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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
NASA NTRS1984

High-resolution observations of radio-source hot spots at 329 MHz

The hot spots of several luminous double radio sources have been observed at 329 MHz with VLBI at fringe spacings ranging from 0.05 to 0.70 arcsec. Two out of two high-redshift (z = about 0.5) sources, but only one of six low-redshift (z = about 0.05) sources, were detected. For the low-redshift source (3C 234) which was detected, either the hot spot is larger at 329 MHz than at 15 GHz, or else the brightness contrast between it and the surrounding lobe is lower at the lower frequency.

Linfield, R.↗
NASA NTRS1986

Interstellar scattering of compact radio sources near supernova remnants

A multifrequency VLBI search for interstellar scattering of extragalactic radio sources near supernova remnants is reported. VLBI observations at 610, 1663, and 4991 MHz were made of compact sources near the supernova remnants CTA 1, G33.6 + 0.1, G74.9 + 1.2, and HB 21, and 610 MHz observations were also made of a source near HB 9. These observations were motivated by the possibility of enhanced cosmic ray-induced turbulence in front of supernova remnants, as expected in 'diffusive' theories of shock wave acceleration. Angular broadening is definitely seen in the case of the source 2013 + 370, which lies within 4 arcmin of the supernova remnant G74.9 + 1.2. Present observations cannot unambiguously attribute the scattering material to the supernova remnant, as the line of sight also passes through the Cygnus OB1 association. The source 1849 + 005 appears to be highly scattered, as fringes were not detected even on short baselines at 5 GHz. This result may be due to the low galactic longitude of this source rather than its proximity to the supernova remnant G 33.6 + 0.1. Broadening was not detected for sources whose lines of sight pass close to the supernova remnants HB 9, HB 21, and CTA 1.

Spangler, S. R.↗
NASA NTRS1987

A complete sample of flat-spectrum radio sources from the Parkes 2.7 GHz survey

A complete sample of about 400 flat-spectrum radio sources from the Parkes 2700 MHz catalog with fluxes of greater than 0.5 is presented. The present sample covers all right ascensions and declinations from +10 deg to -45 deg, but excludes the galactic plane. No significant difference is found in the number-magnitude or number-redshift distributions for the flat spectrum and steep-spectrum radio quasars, in agreement with previous results for stronger sources.

Savage, A.↗
NASA NTRS1988

Compact radio sources in the galactic plane

The preliminary results of a search for infrared counterparts to compact continuum radio sources found within 0.5 deg of the Galactic plane are presented. Out of 75 positions searched, nine very red sources, each with a probability of less than 0.01 of being a random field star, were found. Most of the nine sources are characterized by deep silicate absorption features at 10 microns and a red energy distribution that continues to rise beyond 25 microns. Six of the sources are best explained as late O or early B stars exciting a compact H II region. The radio emission is close to that expected for the rate of emission of ionizing photons for a normal main-sequence star. Although these H II regions are compact, they are not sufficiently dusty in the immediate environment of the star to significantly reduce the ionizing flux. Three of the sources show considerably more emission than expected for a main-sequence star, indicating they are either hotter than a main-sequence star with the same luminosity or the radio emision is nonthermal and coming from a more exotic object.

Jones, Terry Jay↗
NASA NTRS1983

Detection of a compact radio source near the center of a gravitational lens - Quasar image or galactic core

By use of a new, very sensitive interferometric system, a faint, compact radio source has been detected near the center of the galaxy that acts as the main part of a gravitational lens. This lens forms two previously discovered images of the quasar Q0957+561, which lies in the direction of the constellation Ursa Major. The newly detected source has a core smaller than 0.002 arc second in diameter with a flux density of 0.6 + or - 0.1 millijansky at the 13-centimeter wavelength of the radio observations. This source could be the predicted third image of the transparent gravitational lens, the central core of the galaxy, or some combination of the two. It is not yet possible to choose reliably between these alternatives.

Gorenstein, M. V.↗
NASA NTRS1996

Compact Radio Sources in NGC 660

The Very Large Array (VLA) was used to map compact radio sources in NGC 660, a starburst galaxy, which is also a strong infrared emitter.

NGC 660 starburst galaxies↗
NASA NTRS1997

Two Variable Radio Sources Near the Position of GRB 940301

We report on the results of a search for a radio counterpart to the strong gamma-ray burst GRB 940301. Observations with the Westerbork Synthesis Radio Telescope of the Compton Telescope error box region of GRB 940301 began on March 4, 1994, at 21 cm and April 2, 1994, at 92 cm. No flux density variations were detected at 92 cm above S= 10 mJy (5 (sigma)) within a period of 1 to 4 months after the burst. However, when we compared the field with Westerbork Northern Sky Survey data, taken two years prior to GRB 940301, we found two radio sources with significantly increased flux densities. These sources, only 17 min. apart, are located at the 2.3 and 2.6(sigma) Compton Telescope confidence contours. Their separation from the Inter Planetary Network annulus virtually excludes association with GRB 940301. Further observations in January 1996 reveal that the sources continued to change in flux density. The relatively large flux density variations at 92 cm, compared to those at higher frequencies, and the inverted spectra in the frequency range from 325-38O MHz make the sources somewhat unusual. Because the sources were already detected at 5 GHz in 1986 most, if not all, of the radio emission is probably associated with activity in Active Galactic Nuclei in distant galaxies.

Galama, T. J.↗
NASA NTRS1987

The triple radio source 0023 + 171 - A candidate for a dark gravitational lens

In the course of a radio-optical search for gravitational lenses, a multiple radio source, 0023 + 171, was discovered, which has two optical counterparts at z = 0.946 separated by 5 arcsec. Their redshifts are the same within the measurement error, and their optical spectra show the same emission lines with the same relative intensities. One interpretation of these measurements is that the images of 0023 + 171 are gravitationally lensed. However, because the radio morphology is complex and the optical objects are faint, the existing data cannot exclude another interpretation, i.e., that the components of 0023 + 171 are two physically associated radio galaxies or two components of a single galaxy. The only possible evidence in favor of a lensing object is extremely faint optical emission located approximately 1 arcsec from one of the images. If this multiple source is indeed gravitationally lensed, the combination of available data imply a mass-to-light ratio for the lensing matter equal to about 1000.

Hewitt, J. N.↗
NASA NTRS1994

Results of a five-year program of multifrequency monitoring of low-frequency variable radio sources

We present the results of a detailed multifrequency monitoring program of low-frequency variable radio sources. This consists of light curves at 318, 430, 606, 880, and 1400 MHz over a 5 year period. The observations were carried out with the Arecibo 305 m radiotelescope and the Green Bank 91 m radio telescope. The spectral characteristics of the variations confirm the general picture that at least two mechanisms are responsible. The first is clearly intrinsic evolution of synchrotron-emitting components as it is manifested by variations that appear first and most strongly at high frequencies, subsequently drifting to lower frequencies with diminished amplitude. The more common type of low-frequency variability, however, dominates at frequencies below approximately 800 MHz, while the variations near 1 GHz are often quite weak. This spectral property is strong evidence that these variations are interstellar refractive scintillation.

Mitchell, K. J.↗
NASA NTRS1973

Extragalactic radio sources - Accurate positions from very-long-baseline interferometry observations

Relative positions for 12 extragalactic radio sources have been determined via wide-band very-long-baseline interferometry (wavelength of about 3.8 cm). The standard error, based on consistency between results from widely separated periods of observation, appears to be no more than 0.1 sec for each coordinate of the seven sources that were well observed during two or more periods. The uncertainties in the coordinates determined for the other five sources are larger, but in no case exceed 0.5 sec.

Rogers, A. E. E.↗
NASA NTRS1977

The small radio source at the galactic center

Very-long-baseline interferometer observations show that the compact radio source at the Galactic Center has dimensions of approximately 200 AU and that about 25% of the emission comes from a region only 10 AU across. There is no evidence for any expansion or contraction of this compact source, with a velocity of at least a few tens of kilometers per second.

Kellermann, K. I.↗
NASA NTRS1980

VLBI measurements of radio source positions at the Jet Propulsion Laboratory

The results of approximately 1300 observations of 67 radio sources are presented. Most of the measurements were made at the stations of the Deep Space Network in California, Spain, and Australia at wavelengths of 13.1 and 3.6 cm, between 1971 and 1978. The formal errors in the derived source positions are generally in the neighborhood of 0.01 seconds of arc and the positions agree fairly well with those previously published.

Purcell, G. H., Jr.↗
NASA NTRS1979

Submilliarcsecond astrometry via VLBI. I - Relative position of the radio sources 3C 345 and NRAO 512

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.

Shapiro, I. I.↗
NASA NTRS1987

Occultation of compact radio sources by the ion tail of Halley's Comet

Enhancements of scintillations of the compact radio sources PKS 2314+03 and 1827-360 were observed at 103 MHz and 408 MHz during 18-21 December 1985 and on 29 March 1986, respectively, when the plasma tail of Halley's Comet swept across them. At 103 MHz the RMS plasma density variation along the tail was 10 and 3.3/cu cm at 0.12 AU and 0.18 AU, respectively, as measured from the comet's position. At 408 MHz it was 1.9/cu cm at 0.036 AU. Comparison of results of these two sets of observations is presented.

Alurkar, S. K.↗
NASA NTRS1982

Confirmation of a conspiracy - Dual-band VLBI maps of the flat-spectrum radio source 2021 + 614

VLBI observations at 3.6 and 13 cm wavelengths of the flat-spectrum radio source 2021 + 614 reveal two components, separated by about 6.5 + or - 0.6 milli-arcseconds along a position angle of 36 deg + or - 6 deg. The dual-band observations show that the two components have spectral indices of opposite signs, indicating that, as for 0735 + 178 (Cotton et al. 1980), the flat spectrum of 2021 + 614 is a result of the superposition of spectra from individual components, each having a peaked spectrum, presumably due to incoherent electron synchrotron radiation. If this situation is typical, then there is too large a percentage of flat-spectrum sources for the individual components in each to have independent characteristics.

Wittels, J. J.↗