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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 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
NASA NTRS1980

A new method of measuring radio source parameters of a partially polarized distributed source from spacecraft observations

An experimental system is analyzed which is designed to determine the angular and polarization properties of low frequency radio sources from measurements made on a spinning spacecraft. The system has been optimized to provide high accuracies for sources at high as well as low elevation angles. Theoretical expressions are derived for the response of this system to a partially polarized point source. Integrations are then carried out to get the system response to a uniform circular distributed source. Data processing techniques are derived so that computer simulations can be carried out to investigate the accuracy of this technique. It is shown that using 24 measurements of a partially polarized source (with Q = U = V = 0.5), taken in one spacecraft rotation, the resulting rms errors in angular position are less than one degree and the errors in determining the Stokes parameters are generally 1-10% for a wide range of source elevations.

Manning, R.↗
NASA NTRS1987

Millimeter-wave spectra and variability of bright, compact radio sources

Observations at 2.7 mm and at 1.5 cm were used to study the millimeter spectra and variability of 176 bright, compact radio sources. More than 20 percent of the flat-spectrum sources, but none of the steep-spectrum sources, were seen to vary at 1.5 cm by at least 30 percent over ten months. This is consistent with the hypothesis that flat-spectrum sources are compact and possibly beamed, while steep-spectrum sources are not. These data can also be used to choose sources for VLBI observations and for calibration of millimeter-wave observations.

Edelson, R. A.↗
NASA NTRS1974

The earth as a radio source - Terrestrial kilometric radiation

Based on Imp 6 and 8 satellite observation data, a comprehensive study of terrestrial kilometric radiation is presented. In the light of these data, the earth appears to be a very intense planetary radio source, with a total power output comparable to the decametric radio emission from Jupiter. Terrestrial kilometric (i.e., about 50-500 kHz) radiation seems to originate from low altitudes in the auroral region.

Gurnett, D. A.↗
NASA NTRS1985

Observations of the radio source G6.6-0.1 positionally coincident with the W28 SNR

This paper reports scaled-array continuum and H76-alpha line observations of the radio source G6.6-0.1 which is positioned at the center of the W28 SNR. The source exhibits a bright core surrounded by extended emission which appears organized into arcs and wisps. G6.6-0.1 is interpreted as a compact H II region with a unique morphology.

Andrews, M. D.↗
NASA NTRS1991

An unusually strong Einstein ring in the radio source PKS1830 - 211

High-resolution radio images of PKS1830 - 211 are obtained to study the possibility that the double structure is a gravitationally lensed object. The VLBI observations, taken from interferometric radiotelescope networks, reveal an elliptical ring that connects two bright spots of similar composition. Because the lens and the lensed object are closely aligned, and because of the structure of the two spots, the source is concluded to be a radio Einstein ring. The source is found to be close to the galactic plane, and the lens and the lensed object are extragalactic. The source is also found to be unusually bright, suggesting that it is aligned with a bright background source or amplified by some mechanism related to a source that is not so bright.

Jauncey, D. L.↗
NASA NTRS1988

Redshifts of southern radio sources. VII

Redshifts and low-resolution spectral data are presented for 47 objects, most of which are QSOs identified with flat-spectrum radio sources from the Parkes 2.7 GHz survey. These data were taken with the 3.9 m Anglo-Australian Telescope using both the IPCS and FORS spectrographs. The total spectral coverage is 3200-9500 A. Three objects are optical counterparts identified with IRAS sources.

White, Graeme L.↗
NASA NTRS1986

Narrow-angle tail radio sources and evidence for radial orbits in Abell clusters

Published observational data on the tail orientations (TOs) of 60 narrow-angle-tail (NAT) radio sources in Abell clusters of galaxies are analyzed statistically using a maximum-likelihood approach. The results are presented in a table, and it is found that the observed TO distributions in the whole sample and in subsamples of morphologically regular NATs and NATs with pericentric distances d greater than 500 kpc are consistent with isotropic orbits, whereas the TOs for NATs with d less than 500 kpc are consistent with highly radial orbits. If radial orbits were observed near the centers of other types of cluster galaxies as well, it could be inferred that violent relaxation during cluster formation was incomplete, and that clusters form by spherical collapse and secondary infall, as proposed by Gunn (1977).

O'Dea, Christopher P.↗
NASA NTRS1986

Simultaneous submillimeter and infrared observations of flat-spectrum radio sources

Observations are presented of 12 extragalactic nonthermal radio emission sources made at eight wavelengths between 1.25 and 1000 microns, including the important submillimeter wavelength of 350 microns. Nine of the objects were detected at 350 microns, roughly double the number of objects of this type that have previously been detected at this wavelength. The majority of the observations were made within a three-week time span. The results, combined with other studies, show that these objects generally have flat energy distributions at wavelengths longer than 1 mm that break in most cases between 350 microns and 1 mm. At wavelengths shorter than 1 mm, the energy distributions fall with a rough power-law behavior with a spectral index of about -1. For many of the objects, there is a definite break or curvature in the energy distributions at wavelengths near 2.2 microns, with the fluxes falling even more steeply into the near infrared. At the time of the observations, both OJ 287 and 3C 273 were undergoing flares. The results are generally consistent with the flares propagating out to longer wavelengths with time.

Roellig, T. L.↗
NASA NTRS1985

Simultaneous dual-wavelength VLBI observations of the compact radio source near the Galactic center

Preliminary results from simultaneous VLBI observations at 3.6 and 13 cm confirm that, in this wavelength range, the size of the compact radio source near the Galactic center varies as lambda-squared, where lambda is the wavelength of observation. These results also establish a flux density upper limit at 3.6 cm of 5 mJy for any source in Sgr A West smaller than 8 mas (80 AU).

Marcaide, J. M.↗
NASA NTRS1977

The compact radio sources in 4C 39.25 and 3C 345

Long-baseline interferometry of the quasars 4C 39.25 and 3C 345 at 10.65 and 14.77 GHz shows that the centimeter radio source in each object is double, with component separations of 0.0020 arcsec (4C 39.25) and 0.0013 arcsec (3C 345 at 1974.5). For each source, the separation is the same at both frequencies, as well as similar to the structure observed at 7.85 GHz (and 5.0 GHz for 4C 39.25). The spectra of the individual components are derived and shown to vary with time approximately as expected for expanding self-absorbed synchrotron sources. The magnetic fields in the components are estimated to be as high as 0.1 gauss, but the structure of the sources appears to be unrelated to the magnetic-field orientation derived from low-resolution polarization measurements. The component separation in 4C 39.25 has not changed for several years, whereas 3C 345 shows rapid expansion.

Shaffer, D. B.↗
NASA NTRS1994

Radiative excitation of molecules near powerful compact radio sources

In a recent paper, Barvainis & Antonucci searched for and failed to detect CO J = 1 goes to 0 absorption from the obscuring torus in the nearby powerful radio galaxy Cygnus A. We show that a plausible explanation for the lack of absorption (assuming that the ionization parameter within the torus is low enough for the gas to be molecular) is that radiative excitation of the CO molecules by the nonthermal radio continuum increases the excitation temperature of the lower rotational levels substantially, reducing the optical depths. The excitation temperature may approach the brightness temperature of the radio source at high enough flux-to-density ratios. Heating of the gas by the nonthermal excitation may also be important. We discuss the region of parameter space in which this excitation mechanism will be important and the implications for observations of obscuring tori.

Maloney, Philip R.↗
NASA NTRS1986

The radio source in the very luminous gamma-ray Seyfert galaxy MCG 8-11-11

VLA high resolution observations of the Seyfert 1.5 galaxy MCG 8-11-11 at 2 and 6 cm show the center of the galaxy to contain a triple radio source of 400 pc, with a less than 30-pc diameter central component that may be identified as the center of the galaxy. The triple source is in turn embedded in more diffuse emission which has a major axis position angle similar to that in the optical galaxy, but differing from the position angle of the triple source by 50 deg. While the overall morphology of the source is S-shaped, the curvature of the triple source at the ends is abrupt rather than continuous and gradual. It appears that the core radio emission in Seyfert 1 and 1.5 galaxies may be weakly correlated with their X-ray emission.

Ulvestad, J. S.↗
NASA NTRS1979

Interstellar scattering, the North Polar Spur, and a possible new class of compact galactic radio sources

Claims for a galactic-latitude dependence of interstellar angular broadening based on interplanetary-scintillation (IPS) observations are investigated. Analysis of the statistics of the angular sizes in an IPS survey shows that there is no evidence for increased angular broadening in the galactic plane. A region of sky about 500 sq deg of arc in area is considered in which significant angular broadening is thought to exist. An association between this region and the nearby North Polar Spur is proposed on the basis of the former's extension off the galactic plane to high latitudes. An evaluation of two-frequency angular-broadening measurements suggests that the data used to support the conclusion about a galactic-latitude dependence are not statistically significant. A study of pulsar data and implications for the angular broadening expected in the interstellar medium for sources at galactic latitudes below + or - 10 deg indicates the possible existence of a previously unsuspected class of compact galactic nonthermal radio sources, designated 'scintars'.

Rickard, J. J.↗
NASA NTRS1994

The radio source around Eta Carinae

We present high spatial resolution radio observations of the peculiar southern star Eta Carinae, made with the Australian Telescope. The images, at 8 and 9 GHz with a resolution of 1.0 arcsec show a source of dimension 10 arcsec and total flux of 0.7 Jy dominated by a strong central peak. The radio emission is unpolarized and offers no support to models which invoke degenerate stars or more exotic objects within the core of Eta Car. In these data we find no evidence for more than one energy source in the core with arcsecond separations as some infrared observations have suggested. Several levels of structure are evident in the radio image, which shows symmetry on the larger scales. Conventional formulae for stellar wind radio sources give a mass loss rate of order 3 x 10(exp -4) Solar Mass/yr based on the radio flux in the central peak, which yields a wind momentum flux of order 20% of the momentum flux available from the star's radiation field. The radio emission at these frequencies is consistent with thermal emission from gas flowing away from a 'luminous blue variable' star (LBV) Eta Car is probably the brightest thermal stellar wind radio source in the sky.

White, S. M.↗
NASA NTRS1978

Relativistic blast-wave model for the rapid flux variations of AO 0235+164 and other compact radio sources

A relativistic blast-wave version of a signal-screen model is developed which can adequately explain the details of the flux-density and structural variations of compact extragalactic radio sources. The relativistic motion implied by flux variations is analyzed with respect to the synchrotron spectrum of the BL Lac object AO 0235+164 observed during outbursts, and a signal-screen model for rapidly expanding shells produced by ultrarelativistic blast waves is examined. The approximate observed structure of the blast wave at three stages in its evolution is illustrated, each stage is described, and the model is applied to the flux density outburst in AO 0235+164 observed in late 1975. The results show that a relativistic blast-wave model can in general reproduce the main features of the observed flux variations in compact sources. Some problems with the proposed model are briefly discussed.

Marscher, A. P.↗
NASA NTRS1991

VLBI phase-referencing for observations of weak radio sources

Phase-referencing is a technique used in VLBI to extend the signal coherence time from a few minutes to a few hours in order to enhance significantly its sensitivity. With this technique, VLBI observations of milliJansky radio sources can be conducted for high-accuracy differential astrometry as well as imaging. We describe the technique in some details and present, as an example, a submilliarcsecond differential astrometric experiment design to identify the star responsible for the weak radio emission in the binary system Algol.

Lestrade, J.-F.↗