Strong Point Sources Research Articles

A directional gamma-ray telescope using coded aperture techniques

A directional detector for γ-ray astronomy has been developed to image sources in the energy range 0.1 to 5 MeV. An array of 35 gain stabilized bismuth germanate detectors, together with a coded aperture mask based on a Uniformly Redundant Array (URA), allows imaging in 4° square sky bins over a 16° X 24° field-of-view. The position of a strong point source, such as the Crab Nebula, can be determined to within ≲1°. A complementary “anti-mask” greatly reduces systematic effects arising from non-uniform background rates amongst the detectors. The telescope has an effective area of 190 cm 2 and an energy resolution of 19.5% FWHM at 662 keV. Results of laboratory tests of the imaging system, including the ability to image multiple sources, uniformity of response over the field-of-view, and the effect of the “anti-mask”, are in good agreement with computer simulations. Features of the flight detector system are described and results of laboratory tests and computer simulations are reviewed. A balloon flight of the telescope is planned for the fall of 1982.

The development of strong 57Co Mössbauer sources for scattering experiments of 57Fe I. The production of large activities of 57Co at the compact cyclotron at Munich

Mössbauer scattering experiments on 57Fe need strong 57Co sources. This paper describes a new and efficient technique to produce 57Co activities of several Ci at the Munich compact cyclotron. Essential to this technique is the development of a rotating target device, which stands a 22 MeV proton beam with currents up to 500 μA. Typical production rates are I Ci 57Co in 10 days. A scattering experiment on a myoglobin crystal proves the advantage of the new strong 57Co point sources.

Evidence for low-energy γ-ray emission close to CG135+1

The COS-B source CG135+1 (ref. 1) is an extremely interesting object because of its possible identification with the QSO 0241+622. Radio observations have identified a strong point source within 2 arc s (ref. 2) of QSO 0241+622. X-ray emissions have also been observed from this region of the sky by the Uhuru (ref. 3), SAS-3 (ref. 2) and HEAO-A (ref. 4) telescopes. The HEAO-A (A1) data indicate that there are two soft X-ray sources in this region of the sky, H0241+62 which contains the QSO, and H0235+60 which includes the radio source GT0236+61 (ref. 5). The OSO-7 data in the energy range 1–40 keV (ref. 6) and measurements using the high-energy X-ray experiment on Ariel-V between 0.26 and 1.2 MeV (ref. 7) have indicated that the photon emission from this region of the sky is of an extremely hard nature, with a power law spectral index of γ≈1.0. However, the poor angular resolution of these telescopes make it uncertain whether the high-energy X-ray fluxes are due to the same celestial object. We now report the results of a balloon flight carried out on 8 October 1978 from Palestine, Texas, in which the region of the sky including the COS-B source CG135+1 was scanned by the Milan/Southampton telescope (MISO telescope). A 5σ excess in the counting rate of the telescope in the energy range 0.15–20 MeV was observed. The origin of this low-energy γ-ray emission is compatible with the measured position of CG135+1.

Nutrient distributions and transport in Long Island Sound

Inorganic nitrogenous nutrient (NH 3, NO 2, NO 3) distributions and transport in Long Island Sound are investigated for both winter and summer conditions with a steady state, one dimensional mass balance model. Nutrient budgets based on horizontal exchange, lateral input from sewage and agricultural sources, and first order biochemical uptake (utilization minus regeneration) are computed for each of 13 regions in the Sound. All nutrient concentrations, principally ammonia, peak sharply in the upper East River where a strong point source exists. Concentration distributions and uptake rates are consistent with previous studies of the nitrogen cycle and productivity of the Sound. However, this paper stresses horizontal exchange and sewage as important and hitherto neglected components of the nutrient budgets in various regions. Estimates of the loss of nitrogen to the sediment are presented for the western, central and eastern regions, from the Sound to the ocean at the eastern end, and into New York Harbor from the upper East River. Sewage effluents are shown to be the prime external source of nutrients for the Sound.

Final SAS-2 gamma-ray results on sources in the galactic anticenter region

Analysis of SAS-2 high energy Gamma ray data from the direction of the galactic anticenter shows that this region is characterized by: a diffuse emission from the galactic plane which has a maximum along b=0 deg and an enhancement toward negative latitudes associated with Gould's Belt, a strong point source in the direction of the Crab nebula, and a second intense localized source near galactic coordinates 195 deg, +5 deg. Gamma ray emission from the Crab source is dominated by a pulsed flux from PSR 0531+21. The total flux above 100MeV is 3.7 + or - 0.8 million/sq cm s. The source near 195 deg, + 5 deg has a flux above 100 MeV of 4.3 + or - 0.9 million/sq cm s. Its spectrum appears flatter than that of the Crab. The diffuse galactic plane emission at negative lattitudes shows a general correlation with the local matter distribution associated with Gould's Belt. The calculated Gamma ray intensity agrees well with the SAS-2 observations.

CCl 3F (Freon -11) as an indicator of transport processes in an urban atmosphere—a case study in Melbourne

In view of the lack of surface uniformity, and the importance of relatively uniform ground level area sources of pollution in an urban environment, point source diffusion experiments are of limited usefulness in studying transport processes in an urban atmosphere. While it is not feasible to carry out controlled area source diffusion experiments, we argue here that CCl 3F is an existing pollutant which for many cities represents a good ground level area source. For cities such as Melbourne where industrial use of CCl 3F occurs, the effect of a few strong point sources superimposed on the area sources may need to be accounted for. As an example of the use of CCl 3F as an urban atmospheric tracer, vertical concentration profiles and simultaneous profiles of relevant meteorological parameters were obtained at Aspendale during a very stable June night. The profiles show a marked variation of concentration with height which is explained qualitatively by the change in wind direction with height and the variation with direction of the source strength (as determined by a source inventory) around Aspendale. The futility of estimating pollutant concentrations at height from ground level measurements is clear from our results, as is the importance of a knowledge of the non-uniformity of wind (both vertically and horizontally) due to local effects, such as cold air drainage.

Explosion of a hydrogen ice sphere due to a pulsed fast neutron source

Using a modified Lax-Wendroff numerical technique, the computer calculations of a hydrogen ice sphere exploding due to an energy deposition by slowed-down fast neutrons radiating from a strong point source are presented. Due to the large energy drop at each collision, the slowing-down process is treated by the approximate method, taking into account only the first three collisions. Calculations include the formation of a shock wave in the air outside the expanding hydrogen sphere. Because of the very large neutron energy and high source intensity, the binding and ionization energy of the hydrogen ice is neglected and is treated as a gas with a specific heat ratio of γ = 3. The neutron source in the numerical example corresponds to the explosion of the d- t micro-bomb of a diameter of about 1.5 mm. Calculations indicate that in order to recover most of the 14 MeV neutron energy, the radius of the hydrogen ice sphere should be of the order of 20 cm.

Modelling urban air pollution

For the usual case of receptors within a city, the area source component of urban air pollution is strongly dominated by the source strength pattern and by transport by the mean wind. Urban area source patterns tend to be fairly uniform and atmospheric diffusion conditions in cities tend to be of the near-neutral type, without the strong diurnal variations found elsewhere. For these reasons a simple area source formula X = c Q/u where X is air pollution concentration, Q is source strength per unit area, and u is average wind speed, is studied. The parameter c is, according to the authors' urban diffusion model, a weak function of city size and should be approximately constant. This formula is compared with means derived from a large quantity of air pollution data, average annual emissions and concentrations of particles for 44 U.S. Cities and SO 2 data for 20 cities, in order to calibrate the constant, c. The constant c is found to equal about 225 for particles, and 50 for SO 2. Standard deviations of c are roughly half their magnitude. The value c = 225 for particles is most representative of the purely meteorological processes governing urban diffusion, since c is low for SO 2 because of the greater influence of strong elevated point sources and of SO 2 decay. It is suggested that this simple formula has wide applicability in air pollution investigations.

Neutron stars and cosmic-ray production

Recent investigations of the properties of neutron stars and of supernova explosions indicate that neutron stars should frequently be formed as supernova remnants. It now appears unlikely that any form of internal energy storage can play an important role in the production of X rays or cosmic rays. If mass infall onto a neutron star occurs at the upper limiting value allowed by radiation stresses, instabilities are likely to make the resulting mass infall, X-ray, gamma-ray, and cosmic-ray production very intermittent. It is shown that such a model may be able to account for many features of the Crab nebula: the energy input in electrons and ions, the character of the fast-moving light ripples or wisps, and the strong point source of ~50 MHz radiation.

21-cm hydrogen-line absorption in the spectra of discrete sources

This paper is a short survey of the results obtained with the Dwingeloo 25-meter radio telescope (beamwidth 0°.56) and the 21-cm receiver on hydrogen-line absorption effects in the spectra of a number of strong point sources. Earlier work on this subject has been done by Hagen, Lilley, and McClain [1], and by Davies and Williams [2]. Some preliminary results of the first part of the present investigation were already published [3].

Additional Interference Fringes Produced by Scattering and Reflexion

IN a recent observation, F. K. Bauchwitz and D. Shoenberg have reported observing a new interference effect. With the eye accommodated for infinity, they observed coloured circular fringes when looking at a strong point source through a thin air-film formed between silvered plates. The interpretation they give is that the light is first multiply reflected between the surfaces and then scattered, and this scattered light is multiply reflected before being observed.

A New Interference Effect

IN the course of a class experiment demonstrating the circular Haidinger fringes produced by monochromatic illumination of a thin film between silvered surfaces, it was noticed that coloured circular fringes could be seen if white light illumination was used. The effect is best seen by looking through the film (for example, a thin air film between silvered glass plates, or a thin mica film silvered on both sides) with the eye accommodated for infinity, at a strong point source such as a 'Pointolite'. For normal illumination, the point source is seen surrounded by several rainbow fringes, as in Fig. 1, while for oblique illumination, a sharp circular white-light fringe is seen passing through the source, with concentric rainbows inside and outside, as in Fig. 2. Unlike the usual monochromatic fringes, these fringes are not influenced by lack of accurate parallelism of the silvered surfaces. The sharpness of the white-light fringe improves with increase of the reflexion coefficient of the surfaces; with plates heavily silvered by the Tolansky technique1, the fringe width is limited only by the size of the source.