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  • 1
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    Meter-wavelength VLBI. I - Cassiopeia A (1974)
    In:  Other Sources
    Details
    Publikationsdatum: 2011-08-16
    Beschreibung: Very-long-baseline interferometric observations of the supernova remnant Cassiopeia A, at 74 MHz with a 12,000-wavelength baseline and at 111 MHz with a 18,500-wavelength baseline, are reported. The fringe amplitudes are strongly varying on a time scale of about 15 to 30 minutes, which is attributed to much the same complex structure as that observed at higher frequencies, plus one other compact source. Due to the poor (u, nu)-plane coverage, the location of the extra source can not be isolated unambiguously, but possibilities are suggested. The source must lie outside the supernova remnant shell, possibly associated with a concentration of emission north of the shell, or lying outside the gap in the northeastern side of the shell. The flux and spectral index deduced for the compact source depend on the assumed size, with a range of 100 Jy and 500 Jy at 74 MHz. If the source is associated with the supernova explosion, it must have been traveling at least 5000 km/sec.
    Schlagwort(e): ASTRONOMY
    Materialart: Astronomical Journal; 79; Nov. 197
    Format: text
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  • 2
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    Submilliarcsecond astrometry via VLBI. I - Relative position of the radio sources 3C 345 and NRAO 512 (1979)
    In:  Other Sources
    Details
    Publikationsdatum: 2019-06-27
    Beschreibung: The relative position and relative proper motion of the radio sources 3C 345 and NRAO 512 are estimated from four sets of VLBI observations spread out over the period from October 1971 to May 1974. The use of phase-connection techniques yields the separation, in 1950.0 coordinates, of the centers of brightness of the compact components of the two sources. An upper bound of 0.0005 arcsec/yr is placed on the relative proper motion (70% estimated confidence limits). Bounds that can be placed on the distances to the two sources are considered, prospects for improvement in the determination of relative position and proper motion of these sources are discussed, and other possible applications of the basic technique are described.
    Schlagwort(e): ASTRONOMY
    Materialart: Astronomical Journal; 84; Oct. 197
    Format: text
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  • 3
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    Radio source positions from very-long-baseline interferometry observations (1976)
    In:  Other Sources
    Details
    Publikationsdatum: 2019-06-27
    Beschreibung: Accurate positions of compact radio sources have been determined from very-long-baseline interferometry (VLBI) observations based on the bandwidth-synthesis technique. The coordinates for 18 extragalactic sources were obtained from sets of observations spread over the period from April 1972 to January 1975; the scatter among the independent determinations of the source coordinates from the separate sets of observations is about 0.05 arcsec, except for the declinations of near-equatorial sources, where the scatter is about 0.15 arcsec. Comparison of these positions with those determined with the Cambridge 5-km radio interferometer shows the rms scatter about the mean difference to be about 0.04 arcsec in each coordinate (no sources of low declination were in common). A similar comparison of the present results with those obtained by the Jet Propulsion Laboratory from separate VLBI observations yields a slightly larger rms scatter, after exlusion of the declinations of the near-equatorial sources. A position is also obtained for the galactic object Beta Persei (Algol), which agrees well with the position given in the FK 4 catalogue.
    Schlagwort(e): ASTRONOMY
    Materialart: Astronomical Journal; 81; Aug. 197
    Format: text
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  • 4
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    Meter-wavelength VLBI. III - Pulsars (1976)
    In:  Other Sources
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    Publikationsdatum: 2019-06-27
    Beschreibung: Observations of pulsars, especially the Crab Nebula pulsar, made in very long baseline interferometry (VLBI) experiments are discussed. Based on a crude 144 MHz visibility curve which is consistent with a Gaussian brightness distribution, the measured visibilities at 196, 111, and 74 MHz were interpreted to yield apparent angular diameters (at half-power) of about 0.03 sec, 0.07 sec, and 0.18 sec, respectively. These sizes scale approximately as wavelength-squared, and the 74 MHz size agrees with recent observations using interplanetary scintillation techniques. The total flux densities lie on the extrapolation from higher frequencies of the pulsing flux densities. Variations in the total flux density up to 25 per cent were observed. A lack of fine structure other than the pulsar in the nebula is indicated by the simple visibility curves. The pulse shapes are similar to single-dish measurements at 196 MHz but reveal a steady, nonpulsing component at 111 MHz. The ratio of pulsing to total power was approximately equal to one-half but varied with time. It was found that four strong, low-dispersion pulsars were only slightly resolved.
    Schlagwort(e): ASTRONOMY
    Materialart: Astrophysical Journal; 207; Aug. 1
    Format: text
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  • 5
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    Meter-wavelength VLBI. II - The observations (1975)
    In:  Other Sources
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    Publikationsdatum: 2019-06-27
    Beschreibung: Observations made during a series of meter-wavelength very-long-baseline (VLBI) experiments conducted during 1971-1973 are reported. A wide variety of objects was observed, including many extragalactic sources known to have compact components, several strong pulsars (especially the Crab nebula pulsar), and a selection of supernova remnants. The experiments are discussed in detail, and tests made to check the consistency of the results are described. The measured correlated flux densities are presented for each source as a function of interferometer baseline and observing frequency. An assessment is made of the measurement errors and of the possibility of confusion within the interferometer beam. The results show that all sources except the pulsars, which are known to have intrinsically small sizes, are resolved on the longest baseline. We also conclude that no supernova remnants except the Crab nebula and Cassiopeia A gave evidence of compact components.
    Schlagwort(e): ASTRONOMY
    Materialart: Astronomical Journal; 80; Nov. 197
    Format: text
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  • 6
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    Very-long-baseline radio interferometry - The Mark III system for geodesy, astrometry, and aperture synthesis (1983)
    In:  Other Sources
    Details
    Publikationsdatum: 2019-06-28
    Beschreibung: Up to 112 megabit/sec from each radio telescope of an interferometric array can be recorded and processed by the Mark III VLBI system. Sample results are given for baseline lengths between three antennas in the U.S. and three in Europe, as well as for the arc lengths between the positions of six extragalactic radio sources. No significant change is detected in any of these quantities. Signals of a given polarization or of pairs of orthogonal polarizations may be recorded in up to 28 contiguous bands, each nearly 2 MHz wide, for mapping the brightness distribution of compact radio sources. The demonstrated ability to record large bandwidths, and to link many large radio telescopes, allows compact sources with flux densities below 1 millijansky to be detected and studied.
    Schlagwort(e): ASTRONOMY
    Materialart: Science; 219; Jan. 7
    Format: text
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  • 7
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    A high declination search at 8 GHz for compact radio sources (1978)
    In:  Other Sources
    Details
    Publikationsdatum: 2019-06-27
    Beschreibung: With the Haystack-NRAO interferometer (baseline length of 20 million wavelengths at 3.8 cm) 37 sources were observed whose declinations were above 50 deg. Seven of these sources have compact cores with diameters smaller than 5 milliarcsec and with correlated flux densities greater than about 0.5 Jy; the remaining sources have no cores with flux densities above about 0.3 Jy, the sensitivity limit of the interferometer. Two of the sources with detected compact cores, 4C 67.05 and 3C 418, were also observed with longer-baseline interferometers; the diameter of the core of 4C 67.05 was estimated to be smaller than 1 milliarcsec and that of 3C 418 to be smaller than 0.4 milliarcsec. All diameter estimates were based on an assumed circular Gaussian distribution of radio brightness and refer to the contour with brightness density e to the -1/2 power times that of the center. Positions for the detected sources were also obtained from the interferometric data, the uncertainty in these coordinate estimates ranging from 0.04 to 0.6 arcsec. The compact core detected in 3C 390.3 was found to lie near the center of this extended (approximately 4 arcmin in diameter) double radio source and to be coincident to within 1 arcsec with an N galaxy previously identified with 3C 390.3.
    Schlagwort(e): ASTRONOMY
    Materialart: Astronomical Journal; 83; June 197
    Format: text
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  • 8
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    VLBI observations of 3C 345 and NRAO 512 in right and left circular polarization (1978)
    In:  Other Sources
    Details
    Publikationsdatum: 2019-06-27
    Beschreibung: In October 1975, the radio telescopes of the Haystack, National Radio Astronomy, and Owens Valley Radio Observatories were used as an interferometer to monitor, at 8 GHz, the right and left circularly polarized radiation emitted by the quasars 3C 345 and NRAO 512. The data for each polarization are used separately to estimate several parameters describing a model of the fine structure of the radio brightness of 3C 345 and, subsequently, the angular separation between 3C 345 and NRAO 512. The results for the two polarizations are in approximate agreement, indicating that to the limit of resolution about 0.5 milliarcsec), the fourth Stokes parameter, V, is not significantly different from zero within the compact components of these radio sources. The corresponding quantitative limits on the degree of circular polarization are 0.08 + or - 0.07 for 3C 345 and 0.01 + or - 0.08 for NRAO 512.
    Schlagwort(e): ASTRONOMY
    Materialart: Astrophysical Journal; vol. 220
    Format: text
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  • 9
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    An unusually strong radio outburst in Algol - VLBI observations (1976)
    In:  Other Sources
    Details
    Publikationsdatum: 2019-06-27
    Beschreibung: For 8 hours during a strong radio flare on Jan. 15, 16, 1975, the close binary system Beta Persei (Algol) was observed with a three-station VLBI array operating at 7850 MHz. The size of the radio source was estimated to have been about 1.7 milliarcsec (0.05 AU), based on a model of a uniformly bright disk. The corresponding brightness temperature was nearly 10 billion K, indicating that the emission was probably nonthermal. There was no evidence for expansion of the source; the upper limit on the rate of any expansion was 100 km/s. The position of the Algol radio source with respect to an extragalactic reference frame was also determined from these observations with an uncertainty of about 0.1 arcsec in each coordinate.
    Schlagwort(e): ASTRONOMY
    Materialart: Astrophysical Journal; 206; June 1
    Format: text
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  • 10
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    Position and morphology of the compact non-thermal radio source at the Galactic Center (1992)
    In:  Other Sources
    Details
    Publikationsdatum: 2019-07-13
    Beschreibung: We have determined with VLBI the position of the compact nonthermal radio source at the Galactic Center, commonly referred to as SgrA*, in the J2000.0 reference frame of extragalactic radio sources. We have also determined the size of SgrA* at 1.3, 3.6, and 13 cm wavelengths and found that the apparent size of the source increases proportionally to the observing wavelength squared, as expected from source size broadening by interstellar scattering and as reported previously by other authors. We have also established an upper limit of about 8 mJy at 3.6 cm wavelength for any ultracompact component. The actual size of the source is less than 15 AU. Fourier analysis of our very sensitive 3.6 cm observations of this source shows no significant variations of correlated flux density on time scales from 12 to 700 s.
    Schlagwort(e): ASTRONOMY
    Materialart: Astronomy and Astrophysics (ISSN 0004-6361); 258; 2; p. 295-301.
    Format: text
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