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At least 415 records · Page 23
All sourcesNASA NTRSDOE OSTI
NASA NTRS1983

Luminosity function and cosmological evolution of X-ray selected quasars

The preliminary analysis of a complete sample of 55 X-ray sources is presented as part of the Medium Sensitivity Survey of the Einstein Observatory. A pure luminosity evolutionary law is derived in order to determine the uniform distribution of the sources and the rates of evolution for Active Galactic Nuclei (AGNs) observed by X-ray and optical techniques are compared. A nonparametric representation of the luminosity function is fitted to the observational data. On the basis of the reduced data, it is determined that: (1) AGNs evolve cosmologically; (2) less evolution is required to explain the X-ray data than the optical data; (3) the high-luminosity portion of the X-ray luminosity can be described by a power-law with a slope of gamma = 3.6; and (4) the X-ray luminosity function flattens at low luminosities. Some of the implications of the results for conventional theoretical models of the evolution of quasars and Seyfert galaxies are discussed.

Maccacaro, T.↗
NASA NTRS1985

X-ray luminosity and the hydrogen-line ratios of quasars and Seyfert galaxies

New observations of the Ly-alpha/H-alpha ratio in a set of X-ray-selected active galactic nuclei and an archival study of International Ultraviolet Explorer (IUE) observations of Ly-alpha in low-redshift quasars and Seyfert galaxies have been used to form a large sample (49 objects) for studying the influence of soft X-rays on the enhancement of Balmer emission in the broad-line region. In common models of broad-line clouds, the Balmer lines are formed deep in the interior, largely by collisional excitation. Heating within the clouds is provided by soft X-ray radiation, while Ly-alpha is formed mainly by recombination after photoionization. The ratio Ly-alpha/H-alpha is expected to depend weakly on the ratio of ionizing ultraviolet luminosity to X-ray luminosity, L(UV)/L(x). If the Ly-alpha luminosity is used as a measure of L(UV), a weak dependence of Ly-alpha/H-alpha on the X-ray luminosity is found (valid at the 94 percent confidence level), similar to previous results. Future definitive studies depend more critically on contemporaneous observations and better internal reddening estimates rather than a larger sample size.

Kriss, G. A.↗
NASA NTRS1985

VLBI monitoring of the superluminal quasar 3C 273, 1977-1982

The compact ratio structure of the quasar 3C 273 has been monitored with a VLBI array at 5.0 and 10.7 GHz at six-month intervals during 1977-1982. THe VLBI monitoring observations, data reduction, calibration, and mapping techniques are described, and hybrid maps from the monitoring program are presented along with a brief discussion of the main features. Results on the source kinematics (superluminal motion of the knots and their time evolution) are given, and synchro-Compton models of the knots are presented and limits to their Doppler factors (delta) derived from X-ray measurements. The constraints on jet geometry provided by the combined measurements of delta and v/c are considered. For the core, the inhomogeneous jet model of Koenigl (1981) is compared with the available data. The relation between 3C 273 and the similar superluminal source 3C 345 is discussed.

Unwin, S. C.↗
NASA NTRS1985

Theoretical quasar emission-line ratios. VII - Energy-balance models for finite hydrogen slabs

The present energy balance calculations for finite, isobaric, hydrogen-slab quasar emission line clouds incorporate probabilistic radiative transfer (RT) in all lines and bound-free continua of a five-level continuum model hydrogen atom. Attention is given to the line ratios, line formation regions, level populations and model applicability results obtained. H lines and a variety of other considerations suggest the possibility of emission line cloud densities in excess of 10 to the 10th/cu cm. Lyman-beta/Lyman-alpha line ratios that are in agreement with observed values are obtained by the models. The observed Lyman/Balmer ratios can be achieved with clouds whose column depths are about 10 to the 22nd/sq cm.

Hubbard, E. N.↗
NASA NTRS1985

The ultraviolet spectra of intermediate-redshift quasars

In the present sample of 21 intermediate redshift quasars, for which IUE Lyman-alpha line flux and UV continuum observations have been conducted, an extrapolation of the continuum to the X-ray region is noted to fall at least 2-sigma above the measured X-ray flux, at 2 keV, for two-thirds of the sample. The strength of Lyman-alpha is well correlated with the luminosity at 1450 A, and an estimate of the covering factor on the basis of the ratio of the number of Lyman-alpha to ionizing protons yields a value for the sample which, at about 0.17, is consistent with the number of observed Lyman discontinuities.

Kinney, A. L.↗
NASA NTRS1984

Hydrogen line ratios in Seyfert galaxies and low redshift quasars

New observations of the Lymal alpha radiation/hydrogen alpha radiation ratio in a set of X-ray selected active galactic nuclei and an archival study of International Ultraviolet Explorer (IUE) observations of Lymal alpha low redshift quasars and Seyfert galaxies have been used to form a large sample for studying the influence of soft X-rays on the enhancement of Balmer emission in the broad line region. In common models of broad line clouds, the Balmer lines are formed deep in the interior, largely by collisional excitation. Heating within the clouds is provided by soft X-ray radiation, while Lymal alpha is formed mainly by recombination after photoionization. The ratio Lymal alpha/Halpha is expected to depend weakly on the ratio of ionizing ultraviolet luminosity to X-ray luminosity (L sub UV/l sub x). If the Lymal alpha luminosity is used as a measure of L sub UV' a weak dependence of Lymal/H alpha on the X-ray luminosity is found similar to previous results.

Kriss, G. R.↗
NASA NTRS1985

X-ray secondary heating and ionization in quasar emission-line clouds

Accurate Monte Carlo computations of the X-ray secondary electron heating, ionization, and excitation of H and He gas in interstellar space and in quasar emission-line clouds, are presented. The fraction of energy deposited in each form is sensitive to the background ionization fraction, x = n(H+)/n(Htot), and can affect the temperature, ionization state, and line emissivities at large depths in X-ray photoionized clouds. Analytic fits are provided for these energy fractions over the range 0.0001-1 for primary electron energies up to many keV. In both broad-line and narrow-line clouds, emission lines sensitive to the energy budget and electron density may be strongly affected.

Shull, J. M.↗
NASA NTRS1986

Multifrequency observations of the superluminal quasar 3C 345

Attention is given to the continuum properties of the superluminal quasar 3C 345, on the basis of radio, optical, IR, and X-ray frequency monitorings, as well as by means of simultaneous multifrequency spectra extending from the radio through the X-ray bands. Radio outbursts, which appear to follow IR-optical outbursts by about one year, first occur at the highest frequencies, as expected from optical depth effects; the peak flux is nevertheless often reached at several frequencies at once. The beginning of outbursts, as defined by mm-measurements, corresponds to the appearance of the three known 'superluminal' components. An increase in the X-ray flux during 1979-1980 corresponds to increased radio flux, while the IR flux changes in the opposite sense.

Bregman, J. N.↗
NASA NTRS1986

How are quasars fueled? Simulating interstellar gas in tidally disturbed galaxies

Whether gravitational tides from companions trigger global instabilities in spiral galaxy disks and thus rapid flows of gas into the nucleus to fuel activity is investigated. An n-body computer program is used to simulate the disk of the spiral galaxy within a much more stable, high-velocity dispersion spherical halo. Under sufficient perturbation, the disk undergoes violent distortions due to the disturber and its self-gravitation. The tidal action of companions was simulated and the tidal strengths at which the instabilities appear to match those of the observed companions of Seyferts and quasars was shown. With the additional modifications planned, the gas flow will be more realistically simulated to compare with observations (e.g., colors, velocity fields) of active galaxies.

Byrd, Gene G.↗
NASA NTRS1986

The effects of electron scattering opacity in the broad emission-line regions of quasars

It is demonstrated that three observational puzzles in quasars, nearly symmetric Lyman-alpha profiles, weakly asymmetric C IV 1549 A, and offsets between the peaks of these high ionization lines and the systematic velocity, can all be explained by the standard model of the physical state of the broad-line region with the addition of the simplest possible kinematics. Electron scattering in the intercloud medium is the essential ingredient that had been previously neglected. It is shown that the profiles depend at least as sensitively on the run of physical conditions as on the velocity law. Approximate analytic representations of the line emissivities based on detailed photoionization models are used to describe that dependence.

Kallman, T. R.↗
NASA NTRS1986

X-ray to infrared continua of optically selected quasars

X-ray-to-IR continuum data for nine optically selected (PG) quasars show a form which can be simply described in terms of two components - a power law of slope about 1 joining smoothly the 1-10-micron IR with the 0.1-10-keV X-ray points and (superimposed on this) a 'big bump' of optical-UV emission. The 'big bump' can be interpreted as thermal emission from an accretion disk. In a unique case where this 'big bump' extends to soft X-rays, the accretion disk parameters can be constrained interestingly.

Elvis, Martin↗
NASA NTRS1987

Superluminal motion in the double-lobed quasar 3C 245

VLBI observations of 3C 245 obtained at 10.7 GHz with the Mark III array at five epochs in 1981-1986 are reported. The data-reduction and model-fitting procedures are explained, and the results are presented in tables, graphs, and maps along with previously published data on 11 other superluminal sources. The central component of the quasar is found to have an apparent transverse velocity of (3.1 + or - 1.4) c/h, assuming H0 = 100h km/s Mpc and q0 = 0.5. From the apparent source orientation (based on R, variability, apparent curvature, and projected linear size) it is inferred that the apparent transverse velocity is anticorrelated with the angle to the line of sight, as predicted by relativistic-beaming models of superluminal motion.

Hough, D. H.↗
NASA NTRS1987

Molecule formation in quasar broad-line cloud gas

Models for the broad-line emitting clouds of quasars typically assume that the clouds have column densities of at most 10 to the 23rd/sq cm. The consequences of relaxing this assumption are examined, and it is shown that: (1) at slightly larger column densities the gas may cool to about 1000 K as a result of molecule formation; (2) in much of the molecule-forming region the temperature may have either of two values, about 1000 K or 6000-8000 K; (3) the strengths of most observable optical lines, including C II semiforbidden 2326-A lines and Fe II lines, are unaffected by such large column densities; and (4) lines from low-ionization species such as Na I are readily formed at large column densities. Observations of such lines provide evidence for large cloud column densities.

Kallman, T.↗
NASA NTRS1987

Can quasars ionize the intergalactic medium?

The intergalactic environmental impact of quasars is studied in the light of recent QSO observations, addressing in particular whether QSOs can ionize the IGM at z greater than 3.0. After deriving the expansion law for cosmological ionization fronts in a Friedmann universe, it is investigated whether the QSO population is sufficient for these ionized regions to overlap by z of roughly 3.5. The effects of cosmological I-fronts on Lyman-alpha clouds in QSO absorption spectra are discussed.

Donahue, Megan↗
NASA NTRS1988

Evidence for accretion disks in highly polarized quasars

The results of a search for thermal components in 11 highly polarized quasars (HPQs) using UVBRI polarimetry and photometry are reported. The 2000-2500 A luminosities of the thermal components are calculated and the estimated luminosities of the broad-line region (BLR) are given in the same wavelength for comparison. The observed optical continua are modeled as a combination of polarized synchrotron emission, unpolarized emission from the BLR, and an unpolarized flat spectral component that may be optically thick thermal emission from an accretion disk. Evidence for thermal emission components is found in three HPQs: PKS 0420-014, B2 1156+295, and 3C 454.3, with marginal evidence in another two, PKS 1510-089 and PKS 2345-167.

Smith, Paul S.↗
NASA NTRS1988

Radio properties of the highly polarized, quiescent quasar OI 287

The highly polarized, quiescent quasar OI 287 has been observed with high resolution at 2 cm and 6 cm in an effort to determine the origin of some of the object's peculiar properties. The results seem to rule out the classification of OI 287 as a blazar. Extrapolation of the radio core spectrum to midinfrared wavelengths fails to predict the infrared flux by at least an order of magnitude. This supports the conclusion that the infrared emission and the radio emission do not originate in the same synchrotron-emitting component. The high optical polarization could be related to possible broad absorption lines in the unobserved UV regime, or it could be caused by reflection into the line of sight by scattering agents distributed in a thin disk.

Ulvestad, James S.↗
NASA NTRS1987

Relativistic beaming and the nuclei of double-lobed quasars

If the simple relativistic beaming model for extragalactic radio sources is at all correct, Doppler boosting may bias us toward choosing those objects pointing nearly at us in flux-density limited samples. In order to minimize any such effects in statistical tests of physical theories of these objects, an attempt is made to define a complete sample of sources with random orientations. For a sample of 26 double-lobed quasars, the dependence of a number of observable features on the angle to the line of sight and on the superluminal velocity is examined, and the distribution of superluminal velocities for the sources is measured. Two new superluminals are reported, and their apparent velocities and nuclear strengths are discussed.

Hough, David H.↗
NASA NTRS1988

Multifrequency observation of the optically violent variable quasar 3C 446

Nearly 20 years of optical and radio monitoring data as well as seven multifrequency spectra of the violently variable quasar 3C 446 are reported. The monitoring data suggest a correlation between the radio and optical outbursts, with the optical flare preceding the radio activity by 400-600 days. Considerable processing occurs in the optical-emitting plasma before it becomes radio-emitting plasma. Within the radio band, outbursts proceed from high to low frequencies. The flat radio spectrum turns over at 3-10 x 10 to the 11th Hz and the continuum steepens with frequency. The X-ray emission lies an order of magnitude above an extrapolation of the optical-UV spectrum and has a harder spectrum. The power is primarily concentrated in the submillimeter and infrared region. The data suggest that the X-rays are produced by the inverse Compton process from an emitting region smaller than but related to the synchrotron-emitting UV-IR region. The characteristic size of the emitting region increases with decreasing frequency.

Bregman, Joel N.↗