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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 145 records · Page 8
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NASA NTRS1975

On the problems associated with the rapidly varying compact nonthermal sources

Difficulties and possible interpretations are discussed for observations of extragalactic compact nonthermal radio sources. The sources considered are those which have been identified optically with normal and peculiar galaxies, quasars, N galaxies, and BL Lacertae objects. Optical, infrared, and radio flux variations in different sources are noted along with difficulties associated with determining angular sizes from interferometric measurements. It is suggested that the ultimate origin of the infrared flux is probably a nonthermal synchrotron mechanism in some sources and absorption and reradiation of ultraviolet synchrotron radiation by dust in others. Model calculations of compact sources are reviewed, emphasizing equipartition of energy, total energies, the importance of the Compton effect, and sources which present severe difficulties for the theory of a canonical incoherent electron-synchrotron source.

Burbidge, G. R.↗
NASA NTRS1977

A compact radio source in the nucleus of M82

The brightest compact radio source in the nucleus of M82 has a linear diameter of about 0.02 parsecs (25 light-days). On the basis of this small size and energy considerations, it seems that this source is probably not a supernova remnant, but is similar in nature to the compact radio sources found in the nuclei of spiral and elliptical galaxies, Seyfert galaxies, radio galaxies, and in quasi-stellar objects.

Geldzahler, B. J.↗
NASA NTRS1977

An angular size for the compact radio source at the galactic center

Simultaneous very-long-baseline (100 million wavelengths) and intermediate-baseline (4 million wavelengths) interferometer observations at a wavelength of 3.7 cm in 1976.2 show that the galactic-center compact radio source has a linear size of approximately 140 AU. The observations also set an upper limit of about 0.1 Jy to the emission from any core of size less than roughly 10 AU at that epoch. The total flux density of the compact source was slightly higher than the value reported previously.

Lo, K. Y.↗
NASA NTRS1978

X-ray and radio emission from the compact galaxy III Zw 2

X-rays from the compact galaxy III Zw 2 have been detected with the rotating modulation collimator on board SAS 3. The (2-10-keV) flux is (4.3 + or - 0.5) by 10 to the -11th power erg/sq cm per sec, which corresponds to a luminosity of 1.4 by 10 to the 45th power erg/s at 530 Mpc. Subsequent radio observations between 1.46 and 90 GHz were made at the NRAO Very Large Array and 36-foot (11-m) installations. The radio spectrum rises sharply toward higher frequencies and, below about 100 GHz, is indicative of a self-absorbed synchrotron spectrum from an extremely compact, rapidly evolving object.

Schnopper, H. W.↗
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 NTRS1979

Mode 1 crack surface displacements for a round compact specimen subject to a couple and force

Mode I displacement coefficients along the crack surface are presented for a radially cracked round compact specimen, treated as a plane elastostatic problem, subjected to two types of loading; a uniform tensile stress and a nominal bending stress distribution across the net section. By superposition the resultant displacement coefficient or the corresponding influence coefficient can be obtained for any practical load location. Load line displacements are presented for A/D ratios ranging from 0.40 to 0.95, where A is the crack length measured from the crack mouth to the crack tip and D is the specimen diameter. Through a linear extrapolation procedure crack mouth displacements are also obtained. Experimental evidence shows that the results are valid over the range of A/D ratios analyzed for a practical pin loaded round compact specimen.

Gross, B.↗
NASA NTRS1978

The central, compact source in the Cygnus A galaxy

Single, optically thin, component models as well as double, optically thick at low frequencies, component models have been examined in trying to explain the central source in the nucleus of Cygnus A. In the course of exploring those models, it is found that this compact radio source may be quite similar to other compact and variable sources in Seyfert galaxies and quasars. Double component models generally do better because they fit the X-rays, and there is a good chance that the X-rays arise in the central source; these models can naturally explain variability which may already have been detected in X-rays. The IR is most likely due to emission by cold dust in the nucleus of Cygnus A. Optical emission is, very likely, primarily thermal, except perhaps at UV wavelengths. A number of observations are suggested to check the predictions of this work.

Kafatos, M.↗
NASA NTRS1979

Absorption models for low-frequency variability in compact radio sources

The consequences of the most plausible version of the absorption model for low-frequency variability in compact extragalactic radio sources are considered. The general restrictions placed on such a model are determined, and observational tests are suggested that can be used either to support the model or to discriminate among its various versions. It is shown that low-frequency variability in compact radio sources can be successfully explained by a class of models in which the flux is modulated by changes in free-free optical depth within an intervening ionized medium. Two versions of such a model are distinguished, one involving large changes in optical depth and the other, small changes. It is noted that while absorption effects are capable of causing rapid flux and structural variations at centimetric wavelengths, the models predict detailed behavior that is in direct conflict with observational data.

Marscher, A. P.↗
NASA NTRS1979

Compact H II regions in the far-infrared

Far-infrared (40-150 microns) observations of W51 (G49.5-0.4), K3-50, DR 21, NGC 7538, and W3(OH) are presented and discussed. At these wavelengths each source is dominated by a small bright component closely associated with a compact H II region. Their spectra can be explained as emission by dust at temperatures of 30-50 K. Some of the objects appear to be optically thick out to wavelengths of about 60 microns. Of the 10 source components in the five regions, at least four may be powered by pre-main-sequence stars. The far-infrared data are consistent with depletion of volatile components of the dust within compact H II regions, but do not clearly require destruction or expulsion of refractory grains. The bulk of the dust which emits primarily in the far-infrared must lie in a dense shell immediately outside the ionized zone. The sources are possible precursers to open clusters with stellar masses in the range from 1,000 to 10,000 solar masses.

Thronson, H. A., Jr.↗
NASA NTRS1979

Mode I crack surface displacements for a round compact specimen subject to a couple and force

Mode I displacement coefficients along the crack surface are presented for a radially cracked round compact specimen, treated as a plane elastostatic problem, subjected to two types of loading; a uniform tensile stress and a nominal bending stress distribution across the net section. By superposition the resultant displacement coefficient or the corresponding influence coefficient can be obtained for any practical load location. Load line displacements are presented for A/D ratios ranging from 0.40 to 0.95, where A is the crack length measured from the crack mouth to the crack tip and D is the specimen diameter. Through a linear extrapolation procedure crack mouth displacements are also obtained. Experimental evidence shows that the results of this study are valid over the range of A/D ratios analyzed for a practical pin loaded round compact specimen.

Gross, B.↗
NASA NTRS1980

Spectroscopy of compact extragalactic X-ray sources

The X-ray spectra of compact extragalactic sources obtained from the HEAO-1 A-2 experiment and the solid-state spectrometer onboard HEAO-2 (the Einstein Observatory) are reviewed. Seyfert spectra are remarkably consistent with characteristic power-law spectra of energy index alpha = 0.7 + .1 over a dynamic range of almost 100 in both luminosity for the whole sample, and energy for individual members. Radio-quiet quasars have similar spectra, perhaps slightly steeper, for the limited sample available. New solid-state spectrometer results for NGC 4151 yield a consistent picture for the geometry of the broad-line clouds in both these related radio-quiet classes of galactic nuclei. Radio-loud objects, especially BL Lacs, are considerably more variable in spectrum as well as luminosity. Direct synchrotron and synchrotron-self-Compton components are consistent with what we observe from these objects. Finally, the role of spectroscopy in addressing the extent to which compact extragalactic nuclei might contribute to the diffuse X-ray background is discussed.

Holt, S. S.↗
NASA NTRS1979

Theories of magnetospheres around accreting compact objects

The paper reviews theoretical models of magnetospheres formed around neutron stars and other strongly magnetized compact objects in the presence of mass accretion from a companion star. Emphasis is placed on the interaction of the accretion process with the magnetic field of the compact object and the formation of a magnetosphere. The following models are discussed: magnetospheres with polar funnels; closed magnetospheres and their instabilities; models of internal flow; and disk accretion magnetospheres.

Vasyliunas, V. M.↗
NASA NTRS1980

A collection of formulas for calculation of rotating blade noise - Compact and noncompact source results

A unified approach is used to derive many of the current formulations for calculation of discrete frequency noise for helicopter rotors and propellers. Both compact and noncompact source formulations are derived. The compact formulations are obtained as the limit of noncompact source results. In particular, the linearized acoustic equations by Hawkings and Lowson, Farassat, Hanson, Woan and Gregorek, Succi, and Jou are derived in this paper. An interesting thickness noise formula by Isom and its recent extension to the near field by Ffowcs Williams are also presented. The paper includes some comparisons of measured and calculated acoustic pressure signatures and spectra for an advanced propeller. The theoretical results are obtained using a computer program developed by the author and P. A. Nystrom.

Farassat, F.↗
NASA NTRS1979

Compact galactic X-ray sources; Proceedings of the Workshop, Washington, D.C., April 20, 21, 1979

The current status and future prospects regarding the study of compact galactic X-ray sources are reviewed; key questions addressable by X-ray observations are outlined, and various proposed types of missions are evaluated in terms of their ability to address these questions. Subjects discussed include degenerate dwarfs and cataclysmic variables, bursters, neutron stars, black holes, and erratic variables, transients, and patrol programs. Specific topics include future X-ray observations of magnetic binaries, the thermonuclear-flash model for X-ray burst sources, neutron star X-ray sources, prospects for experimental research on black holes in binary systems, and compact X-ray source observations after Explorer with the LAMAR.

Lamb, F.↗
NASA NTRS1980

Time-dependent radio fine structure of the compact sources NRAO 150 and 4C 39.25

Very long baseline interferometer observations at 7.85 GHz have been used to probe the milliarcsecond structure of the unidentified, very compact radio source NRAO 150 and QSO 4 C 39.25. NRAO 150 exhibited no structural variations from 1972 to the end of 1974. A model with two circular Gaussian components fits the data well. NRAO 150 had a flux density of 7.6 plus or minus 0.5 Jy in the compact component; 4 C 39.25 showed a two-component structure, the components having a separation of (2.02 plus or minus 0.05 arc sec) x 10 to the -3rd power. The upper bound on the speed of transverse separation is 0.0001 arc sec per year or less than 2.7 c. From the spectrum there are also indications of a third, larger component.

Baath, L. B.↗
NASA NTRS1981

Effects of gamma-ray, neutrino, and particle production on the energetics and dynamics of compact extragalactic radio sources

Consideration is given to particle production and high-energy radiation within apparently superluminal radio components of extragalactic radio sources forming within the apparent region of nuclear activity of a quasar or active galaxy. The physical conditions in compact components observed as radio emitters are derived for the quasars 3C 273 and 3C 345 and extrapolated to those of initial components of sizes on the order of 10 to the 15th cm on the basis of two-dimensional relativistic jet and relativistic three-dimensional models of component expansion. Probabilities that a given particle avoids an inelastic collision in the relativistic plasma are calculated for both cases which show that collisions which produce particles and radiation may be very important during the formation of a compact radio component. The consequences of electron-positron production, bremsstrahlung and proton-proton inelastic collisions ultimately giving rise to neutrinos and gamma rays for the development and energetics of the radio component are then examined, and upper limits to the amount of energy which can be channeled into radio components from an active region without giving rise to a high-energy X-ray source are derived.

Vestrand, W. T.↗
NASA NTRS1981

Stress-intensity factors and crack-opening displacements for round compact specimens

An improved method of boundary collocation based on the complex variable analysis of Muskhelishvili (1953) is applied to the two-dimensional stress analysis of round compact specimens with pin-loaded holes. Stress intensity factors and crack opening displacements are calculated for various round compact specimens as a function of crack-length-to-specimen-width ratios. A wide-range equation for the stress intensity factors and equations for crack-surface displacements and load-point displacements are developed.

Newman, J. C., Jr.↗
NASA NTRS1982

Large scale X-ray and radio structures associated with compact extragalactic sources

Knots of X-ray emission have been detected within 20 arcmin of five compact sources initially selected from the MPIfR north polar 5 GHz survey. Two of the knots have also been detected at centimeter wavelengths and probably have nonthermal spectra. They appear to be associated with the compact sources since the probability of serendipitous discovery at the observed flux levels is low. While the apparent association may be due to colocation of the sources in a distant supercluster, it is suggested on the basis of overall alignment, and possible correlations with structures in the respective central sources, that the association may be similar to that found in extended radio sources. The observed emission may thus be due to synchrotron or inverse Compton radiation, the energy being supplied by jets from the central source.

Biermann, P.↗