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At least 541 records · Page 30
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NASA NTRS1981

Galactic disk absorption lines in the spectrum of the quasar 3C 273

It is pointed out that the line of sight to 3C 273 is unique because of the possibility of obtaining both radio and optical absorption line measurements. This information may make it possible to understand in detail the structure of the interstellar medium along the line of sight. To take advantage of this possibility, high-resolution optical spectra were obtained of 3C 273. The observations were made using the echelle spectrograph at the Cassegrain focus of the 4 m telescope at the Cerro Tololo Inter-American Observatory in 1980 January. Both Na D and Ca K lines were obtained on the same plate. In the visual lines, the galactic disk absorption shows a main component at 20 km/sec with a trailing wing in the negative velocities.

Cowie, L. L.↗
NASA NTRS1981

The Lyman-alpha/H-alpha ratio in solar flares and quasars

Constant temperature and density solar flare models are constructed with temperature and hydrogen density values that reflect reasonable nonlinear averages of those parameters in the depth dependent solar flare chromosphere models of Lites and Cook (1979). Acceptable values of the intensity ratios L-alpha/H-alpha and H-beta/H-alpha correspond to temperatures from about 9000 to 13,000 K, and hydrogen densities from 10 to the 11th to 10 to the 15th cu cm. The H-alpha and Ly-alpha source functions are thermalized at depths consistent with those inferred from independent studies, although the observed Ly-alpha/H-alpha ratio does not necessarily imply an electron temperature appropriate to the Planck function ratio. It is also shown that the value of Ly-alpha/H-alpha depends on the temperature, hydrogen density, and the optical depth of the emitting chromospheric layer.

Canfield, R. C.↗
NASA NTRS1981

Thermal instabilities in radiatively driven winds - Application to emission line clouds of quasars and active galactic nuclei

It is shown that radiatively driven, optically thin winds from active galactic nuclei are thermally unstable, provided that the mass loss rates are not smaller than about 50 solar masses/year. Clouds form at distances of less than about 1 pc, with electron densities greater than 100 million/cu cm, temperatures of more than 10,000 K, and radii between 3 x 10 to the 14th and 10 to the 15th cm. These values agree with the values deduced from observations. Since the clouds are formed in a high velocity wind, this model, as does the model by Eilek and Caroff (1979), avoids problems of disruption inherent in using radiation pressure to accelerate clouds from rest to velocities up to 0.1 c needed to explain the emission line widths. The thermal balance of the gas is discussed, the criteria for thermal instability are given, and a perturbation analysis is made. This analysis is restricted to objects with bolometric luminosities lower than approximately 2 x 10 to the 46th ergs/sec. For more luminous objects, only winds slightly optically thick to electron scattering can be unstable, provided one can extend this analysis to the optically thick case.

Beltrametti, M.↗
NASA NTRS1981

Theoretical quasar emission line ratios. IV - General asymptotic escape probabilities and the effects of linear Stark broadening

Established techniques permitting the evaluation of exact asymptotic forms of photon escape probabilities for a wide range of absorption coefficient profile types are used for QSO radiative transfer effect studies, and it is shown that the exact formulae are easily evaluated. It is found that some of the commonly used expressions overestimate the true value by a factor of about two, and a computationally expedient approximate expression is derived for the asymptotic form of the photon escape probability when linear Stark broadening contributes to the line absorption coefficient profile in hydrogenic lines. Attention is given to Lyman-alpha, for the case in which partial redistribution, over the linear Stark component of the line absorption coefficient profile, dominates single-flight photon escape.

Puetter, R. C.↗
NASA NTRS1982

The heating of dust in the broad-line regions of active galaxies and quasars

The present investigation is concerned with the relevant energy sources which heat (and, in many cases, destroy) the dust grains in the emission-line clouds (ELCs) that give rise to the broad-line emission seen in active extragalactic objects. The heating rates from the external radiation field, trapped line radiation, and the diffuse bound-free continua are compared. The relevant grain parameters are considered, taking into account the composition, size, and emission and absorption cross sections. Three distinct regions of an ELC are identified, including the cloud face, the extended ionized zone, and the neutral zone. The thermal emission of the dust in each of these regions is shown to be different. An upper limit to the heating is provided by an LTE model which is discussed.

Rudy, R. J.↗
NASA NTRS1983

A thermal interpretation of the X-ray spectra of quasars, active galactic nuclei, and Cygnus X-1

It is shown that spherically accreting black hole models emitting bremsstrahlung radiation provide an excellent fit to the observed X-ray spectrum of typical active galactic nuclei, as well as of 3C 273 and Cygnus X-1. The spectral slope remains approximately constant under changes of luminosity, as required by observations. Only one parameter, T(max), is needed for the fit.

Meszaros, P.↗
NASA NTRS1983

An optical search for 'X-ray quiet' quasars

Recent progress is described in an ongoing program to develop criteria for selecting faint QSOs in several tens of square degrees of sky previously surveyed by the Einstein X-ray Observatory. The physical properties of the selected QSOs are discussed in terms of the difficulties they present for QSO X-ray detection techniques. A series of F-plates for about two dozen fields have been obtained, but only a few of the X-ray images have been analyzed. The analysis of the plates consists of scanning several times with a binocular microscope. The number of positively identified QSOs per plate is 3, with a limiting magnitude of about 21 B. For about one half of the candidates, more than one emission line is identified at a consistent redshift, confirming that the object is a QSO. A table is presented which lists some of the flux density and luminosity data for the QSO objects detected thus far.

Anderson, S. F.↗
NASA NTRS1985

Theoretical quasar emission-line ratios. VIII - Implications of free-free emission for QSO clouds

A range of simple theoretical models is presented which explores the effects of high densities in the broad-line clouds on near-IR free-free emission. These calculations demonstrate in a qualitiative way that large near-IR fluxes can result from high densities without compromising the observed hydrogen line spectrum. It is suggested that high density in the emission-line region provides a plausible explanation of several features seen in the spectra of QSOs and active galaxies, including (1) the 3-micron bump and/or the near-IR spectral break seen in many QSOs, (2) the low observed Pa-alpha broad-line fluxes seen in some objects, and (3) the highly depressed Br-gamma broad-line fluxes seen in active galaxies. It is also found that free-free emission in such clouds can be an important coolant.

Puetter, R. C.↗
NASA NTRS1986

The central engine of quasars and active galactic nuclei Hadronic interactions of shock-accelerated relativistic protons

The detailed dynamics are presented of a model for AGNs and QSOs based on spherically symmetric accretion onto a black hole and dissipation of the accretion energy in a collisionless shock. The basic assumptions and equations relevant for the model are presented, and the relations between the physical parameters are given in terms of the shock parameters. The latter are calculated for varying Mach numbers, and the resulting scaling laws are discussed and compared to the data, in particular the recently discovered correlations between luminosity and mass. The model can naturally produce ultrarelativistic electrons with flat spectra, a feature which, when coupled with e(+)-e(-) pair production, can account for the observed continuum radiation from these objects over essentially the whole electromagnetic spectrum. The model also predicts the emission of high-energy neutrinos with a luminosity roughly equal to that of the photons.

Kazanas, D.↗
NASA NTRS1986

An archival study of IUE observations of low redshift quasars

The emission line and continuum characteristics of a sample of 18 low redshift QSOs from the IUE Archives are presented. The spectral index is found to have a mean value of 1.22, which is considerably steeper than the index of the same spectral region for high redshift QSOs. The line ratios L alpha/H beta and C IV/L alpha for the sample, 9.4 + or - 1.9 and 0.61 + or - .06, respectively, are consistent with the values of the photoionization models. When the low redshift sample is extended with Seyfert galaxies, intermediate redshift QSOs and high redshift QSOs, the equivalent width of L alpha and CIV anticorrelates with the luminosity at 1450 A. The slope of the anticorrelation is seen to be steeper for the higher redshift samples than for the lower redshift samples.

Cheng, F. H.↗
NASA NTRS1986

Quasar evolution and luminosity function - An X-ray perspective

X-ray properties of optically selected QSOs are reviewed, and their implications for the QSO evolution and luminosity function are summarized. Analyses of the explicit dependence of the X-ray to optical luminosity ratio parameter alpha (a,x) on redshift z and optical luminosity Lo are described, and results from a recent analysis of complete, magnitude-limited samples of optically selected QSOs are presented. It is shown that probably all optically-selected QSOs are X-ray loud and that comparisons of the optical evolution and luminosity function, combined with the alpha(o,x)(z,Lo) dependence, with X-ray-selected samples are numerically sensitive to the details of the input.

Avni, Y.↗