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At least 433 records · Page 24

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NASA NTRS1988

Relativistic beaming and the nuclei of double-lobed radio quasars

A complete sample of double-lobed 3CR quasars with minimal orientation bias is defined. Properties of these objects on the greater than kpc scale are consistent with the simple beaming model for their nuclei. VLBI maps of six nuclei reveal 'core-jet' structures. Both nuclei mapped at multiple epochs are modestly superluminal. The distribution of apparent velocities in this sample will provide a stringent test of beaming.

Hough, D. H.↗

NASA NTRS1989

Emission-line properties of optically and radio-selected complete quasar samples

Spectrophotometry of two complete samples of quasars, one obtained from the flat radio spectra of the objects and the other from their optical colors, is presented. It is confirmed that the equivalent widths of the major UV emission lines decrease with increasing continuum luminosity (the Baldwin effect) and that the (Ly-alpha + N V)/C IV and 1909 A/C IV intensity ratios are also luminosity-dependent by virtue of the C IV luminosity dependence. It is shown that the Mg II/C IV intensity ratio and possibly the strengths of the blue Fe II emission features and the FWHM of the 1909 A blend are luminosity-dependent. These results generally support the Mushotzky and Ferland (1984) interpretation of the Baldwin effect in terms of a weak inverse correlation between the continuum luminosity and ionization parameter.

Baldwin, J. A.↗

NASA NTRS1989

Accurate galactic N(H) values towards quasars and AGN

Integrated Galactic 21-cm column densities toward about 174 quasars and AGN are presented. The data are corrected for stray radiation, using the technique of Lockman et al. (1986). Because of the 21-arcmin beam size of the 140-ft telescope used in the study, the uncertainty in N(H) due to angular variations in the H I of the Galaxy at high latitudes is minimized. The column densities are accurate to about 1 X 10 to the 19th atoms/sq cm, or 5 percent, whichever is larger. Errors above N(H) of about 4 X 10 to the 20th atoms/sq cm are dominated by opacity uncertainties.

Elvis, Martin↗

NASA NTRS1989

On the ratio of the infrared-to-ultraviolet continuum to the X-rays in quasars and active galaxies

A sample of 122 quasars and Seyfert galaxies was analyzed and it was found that the NIR 7500 A rest frame luminosity is more linearly related to the X-ray luminosity than is the optical luminosity. It is also found that the mean spectral slope between 7500 and 4200 A in the rest frame monotonically flattens with luminosity over more than three orders of magnitude. The present results support models which attribute the IR and X-ray continua to linked nonthermal processes such as synchrotron self-Compton, while the blue-UV bump comes from a different source, such as an accretion disk.

Mushotzky, R. F.↗

NASA NTRS1989

MG 1654+1346 - An Einstein Ring image of a quasar radio lobe

VLA radio images of MG 1654+1346, a candidate for an Einstein Ring gravitational lens, reveal two components which are separated by about 7 arcsec. Two optical counterparts, a 19-mag galaxy coincident with the center of the radio ring and a blue 21-mag stellar object located about 3 arcsec northeast of the galaxy, are identified. A model representing the source as a quasar with two extended radio lobes and the lens as an elliptical potential at the position of the galaxy is found to reasonably reproduce the radio observations.

Langston, G. I.↗

NASA NTRS1989

The Lyman edge test of the quasar emission mechanism

The Lyman edge region in the spectra of 11 high-redshift quasars is studied using data designed to detect broadened Lyman edge absorption predicted by thermal models of the 'big blue bump' continuum component, especially thin accretion disks. Three objects have broadened, partial edges near the systemic redshifts, as expected for thermal models. In two cases, however, narrow Lyamn lines with the edge absorption, ruling out the disk interpretation. Thus, the data are negative for thermal models, at least for opaque thin accretion disks.

Antonucci, R. R. J.↗

NASA NTRS1989

A complete sample of double-lobed radio quasars for VLBI tests of source models - Definition and statistics

A complete, flux-density-limited sample of double-lobed radio quasars is defined, with nuclei bright enough to be mapped with the Mark III VLBI system. It is shown that the statistics of linear size, nuclear strength, and curvature are consistent with the assumption of random source orientations and simple relativistic beaming in the nuclei. However, these statistics are also consistent with the effects of interaction between the beams and the surrounding medium. The distribution of jet velocities in the nuclei, as measured with VLBI, will provide a powerful test of physical theories of extragalactic radio sources.

Hough, D. H.↗

NASA NTRS1991

A high-redshift IRAS galaxy with huge luminosity - Hidden quasar or protogalaxy?

An emission line galaxy with the enormous far-IR luminosity of 3 x 10 to the 14th solar has been found at z = 2.286. The spectrum is very unusual, showing lines of high excitation but with very weak Lyman-alpha emission. A self-absorbed synchrotron model for the IR energy distribution cannot be ruled out, but a thermal origin seems more plausible. A radio-quiet quasar embedded in a very dusty galaxy could account for the IR emission, as might a starburst embedded in 1-10 billion solar masses of dust. The latter case demands so much dust that the object would probably be a massive galaxy in the process of formation. The presence of a large amount of dust in an object of such high redshift implies the generation of heavy elements at an early cosmological epoch.

Rowan-Robinson, M.↗

NASA NTRS1991

Persistence and change in the soft X-ray spectrum of the quasar PG 1211 + 143

The present study examines two Einstein and three Exosat observations of PG 1211 + 143 over a 6-yr baseline which show that strong steep-spectrum low-energy X-ray emission is a persistent feature of this quasar. Exosat observations of PG 1211 + 143 detected an increase of a factor of 2.3 in its steep soft X-ray flux during 18 days. It is concluded that the bulk of the soft X-ray emission of PG 1211 + 143 comes from a region less than 5 x 10 to the 16th cm across. In another interval of 193 days, the soft X-rays decreased by a factor of 3.7. In the same interval the hard X-rays decreased by a factor of 1.6 + or - 0.05, which suggests a connection between the two energy regimes and argues against variable absorption causing the soft X-ray variations. It is contended that in order to decrease in luminosity so rapidly, a thermal source in PG 1211 + 143 would have to be optically thick to both electron scattering and free-bound absorption.

Elvis, Martin↗

NASA NTRS1991

The dust enshrouded quasar in the ultraluminous galaxy Markarian 463 - Radio, near-infrared, and optical imaging

New imaging data are presented at radio, near-infrared, and optical wavelengths to elucidate the processes associated with the ongoing merger and nuclear activity in Mrk 463. Imaging at 20 cm reveals previously unknown components 4 arcsec north and 18 arcsec south of Mrk 463E, and a source 10 arcsec to the northwest which is believed to be linked to the nuclear region. The western nucleus, Mrk 463W, was detected at 6 and 20 cm; it has a radio luminosity comparable to that of a moderately luminous Seyfert galaxy or a highly luminous starburst galaxy. Near-infrared images at wavelengths less than 2.3 microns show the two nuclei; only Mrk 463E has been detected at 3.7 microns. Brightness profiles of Mrk 463E are unresolved at K and L-prime, but extended at J. It is inferred from the fact that radio continuum components 4-18 kpc from the nucleus are aligned with previously known 0.05-1.5 kpc radio structure and extended conical O-III forbidden omission lines that Mrk 463E is powering weak radio lobes and therefore exemplifies a transition between the confined linear sources in Seyfert galaxies and the extended lobes in classical quasars and radio galaxies.

Mazzarella, J. M.↗

NASA NTRS1992

Expected level of self-Compton scattering in radio loud quasars

Radio-loud quasars usually contain parsec-scale nonthermal jets. The most compact emission region ('the core'), and perhaps some of the moving 'knots', are expected to be efficient producers of inverse Compton scattered X-rays and gamma-rays since many of the synchrotron photons will upscatter before escaping. Through multifrequency flux density observations and Very Long Baseline Interferometry (VLBI) measurements of angular sizes, one can predict the flux density of this self-Compton high-energy emission. It is not always the case that the brightest synchrotron sources are also the brightest X-ray and gamma-ray sources. Perhaps a better predictor of high-energy brightness is the ratio of hard X-ray to high-frequency radio emission. Using the synchrotron self-Compton relations, we predict the gamma-ray fluxes of several sources we expect to be detected by the Energetic Gamma Ray Experiment Telescope (EGRET). More accurate predictions will be made when we complete a program of contemporaneous radio-submillimeter and X-ray observations during the course of the EGRET all-sky survey.

Bloom, Steven D.↗

NASA NTRS1992

Optimal extraction of quasar Lyman limit absorption systems from the IUE archive

The IUE archive contains a wealth of information on Lyman limit absorption systems (LLS) in quasar spectra. QSO spectra from the IUE data base were optimally extracted, coadded, and analyzed to yield a homogeneous samples of LLS at low red shifts. This sample comprise 36 LLS, twice the number previously analyzed low z samples. These systems are ideal for the determination of the origin, redshift evolution, ionization, velocity dispersions and the metal abundances of absorption systems. Two of them are also excellent targets for the primordial Deuterium to Hydrogen ratio.

Tytler, David↗

NASA NTRS1992

Detection of high-energy gamma radiation from quasar 3C 279 by the EGRET telescope on the Compton Gamma Ray Observatory

Intense gamma radiation has been observed from the direction of the quasar 3C 279 throughout the energy range from 30 MeV to over 5 GeV by the Energetic Gamma Ray Experiment Telescope (EGRET) during the period June 15-28, 1991. Its spectrum is well represented by a photon differential power-law exponent of 2.0 +/- 0.1, with a photon intensity above 100 MeV of (2.8 +/- 0.4) x 10 exp -6/sq cm s. For E is greater than 100 MeV, the 2-sigma upper limits were 1.0 x 10 exp -6/sq cm s in 1973 from the SAS 2 observations and 0.3 x 10 exp -6/sq cm s for the combined 1976, 1978, and 1980 COS B observations. Hence, there has been a large increase in high-energy gamma-ray intensity relative to the earlier times, as there has been in the radio, infrared, optical, and X-ray ranges. This source is the most distant and by far the most luminous gamma-ray source yet detected.

Hartman, R. C.↗

NASA NTRS1992

The soft X-ray excess in Einstein quasar spectra

An SNR-limited subsample of 14 quasars from the Wilkes and Elvis (1987) sample is presently investigated for low-energy excess above a high-energy power law in the X-ray spectra obtained by the Einstein Imaging Proportional Counter. A significant excess that is 1-6 times as strong as the high-energy component at 0.2 keV is noted in eight of the 14 objects. In the case of 3C273, multiple observations show the excess to be variable.

Masnou, J. L.↗

NASA NTRS1992

A complete spectral analysis of the flare of the quasar 3C 273

The 1983 quasar outburst of the source 3C 273 is investigated by studying several months of radio-frequency and UV data that represent about six decades of frequency. The data are analyzed to account for the differences inherent in various photometric systems, and adjustments are made for reddening and line emission. Individual wavelengths are delineated in light curves which indicate distinct periods of burst activity for specific wavelengths. The spectrum of the flare is characterized by a trend in which the critical wavelength evolves toward longer wavelengths as the outburst progresses. It is suggested that the data are consistent with a model of an expanding gas cloud that emits Doppler-boosted synchrotron radiation. The spectral curves, light curves, and the spectral index are of use in the analysis of similar phenomena when VLBI data of superluminal motion are incorporated.

Sadun, Alberto C.↗

NASA NTRS1992

Quasar absorption lines with a nonzero cosmological constant

Quasar absorption lines in flat universes with nonzero cosmological constant Lambda are examined and compared with more conventional zero Lambda universes. Various evolution effects for intergalactic absorbers and the observed number density evolution of each absorption system are examined in order to discriminate between evolution effects and the cosmological models. An interesting interaction between Lambda effects and cosmic absorption phenomena is explored. Equations describing IGM absorption statistics are developed for nonzero Lambda cosmologies, both for unevolving absorber populations and some of the more popular physical models for the IGM and intergalactic clouds which include the effects of the absorber evolution.

Turner, Edwin L.↗

NASA NTRS1992

Accurate positions and finding charts for 528 high-redshift, luminous quasars

Finding charts and J2000 positions accurate to about 1 arcsec are presented for the 528 high-redshift (z is greater than 1), luminous (MV is less than -25.5) quasars investigated in the HST Snapshot Survey. A histogram showing the distribution of the differences between the positions determined by the Space Telescope Science Institute Astrometric Support Program and the catalog positions is provided.

Schneider, Donald P.↗

NASA NTRS1993

Ionization states of metallic absorption-line systems in continua of quasars

Ionization states of metallic absorption-line systems in continua of quasars (QSO's) are studied, assuming that the metallic lines arise in gaseous halos of high-redshift galaxies in photoionization equilibrium under the background UV radiation, and constraints on the intensity and spectral shape of the UV radiation are obtained. Then a structure of absorbers suitable for all of the metallic absorption line systems are discussed.

Denda, Kiyomi↗