region on the detector through which the source passed during the satellite's wobble. By measuring the positions of the individual image centroids, I found clear evidence for systematic offsets from a common mean by up to approximately +/- 3 sec in both right ascension and declination as a function of phase in the satellite wobble. Shifting the subimages to a common center and then restacking them into a single image measurably improved the symmetry of the point-spread function. HRI observations are wobbled primarily to smooth out variations in the pixel-to-pixel sensitivity of the detector and also to extend the lifetime of the microchannel plates in the detector since these decay at a given location as a function of the number of photons detected. However, the elongations introduced by the aspect errors inhibit the identification of possible extended X-ray emission associated with sources such as pulsars and active galactic nuclei. In light of these results, I suggest that until the aspect errors are understood, observations of compact sources, where this effect may be important, should not be wobbled.

Analysis of the long-term polarization behaviour of BL Lac

Abstract: An analysis is performed of polarimetric and photometric observations of BL Lac carried out in 1969–1991 at the Astronomical Institute of St. Petersburg State University. The distribution of polarization directions certainly points to the existence of the preferred direction of polarization ($\theta= 22\degr$) close to the direction of the jet observed by VLBI. High polarization degree and colorimetric data are evidence of the synchrotron nature of the variable source. The relative Stokes parameters are distributed normally over some mean value. The correlations between different parameters characterizing the flux density and polarization of the central point source allow us to consider that in BL Lac there is a continually acting source of polarized radiation on which the sources with randomly distributed polarization directions are superimposed.

An α-stable approach to the study of the P(D) distribution of unresolved point sources in CMB sky maps

Abstract: We present a new approach to the statistical study and modelling of number counts of faint point sources in astro- nomical images, i.e. counts of sources whose flux falls below the detection limit of a survey. The approach is based on the theory of α-stable distributions. We show that the non-Gaussian distribution of the intensity fluctuations produced by a generic point source population - whose number counts follow a simple power law - belongs to the α-stable family of distributions. Even if source counts do not follow a simple power law, we show that the α-stable model is still useful in many astrophysical scenarios. With the α-stable model it is possible to totally describe the non-Gaussian distribution with a few parameters which are closely related to the parameters describing the source counts, instead of an infinite number of moments. Using statisti- cal tools available in the signal processing literature, we show how to estimate these parameters in an easy and fast way. We demonstrate that the model proves valid when applied to realistic point source number counts at microwave frequencies. In the case of point extragalactic sources observed at CMB frecuencies, our technique is able to successfully fit the P(D) distribution of deflections and to precisely determine the main parameters which describe the number counts. In the case of the Planck mission, the relative errors on these parameters are small either at low and at high frequencies. We provide a way to deal with the presence of Gaussian noise in the data using the empirical characteristic function of the P(D). The formalism and methods here presented can be very useful also for experiments in other frequency ranges, e.g. X-ray or radio Astronomy.