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 1,063 records · Page 59
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

The Hopkins ultraviolet telescope

The Shuttle is scheduled to carry the Hopkins UV Telescope (HUT) aloft in 1986. HUT will be one of three telescopes on the mission and will be sensitive to the 450-1800 A interval. Its primary goals will be observations of Halley's Comet, the planets, distant galaxies and quasars. It will be controlled by Spacelab personnel and features a 0.9 m primary mirror, a spectrograph on the focal plane, and an Si intensified image television camera for on-line image monitoring. The main component of the spectrograph will be a 1024 channel self-scanning Reticon photodiode array coupled by optic fibers to the microchannel detector plates. HUT is presently undergoing qualification testing.

Davidsen, A. F.↗
NASA NTRS1983

Fundamental processes in pair plasmas

The various processes that produce and destroy electron-positron pairs are reviewed, and the timescales of these processes are compared to thermalization, accretion, and cooling timescales. The various radiation spectra produced by relativistic, thermal plasmas are considered. Recent results for the equilibria available to finite, thermal relativistic plasmas with and without embedded magnetic fields are reviewed. Such plasmas, in steady state, have maximum temperatures, luminosities, and field strengths, useful diagnostics for interpreting quasars and active galaxies.

Lightman, A. P.↗
NASA NTRS1985

Stimulated Compton conversion of Langmuir waves by relativistic electron beams

The scattering of a Langmuir wave by a relativistic electron to produce a transversely polarized high-frequency EM wave by an inverse Compton process is investigated theoretically. The results of numerical computations are presented in diagrams and graphs, and it is shown that although net growth rates are predicted for several classes of Langmuir spectra, they are at least several orders of magnitude too small to account for the intense high-frequency emission seen in recent beam-plasma experiments (Benford et al., 1980; Kato et al., 1983) unless some unknown coherence mechanism is focusing the inverse-Compton emission. The implications of the mechanism for astrophysical beam-plasma systems such as quasars and radio galaxies are indicated.

Newman, D. L.↗
NASA NTRS1985

Consequences of hot gas in the broad-line region of active galactic nuclei

Models for hot gas in the broad line region of active galactic nuclei are discussed. The results of the two phase equilibrium models for confinement of broad line clouds by Compton heated gas are used to show that high luminosity quasars are expected to show Fe XXVI L alpha line absorption which will be observed with spectrometers such as those planned for the future X-ray spectroscopy experiments. Two phase equilibrium models also predict that the gas in the broad line clouds and the confining medium may be Compton thick. It is shown that the combined effects of Comptonization and photoabsorption can suppress both the broad emission lines and X-rays in the Einstein and HEAO-1 energy bands. The observed properties of such Compton thick active galaxies are expected to be similar to those of Seyfert 2 nuclei. The implications for polarization and variability are also discussed.

Kallman, T.↗
NASA NTRS1985

O VIII resonant absorption in PKS 2155 - 304 - A hot wind

The broad X-ray absorption feature in PKS 2155 - 304 could be due to O VIII Ly-alpha resonant absorption in a wind with outflow speeds up to about 30,000 km/s. The properties of such a wind are calculated. A homogeneous, spherically symmetric wind would involve a mass flux of about a 1000 solar masses/year, but this estimate could be greatly reduced if the outflow were beamed, and/or the absorbing gas were clumped into small clouds. In the latter case, the O VIII absorption offers clues to the physical state of thermal material closer to the central energy source than the clouds which produce optical emission lines in quasars.

Krolik, J. H.↗
NASA NTRS1985

A VLBI survey at 2.29 GHz

VLBI observations at 2.29 GHz with fringe spacings of about 3 milliarcsec have been performed on 1398 radio sources spread over the entire sky. 917 sources were detected, including 93 percent of the identified BL Lacertae objects, 86 percent of the quasars, and 36 percent of the galaxies. The resulting catalog of compact radio sources is useful for various astrophysical studies and in the formation of VLBI celestial reference frames.

Preston, R. A.↗
NASA NTRS1985

A VLBI survey at 2.29 GHz

The Deep Space Network (DSN) is establishing a high-accuracy VLBI celestial reference frame. The results of a search for suitable radio sources to be used in constructing this frame are given. The VLBI observations using DSN baselines at 2.29 GHz with fringe spacings of about 3 milliarcseconds have been performed on 1398 radio sources spread over the entire sky. Of those, 917 sources were detected including 93% of the identifed BL Lacertae objects, 86% of Quasars and 36% of galaxies. The resulting catalog of compact radio sources is also useful for various astrophysical studies and in the formation of VLBI celestial reference frames.

Morabito, D. D.↗
NASA NTRS1985

Narrow-band forbidden O III imaging of the QSO 4C 18.68 - A tidal tail revealed?

The z = 0.313 quasar 4C 18.68 and its surroundings in the light of redshifted forbidden O III were imaged. The QSO appears much more extended than in broad-band images, with a large halo and possibly a faint tidal tail discerned. A broad-band I CCD image taken in 1 - 1.2 arcsec seeing resolves the QSO from a companion galaxy 3.6 arcsec distant, similar to many others seen in the field. It is suggested that the elongated and asymmetric shapes of some QSOs may be due to not-quite-resolved companion galaxies, and that caution be exercised in interpreting luminosity and color profiles as evidence for underlying spiral or elliptical structure.

Shara, M. M.↗
NASA NTRS1985

A comparison between Lageos laser ranging and very long baseline interferometry determined baseline lengths

NASA's Crustal Dynamics Project (CDP) has the objective to improve the understanding of geodynamics by measuring crustal deformation, tectonic motion, and polar motion and earth rotation. Three different approaches are utilized for obtaining these measurements. One is based on satellite laser ranging (SLR), while another makes use of very long baseline interferometry (VLBI) which uses reception of radio signals from quasars. The third approach involves laser ranging to the moon. An important part of the CDP is to compare baselines periodically or the straight-line distance between two points on the earth's surface as determined by either SLR or VLBI. Attention is given to the SLR analysis, the VLBI analysis, a local survey, and error sources. A table is presented with the baselines between SLR survey markers as measured by VLBI and SLR.

Kolenkiewicz, R.↗
NASA NTRS1985

The radio-optical-X-ray spectral flux distributions of blazars

Ten confirmed and probable BL Lac objects were searched for X-ray emission using the Einstein Observatory. All 10 were detected. These data were combined with existing data to form a catalog of 99 confirmed (by measured, high, optical linear polarization) and probable blazars (BL Lac objects and highly polarized quasars). Radio, optical, and X-ray observations are available for 77 of these. One of the ten program objects was observed at four epochs during large radio and optical outbursts. The correlations of radio, optical, and X-ray luminosities are examined.

Ledden, J. E.↗
NASA NTRS1985

Are some BL Lac objects artefacts of gravitational lensing?

It is proposed here that a significant fraction of BL Lac objects are optically violently variable quasars whose continuum emission has been greatly amplified, relative to the line emission, by pointlike gravitational lenses in intervening galaxies. Several anomalous physical and statistical properties of BL Lacs can be understood on the basis of this model, which is immediately testable on the basis of absorption line studies and by direct imaging.

Ostriker, J. P.↗
NASA NTRS1986

Infrared astronomy after IRAS

The development of infrared astronomy in the wake of IRAS is discussed. Attention is given to an overview of next generation infrared telescope technology, with emphasis on the Space Infrared Telescope Facility (SIRTF) which has been built to replace IRAS in the 1990s. Among the instruments to be included on SIRTF are: a wide-field high-resolution camera covering the infrared range 3-30 microns with large arrays of detectors; an imaging photometer operating in the range 3-700 microns; and a spectrograph covering the range 2.5-200 microns with resolutions of 2 and 0.1 percent. Observational missions for the SIRTF are proposed in connection with: planetary formation; star formation; cosmic energy sources; active galactic nuclei; and quasars.

Rieke, G. H.↗
NASA NTRS1986

Will cosmic strings be discovered using the Space Telescope?

Cosmic strings are topologically stable defects in the vacuum of space which may be produced by a phase transition in the early universe. Here, it is suggested that observations of very distant galaxies are a more useful means of discovering strings than quasar observations. It is argued that if there is only one string out to redshift z - about 1 the probability that it crosses a random image obtained using the Wide Field Camera (WFC) of the Space Telescope is about 0.0001. In order to discover a cosmic string the Space Telescope WFC will be required to operate almost continuously in primary and serendipity modes, and a cosmic string, if it exists, may be discovered within the first few years of operation.

Pacynski, B.↗
NASA NTRS1986

Optical identifications of radio sources with accurate positions using the United Kingdom Schmidt Telescope (UKST) IIIa-J plates

Several programs are making use of UKST Sky Survey plates to identify southern radio sources. The fine-grain modern plates and accurate radio positions give a much improved identification rate. It seems that it will very soon be possible to determine whether or not there is a quasar redshift cut-off at z of about 4. There is an urgent need for more accurate fundamental reference star positions in the South.

Savage, A.↗
NASA NTRS1985

Space telescope

This paper describes the Space Telescope, a program of the National Aeronautics and Space Administration (NASA). The Space Telescope is the largest and most powerful optical and ultraviolet astronomical observatory to be operated in space. Through the remote eyes of this telescope, astronomers will look further into space and time to produce data including imagery of unequaled quality of galaxies, star systems, quasars and other objects of scientific interest. The Space Telescope will do this by observing the sky from a nominal 550 kilometer low earth orbit via an optical system with a 2.4 meter diameter primary mirror. Additionally, this observatory has a precision pointing control system that is capable of maintaining a locked state on an object for extended periods accurately to within 0.01 arc seconds. The observatory instrument complement, optical system and pointing control system is presented.

Wojtalik, F. S.↗
NASA NTRS1986

Limits of X-ray variability in active galactic nuclei

A significant correlation is demonstrataed here between the X-ray luminosity and the timescale of X-ray variability for Seyfert galaxies and quasars. This is interpreted as evidence that the emitting plasma is near the limit of being dominated by electron-positron pairs. BL Lac objects do not follow this pattern; this may be due either to relativistic beaming or to the differing importance of the pair production process.

Barr, P.↗
NASA NTRS1986

The linear polarization of 3C 345 at milliarcsecond resolution

A polarization-sensitive 6 cm map of the superluminal quasar 3C 345 with angular resolution 2.5 x 4.7 mas was made using four stations of the US VLBI network. The optically thick core is essentially unpolarized, while both of the knots in the jet are highly polarized. The inner knot C3 has m = 11 percent at electric vector position angle chi = 22 deg, and the outer knot C2 has m = 6 percent at chi = 83 deg. Thus the magnetic field appears to be parallel to the jet near its base and to change direction down the jet.

Wardle, J. F. C.↗
NASA NTRS1986

The detection of distant cooling flows and the formation of dark matter

Cooling flows involving substantial mass inflow rates appear to be common in many nearby rich and poor clusters and in isolated galaxies. The extensive optical and ultraviolet filaments produced by the thermal instability of large flows are detectable out to redshifts greater than 1. It is proposed that this may explain the extended optical line emission reported in, and around, many distant radio galaxies, narrow-line quasars, and even nearby normal and active galaxies. An important diagnostic to distinguish cooling flows from other possible origins of emission line filaments is the presence of extensive regions at high thermal pressure. Other evidence for distant cooling flows and the resultant star formation is further discussed, together with the implications of cooling flow initial-mass functions for galaxy formation and the nature of 'dark' matter.

Fabian, A. C.↗