Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “QUASARS”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,099 records · Page 61
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

A dynamical model for the central engine of QSOs and active galactic nuclei

A model for the energy generation in quasars and active galaxies is proposed based on (quasi)spherical accretion and a shock as a means for randomizing the inflowing kinetic energy. According to the model, most of the accretion energy is converted into relativistic protons at the shock, which in turn can provide the necessary pressure to self-consistently support it if their energy loss time scale by nuclear collisions is longer than the free fall time scale. The shock can thus be characterized as a 'relativistic proton radiative shock' in analogy with similar accretion shocks on white dwarfs.

Kazanas, Demosthenes↗
NASA NTRS1986

Comptonization in supercritical winds. I - Spectral evolution. II - Dynamics and observational diagnostics

The physics governing spectral evolution in steady, fully ionized, radiatively driven winds from compact objects is examined. A largely analytic theory is developed to describe the effects of diffusion, adiabatic cooling, and repeated Compton scattering on the emergent radiation spectrum. Analytic forms for the wind temperature profile are derived, and spectral solutions obtained by considering a modified, time-independent Kompaneets equation are presented subject to a variety of initial conditions. A probabilistic formalism which describes the effect of diffusion photons travelling beyond the trapping radius is developed to allow the calculation of spectra observed at infinity. The second paper focuses on the dynamics of supercritical winds and on observational diagnostics which may be used to constrain model parameters in the contexts of specific astrophysical systems, including X-ray burst sources and mass-losing quasars.

Becker, Peter A.↗
NASA NTRS1987

Gravitational lenses and dark matter - Theory

Theoretical models are presented for guiding the application of gravitational lenses to probe the characteristics of dark matter in the universe. Analytical techniques are defined for quantifying the mass associated with lensing galaxies (in terms of the image separation), determining the quantity of dark mass of the lensing bodies, and estimating the mass density of the lenses. The possibility that heavy halos are made of low mass stars is considered, along with the swallowing of central images of black holes or cusps in galactic nuclei and the effects produced on a lensed quasar image by nonbaryonic halos. The observable effects of dense groups and clusters and the characteristics of dark matter strings are discussed, and various types of images which are possible due to lensing phenomena and position are described.

Gott, J. Richard, III↗
NASA NTRS1986

Astrophysics

The instrumentation, accomplishments and future activities in space astrophysics observations are explored. The Solrad, Orbiting Solar Observatory, Solar Maximum Mission, Orbiting Astronomical Observatory, Extreme Ultraviolet Explorer, Uhuru, HEAO-2, IRAS and ESA SAS spacecraft missions and data are summarized. Observations which identified the black hole in Cygnus-1 are described and known gravity-controlled phenomena in the universe are reviewed. Observations at different electromagnetic wavelengths and by particle detectors are outlined, and proposed gravity wave and neutrino detectors are discussed. The implications of the angular, spectral and temporal resolution, polarization, spectral range and observational limits of the instrumentation and data are considered. Current topics of interest in studies of the cosmic background radiation, distance scales, quasars and pulsars, star formation, and coronal heating are outlined. Finally, the design features and observational goals of the NASA Great Observatories space platforms are described.

Gilman, David A.↗
NASA NTRS1986

Polarization distributions in compact radio sources

Intensity and polarization maps of active galactic nuclei, obtained with marcsec resolution at 5 GHz using the Mark III VLBI array during 1981-1982, are presented. The techniques of high-resolution polarization synthesis are described, and maps for the superluminal quasar 3C345, the superluminal galaxy 3C120, and the BL Lac objects 0735 + 178 and OJ287 are characterized, with an emphasis on the relationship between arcsec-scale and marcsec-scale jets.

Roberts, David H.↗
NASA NTRS1987

Detection of the He I 10830-A line in the broadline radio galaxy 3C 120

The first measurement of He I 10830 A flux in a broadline radio galaxy (BLRG), 3C 120, is reported. The flux is 2.7 x 10 to the -13th erg/sq cm/s. The relative strengths of the 10830 A line and other prominent broad emission lines are 10830 A/He I 5876 A/H-beta/H-alpha = 0.64/0.27/1.00/6.3. Reddening is ruled out as the single source of the large 5876 A/H-beta ratio, and the necessity for either an enhanced helium abundance or an extended He II is obviated. Rather, the line ratios indicate that significant collisional enhancement of 5876 A and H-beta occurs, identical to that observed in Seyfert I galaxies and quasars.

Rudy, Richard J.↗
NASA NTRS1987

Beaming in blazars

Relativistic beaming of radiation is generally invoked to explain the extreme properties of blazars. The simplest synchrotron-self-Compton (SSC) model is applied to all blazars that have been observed in the X-ray region and have known VLBI sizes, and the minimum kinematic Doppler factor for each source is derived. All high polarization quasars (HPQs), but only half of the 30 BL Lac objects in the sample require relativistic enhancement of radiation due to beaming effects. The different Doppler factor distributions for the two classes of sources argue against the hypothesis that BL Lacs could be low-luminosity HPQs whose strong emission lines are swamped out by the boosted nonthermal continuum. Thus HPQs and BL Lacs could be intrinsically different in their physical structure.

Madau, P.↗
NASA NTRS1986

Navigating highly elliptical earth orbiters with simultaneous VLBI from orthogonal baseline pairs

Navigation strategies for determining highly elliptical orbits with VLBI are described. The predicted performance of wideband VLBI and Delta VLBI measurements obtained by orthogonal baseline pairs are compared for a 16-hr equatorial orbit. It is observed that the one-sigma apogee position accuracy improves two orders of magnitude to the meter level when Delta VLBI measurements are added to coherent Doppler and range, and the simpler VLBI strategy provides nearly the same orbit accuracy. The effects of differential measurement noise and acquisition geometry on orbit accuracy are investigated. The data reveal that quasar position uncertainty limits the accuracy of wideband Delta VLBI measurements, and that polar motion and baseline uncertainties and offsets between station clocks affect the wideband VLBI data. It is noted that differential one-way range (DOR) has performance nearly equal to that of the more complex Delta DOR and is recommended for use on spacecraft in high elliptical orbits.

Frauenholz, Raymond B.↗
NASA NTRS1987

Multiepoch VLBI observations of 4C 39.25 - Superluminal motion amid stationary structure

Eight VLBI maps of the quasar 4C 39.25 are presented, covering the time interval November 1979 to July 1985. During this period the compact components at the eastern and western ends of the source remained roughly stationary with respect to each other. A third component emerged from the western component in 1982 and proceeded to move (relative to the other two components) eastward at a rate of 0.16 + or - 0.02 marcsec per year. This corresponds to an average apparent speed between 3.2c and 8.4c for Hubble's constant between 100 and 50 km/s Mpc and q0 between 1 and 0. This superluminal motion contrasts with the stationary structure observed in the 1970s and also still observed between the eastern and western ends of the source. Possible explanations include superluminal feeding of a stationary compact counterpart to a classical radio lobe, an obstacle in a relativistic jet, a relativistic jet which alternatively expands and contracts owing to gradients in the confining pressure, and gravitational lensing of the compact radio emission.

Shaffer, David B.↗
NASA NTRS1986

Operation of the CCD/Transit Instrument (CTI)

The fully automated imaging survey telescope at Kitt Peak known as the CCD/Transit Instrument (CTI) has no moving parts, and employs two CCDs aligned east-west in the focal plane that are operated in the time delay and integrate mode, at the apparent sidereal rate, in order to produce a strip image of the sky. This image is 8.25-arcmin wide in declination and 8 hr in duration, to yield about 15 sq deg of sky from each night's survey; this, over the course of a year, represents more than 40 sq deg in a continuous strip. The CTI is addressing such astronomical tasks as the determination of the supernova production rate by counting, galactic structure investigations, and the definition of a complete sample of quasars. Attention is given to the CTI's bimetallic, thermally self-compensating structure and three-mirror wide-field optical system.

Mcgraw, J. T.↗
NASA NTRS1986

The CCD/Transit Instrument (CTI) data-analysis system

An account is given of the Kitt Peak CCD/Transit Instrument's software for the automated archiving, analysis, and interrogation of each observation night's nearly 450 Mbytes of image data. The analysis phase is performed by a pair of linked algorithms, one of which operates on the absolute pixel-values and the other on the spatial derivative of the data, so that both isolated and merged images are reliably detected in a single pass. This is in due course followed by the automatic comparison of each new detection with a set of predefined templates in parameter space, in order to find such interesting objects as supernovae, quasars, and variable stars.

Cawson, M. G. M.↗
NASA NTRS1987

Gravitational lensing by an ensemble of isothermal galaxies

Calculation of 28,000 models of gravitational lensing of a distant quasar by an ensemble of randomly placed galaxies, each having a singular isothermal mass distribuiton, is reported. The average surface mass density was 0.2 of the critical value in all models. It is found that the surface mass density averaged over the area of the smallest circle that encompasses the multiple images is 0.82, only slightly smaller than expected from a simple analytical model of Turner et al. (1984). The probability of getting multiple images is also as large as expected analytically. Gravitational lensing is dominated by the matter in the beam; i.e., by the beam convergence. The cases where the multiple imaging is due to asymmetry in mass distribution (i.e., due to shear) are very rare. Therefore, the observed gravitational-lens candidates for which no lensing object has been detected between the images cannot be a result of asymmetric mass distribution outside the images, at least in a model with randomly distributed galaxies. A surprisingly large number of large separations between the multiple images is found: up to 25 percent of multiple images have their angular separation 2 to 4 times larger than expected in a simple analytical model.

Katz, Neal↗
NASA NTRS1987

Gravitational microlensing and gamma-ray bursts

Models of gravitational microlensing at small and large optical depth are presented, with a special emphasis on the time delays between microimages. The time distribution of microimage arrival times is random when the microobjects responsible for the lensing are randomly distributed. If the microobjects are clustered in space, then microimages are clustered in time. In some models there are many pairs of microimages with a very small time separation. It is proposed that the recently discovered recurrent soft gamma-ray burster GB 790107, for which more than 50 bursts were detected, is a single event multiply imaged due to gravitational microlensing by an ensemble of nonrandomly distributed microobjects, perhaps a rich cluster of galaxies. Some observational tests of this hypothesis are proposed. If correct, the hypothesis requires the burster to be at a cosmological distance, perhaps farther away than the most distant quasars.

Paczynski, Bohdan↗
NASA NTRS1987

The origin of the diffuse X-ray background

The Einstein Deep Survey fields are used to reexamine the problem of the origin of the extragalactic diffuse X-ray background. An approach based on fluctuation analysis is adopted, and a sophisticated Monte Carlo scheme is used to compare the X-ray images with a variety of models representing point and diffuse source contributions. Results are presented for two classes of models: those which determine the flux cutoff beyond which pointlike sources fail to contribute significantly if the log N-log S slope is fixed at the measured value of -1.5, and those which set limits on the slope of the log N-log S relation below its last measured point at the Deep Survey threshold. In both cases, it is found that the known population of X-ray-emitting quasars and any reasonable extrapolations therefrom do not comprise the entire X-ray background.

Hamilton, T. T.↗
NASA NTRS1987

Spectroscopy of the extranuclear line-emitting regions associated with the gravitational lens system 2016 + 112

Moderate-resolution slit spectroscopy of the extended emission-line objects A1 and B1 located within 5 arcsec of the triple radio source 2016 + 112 shows that at least one is physically associated with the z = 3.273 quasar thought to be multiply imaged by a foreground galaxy. The data and theoretical models unanimously favor the identification of A1 and B1 with two separate clouds of ionized gas that lie within a few kiloparsecs of the primary A/B QSO and radio source. The available evidence does not support the view that they are multiple images of a single object or their classification as separate (companion) galaxies.

Schneider, D. P.↗
NASA NTRS1987

Four-shooter - A large format charge-coupled device camera for the Hale telescope

The Hale telescope has employed an astronomical camera using four 800x800 CCDs in an optical arrangement that allows the imaging of a contiguous 1600-pixel-square region of the sky; reimaging optics are then used to yield a scale of 0.33 arcsec/pixel. Optical coatings are incorporated to yield a throughput at peak efficiency of nearly 50 percent, including telescope losses. This camera can be used in a scanning mode, in which the telescope tracking rate is offset and the charge is clocked in the chips in such a way that the charge image remains aligned with the optical image. Attention is given to the results of a survey for high-redshift quasars using this equipment, which has produced images for the most distant galaxy clusters yet discovered.

Gunn, James E.↗
NASA NTRS1987

Short baseline phase delay interferometry

The high precision of the phase delay data type allows angular navigation accuracy on relatively short baselines to compete with the angular accuracy achieved with long baseline group delay measurements. Differential phase delay observations of close quasar pairs on both a 5.9-km baseline (DSS 12-DSS 13) and a 253-km baseline (DSS 13-Owens Valley Radio Observatory) have been performed to study the potential navigational precision and accuracy of the short baseline interferometry. As a first step toward demonstration of a connected element system at Goldstone, the DSS 12-DSS 13 baseline was operated coherently, distributing a common frequency reference via a recently installed fiber optic cable. The observed phase delay residuals of about 10 psec or less on both baselines appear to be dominated by short term troposphere fluctuations, and correspond to navigational accuracies of well below 50 nrad for the 253-km baseline. Additional experiments will be required to probe the full range of systematic errors.

Edwards, C. D.↗
NASA NTRS1987

Nonlinear, relativistic Langmuir waves in astrophysical magnetospheres

Large amplitude, electrostatic plasma waves are relevant to physical processes occurring in the astrophysical magnetospheres wherein charged particles are accelerated to relativistic energies by strong waves emitted by pulsars, quasars, or radio galaxies. The nonlinear, relativistic theory of traveling Langmuir waves in a cold plasma is reviewed. The cases of streaming electron plasma, electronic plasma, and two-streams are discussed.

Chian, Abraham C.-L.↗