Point source

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

Optoacoustic methods for frequency calibration of ultrasonic sensors

Abstract: The frequency response of ultrasonic detectors is commonly calibrated by finding their sensitivity to incident plane waves at discrete frequencies. For certain applications, such as the emerging field of optoacoustic tomography, it is the response to point sources emitting broadband spectra that needs to be found instead. Although these two distinct sensitivity characteristics are interchangeable in the case of a flat detector and a point source at infinity, it is not the case for detectors with size considerably larger than the acoustic wavelength of interest or those having a focused aperture. Such geometries, which are common in optoacoustics, require direct calibration of the acoustic detector using a point source placed in the relevant position. In this paper, we report on novel cross-validating optoacoustic methods for measuring the frequency response of wideband acoustic sensors. The approach developed does not require pre-calibrated hydrophones and therefore can be readily adopted in any existing optoacoustic measurement configuration. The methods are successfully confirmed experimentally by measuring the frequency response of a common piezoelectric detector having a cylindrically focused shape.

On The Detection of Artefacts in Spectro-Astrometry

Abstract: We demonstrate that artificial bipolar structure can be detected using spectroastrometry when the point spread function (PSF) of a point source suffers distortion in a relatively wide slit. Spectro-Astrometry is a technique which allows us to probe the spatial structure of astronomical sources on milliarcseond (mas) scales making it possible to detect close binaries and to study the geometry and kinematics of outflowing gas on scales much smaller than the seeing or the diffraction limit of the telescope. It is demonstrated that distortion of the PSF, caused by tracking errors of the telescope or unstable active optics during an exposure can induce artificial signals which may be misinterpreted as a real spectro-astrometric signal. Using simulations we show that these may be minimised by using a narrow slit relative to the seeing. Spectra should be obtained at anti-parallel slit position angles (e.g., 0 ◦ and 180 ◦ ) for comparison in order to allow artificial signatures to be identified.

Observing Extended Sources with the \Herschel SPIRE Fourier Transform Spectrometer

Abstract: The Spectral and Photometric Imaging Receiver (SPIRE) on the European Space Agency's Herschel Space Observatory utilizes a pioneering design for its imaging spectrometer in the form of a Fourier Transform Spectrometer (FTS). The standard FTS data reduction and calibration schemes are aimed at objects with either a spatial extent much larger than the beam size or a source that can be approximated as a point source within the beam. However, when sources are of intermediate spatial extent, neither of these calibrations schemes is appropriate and both the spatial response of the instrument and the source's light profile must be taken into account and the coupling between them explicitly derived. To that end, we derive the necessary corrections using an observed spectrum of a fully extended source with the beam profile and the source's light profile taken into account. We apply the derived correction to several observations of planets and compare the corrected spectra with their spectral models to study the beam coupling efficiency of the instrument in the case of partially extended sources. We find that we can apply these correction factors for sources with angular sizes up to \theta_{D} ~ 17". We demonstrate how the angular size of an extended source can be estimated using the difference between the sub-spectra observed at the overlap bandwidth of the two frequency channels in the spectrometer, at 959< u<989 GHz. Using this technique on an observation of Saturn, we estimate a size of 17.2", which is 3% larger than its true size on the day of observation. Finally, we show the results of the correction applied on observations of a nearby galaxy, M82, and the compact core of a Galactic molecular cloud, Sgr B2.

Calculating the Absorbed Dose from Radioactive Patients: The Line-Source Versus Point-Source Model

Abstract: In calculations of absorbed doses from radioactive patients, the activity distribution in such patients is generally assumed to be an unattenuated point source and the dose to exposed individuals at a given distance is therefore calculated using the inverse square law. In many nuclear medicine patients, the activity distribution is widely dispersed and does not simulate a point source. In these cases, a line-source model is proposed to more accurately reflect this extended activity distribution. Methods: Calculations of dose rate per unit activity were performed for a point source and for line sources of lengths of 20, 50, 70, 100, and 174 cm, and the ratios of line-source values to point-source values were calculated. In addition, radionuclide-independent conversion factors, to convert exposure rate constants to dose rates per unit activity, for these line-source lengths at various distances were determined. Results: The calculated values, substantiated by published data, indicate that the inverse square law approximation is not valid for a line source until a certain distance is reached, dependent on the length of the line source. For the 20-, 50-, 70-, 100-, and 174-cm line sources, the dose rate values estimated by the inverse square law approximation are within approximately 10% of the values estimated using the line-source approach at distances of 20, 45, 60, 85, and 145 cm, respectively. At closer distances, use of the point-source model for a patient with an extended activity distribution will overestimate the radiation absorbed dose to exposed individuals, sometimes by a very significant amount. Conclusion: The line-source model is a more realistic and practical approach than the traditional point-source model for determining the dose to individuals exposed to radioactive patients with widespread activity distributions.

Discovery of a Young, Energetic 70.5 ms Pulsar Associated with the TeV Gamma-Ray Source HESS J1837–069

Abstract: We report the discovery of 70.5 ms pulsations from the X-ray source AX J1838.0–0655 using the Rossi X-Ray Timing Explorer ( RXTE). PSR J1838–0655 is a rotation-powered pulsar with spin-down luminosity Ė = 5.5 × 1036 ergs s−1, characteristic age τc ≡ P/2 = 23 kyr, and surface dipole magnetic field strength Bs = 1.9 × 1012 G. It coincides with an unresolved INTEGRAL source and the extended TeV source HESS J1837–069. At an assumed distance of 6.6 kpc by association with an adjacent massive star cluster, the efficiency of PSR J1838–0655 converting spin-down luminosity to radiation is 0.8% for the 2–10 keV ASCA flux, 9% for the 20–300 keV INTEGRAL flux and ~3% for the >200 GeV emission of HESS J1837–069, making it a plausible power source for the latter. A Chandra X-ray observation resolves AX J1838.0–0655 into a bright point source surrounded by a 2' diameter, centrally peaked nebula. The spectra of the pulsar and nebula are each well fitted by power laws, with photon indices Γ = 0.5(0.3–0.7) and Γ = 1.6(1.1–2.0) , respectively. The 2–10 keV X-ray luminosities of the pulsar and nebula are LPSR = 4.6 × 1034 d26.6 ergs s−1 and LPWN = 5.2 × 1033 d26.6 ergs s−1. A second X-ray source adjacent to the TeV emission, AX J1837.3–0652, is resolved into an apparent pulsar/PWN; it may also contribute to HESS J1837–069. The star cluster RSGC 1 may have given birth to one or both pulsars, while fueling TeV emission from the extended PWN with target photons for inverse Compton scattering.