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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.

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At least 415 records · Page 23
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NASA NTRS1982

On the origin of relativistic particles and gamma-rays in quasars

A model for a class of quasars and active galactic nuclei is proposed in which a shock around a massive black hole randomizes the infall kinetic energy of spherically accreting matter producing a nonthermal spectrum of high energy protons. It is suggested that these protons are responsible for the secondary production (via proton decay) of the radio emitting high energy electrons and also of high energy gamma-rays (via proton decay and inverse Compton interactions of the electrons). The correlation between radio and gamma-ray emission implied by the model is in good agreement with observations of 3C273. Observation of the flux of high energy neutrinos from quasars may provide a test for the model.

Protheroe, R. J.↗
NASA NTRS1982

Goddard X-ray astronomy contributions to the IAU/COSPAR (1982)

The relation of X-ray flux to both the continuum flux in the optical and radio bands, and to the line emission properties of these objects were studied. The Einstein Observatory, because of increased sensitivity and improved angular resolution, increased substantially the number of known X-ray emitting active galactic nuclei. The Einstein imaging instruments detected morphology in AGN X-ray emission, in particular from jetlike structures in Cen-A, M87, and 3C273. The improved energy resolution and sensitivity of the spectrometers onboard the Observatory provide information on the geometry and ionization structure of the region responsible for the broad optical emission lines in a few AGN's. This information, combined with theoretical modeling and IUE and optical observations, allows the construction of a moderately detailed picture of the broad line region in these objects.

Holt, S. S.↗
NASA NTRS1982

The passage of a star by a massive black hole

The gridless, smoothed particle hydrodynamic method of Lucy (1977) and Gingold and Monaghan (1977, 1980, a, b, 1981) is used to solve the three-dimensional problem posed by the effects on a 1.0 solar mass star of a 10,000 solar mass black hole's passage, in an initially parabolic orbit with a variety of pericenter distances. It is found that the tidal forces induce rotation and radial and nonradial pulsations in the star, and that the loss of orbital energy to these internal motions leads to the star's capture by the black hole. While the outer layers of the star are disrupted at small pericenter distances, the entire star is destroyed at even smaller distances. These results are applicable to X-ray sources, active galactic nuclei, and quasars

Nolthenius, R. A.↗
NASA NTRS1983

Prominent ultraviolet emission lines from Type 1 Seyfert galaxies

IUE Explorer data with a 6-A resolution are reported for Ly-alpha, C IV 1550 A, C III semiforbidden line 1909 A, and Mg II 2800 A, as well as the continuum at 1450 A, for 20 Seyfert galaxies and one quasar. Good correlation is found between the continuum and the line fluxes, indicating that photoionization is the dominant heating mechanism for these active galactic nuclei. A combination of the data for Seyferts and high redshift quasars covering five orders of magnitude in continuum luminosity shows the lower luminosity objects to have higher equivalent widths for their emission lines, suggesting that the covering factor increases with decreasing luminosity.

Wu, C.-C.↗
NASA NTRS1983

Compton heated winds and coronae above accretion disks. I Dynamics

X rays emitted in the inner part of an accretion disk system can heat the surface of the disk farther out, producing a corona and possibly driving off a strong wind. The dynamics of Compton-heated coronae and winds are analyzed using an approximate two-dimensional technique to estimate the mass loss rate as a function of distance from the source of X rays. The findings have important dynamical implications for accretion disks in quasars, active galactic nuclei, X ray binaries, and cataclysmic variables. These include: mass loss from the disk possibly comparable with or exceeding the net accretion rate onto the central compact object, which may lead to unstable accretion; sufficient angular momentum loss in some cases to truncate the disk in a semidetached binary at a smaller radius than that predicted by tidal truncation theories; and combined static plus ram pressure in the wind adequate to confine line-emitting clouds in quasars and Seyfert galaxies.

Begelman, M. C.↗
NASA NTRS1983

VLA observations of a complete sample of extragalactic X-ray sources. II

A complete sample of 35 X-ray selected sources found with the Einstein Observatory has been observed with the Very Large Array at 6 cm to investigate the relationship between radio and X-ray emission in extragalactic objects. Detections include three active galactic nuclei (AGNs), two clusters or groups of galaxies, two individual galaxies, and two BL Lac objects. The frequency of radio emission in X-ray selected AGNs is compared with that of optically selected quasars using the integral radio-optical luminosity function. The result suggests that the probability for X-ray selected quasars to be radio sources is higher than for those optically selected. No obvious correlation is found in the sample between the richness of X-ray luminosity of the cluster and the presence of a galaxy with radio luminosity at 5 GHz larger than 10 to the 30th ergs/s/Hz.

Schild, R.↗
NASA NTRS1983

Optical characteristics of young quasars as sources of the cosmic X-ray background

The sources which dominate the thermal cosmic X-ray background cannot have X-ray spectra similar to the power laws measured for bright active galactic nuclei. The optical consequences of this disparity are pursued by considering a standard model for the photoexcitation and heating of the line emitting gas surrounding a central source (e.g., such as a quasar). The optical line emission to be associated with compact young quasar sources having the same X-ray spectrum as the X-ray background is found to be substantially different from that characteristic of typical quasars. Implications on quasar source counts and the identification of such new objects are discussed.

Boldt, E.↗
NASA NTRS1983

Spatial Fluctuations in the Diffuse Cosmic X-Ray Background

The bright, essentially isotropic, X-ray sky flux above 2 keV yields information on the universe at large distances. However, a definitive understanding of the origin of the flux is lacking. Some fraction of the total flux is contributed by active galactic nuclei and clusters of galaxies, but less than one percent of the total is contributed by the or approximately 3 keV band resolved sources, which is the band where the sky flux is directly observed. Parametric models of AGN (quasar) luminosity function evolution are examined. Most constraints are by the total sky flux. The acceptability of particular models hinges on assumptions currently not directly testable. The comparison with the Einstein Observatory 1 to keV low flux source counts is hampered by spectral uncertainties. A tentative measurement of a large scale dipole anisotropy is consistent with the velocity and direction derived from the dipole in the microwave background. The impact of the X-ray anisotropy limits for other scales on studies of large-scale structure in the universe is sketched. Models of the origins of the X-ray sky flux are reviewed, and future observational programs outlined.

Shafer, R. A.↗
NASA NTRS1983

The Einstein Observatory Medium Sensitivity Survey - Optical identifications for a complete sample of X-ray sources

It is suggested that virtually all the X-ray sources in the Einstein Observatory's Medium Sensitivity Survey flux range can be identified with objects visible on the POSS, on the basis of the complete identification of all sources north of -25 deg declination. There is no evidence for a significant population of 'blank field' X-ray sources at this flux level, and therefore no evidence for any new X-ray source class with very high L(x)/L(v). Most of the quasars detected in the present survey are spectroscopically similar to optical or radio-selected quasars. About 25 percent of the quasar sample, however, had reddish colors, and permitted lines dominated by a narrow-line component. These objects form a second sequence of active galactic nuclei, distinct in their optical properties from the broad line objects.

Stocke, J. T.↗
NASA NTRS1983

Shocks in spherically accreting black holes - A model for classical quasars

Spherically symmetric accretion onto black holes is thought to be very inefficient if the flow is nondissipative because of the inadequacy of compression heating and the short time available to radiate. It is shown that standing shock solutions exist where the downstream protons are heated to temperatures at which the fluid becomes almost collisionless. The material then moves inwards at the much slower diffusion rate given by the p-e energy exchange time scale. For a given accretion rate, a self-consistent shock radius is found by matching the flow equations above and below that value, showing that such shocks can be self-sustaining. Efficiencies reaching up to 0.1-0.3 can be achieved, with most of the luminosity in the hard X-ray and gamma-ray, as well as optical-IR ranges. This provides a new model for the classical (radio-quiet) quasars and active galactic nuclei and possibly for galactic sources such as Cyg X-1.

Meszaros, P.↗
NASA NTRS1983

X-ray sources of cosmological relevance

The results of extragalactic observations made with the imaging proportional counter (IPC) on board the Einstein Observatory satellite are reported. Quasars were found to be strong X ray sources with correlations between the X ray and optical luminosities; strong radio emissions also correlated well with X ray luminosities. The data indicate that the ratio of the X ray and optical luminosities is inversely proportional to the optical luminosity. Active galactic nuclei are also strong X ray sources. A continuity exists between the X ray behavior of Seyfert galaxies and quasars, while BL Lac objects exhibit X ray variability exceeding that of quasars, although some BL Lac objects may be quasars. The X ray data revealed evolutionary phases in galactic cluster structures.

Maccacaro, T.↗
NASA NTRS1983

O IV temperature determination for NGC 7662

A novel method is presented for the determination of electron temperatures in the high excitation zones of planetary nebulae and active galactic nuclei, using the ratio of the O IV line at 25.87 microns to those near 1400 A to calculate a temperature within the He III zone of the galaxy NGC 7662. The observed fluxes yield a lower bound to the temperature of 14,730 K, which is noted to be significantly higher than the O III temperature of about 12,000 K.

Shure, M. A.↗
NASA NTRS1983

Einstein X-ray observations of optical-radio selected areas

Einstein Observatory Imaging Proportional Counter X-ray observations of fields in SA 57, SA 68, and U Mi that possess deep optical and radio coverage have been conducted with a sensitivity comparable to that of the Einstein Observatory's medium sensitivity survey of serendipitous X-ray sources. The number of X-ray sources detected is consistent with a log N(greater than S) - log S slope of about -1.5. The fraction of active galactic nuclei in the X-ray sample is less than about 50 percent, with the rest being composed of clusters and normal galaxies, consistent with the medium sensitivity survey.

Katgert, P.↗
NASA NTRS1984

Optical characteristics of young quasars as sources of the cosmic X-ray background

The sources which dominate the thermal cosmic X-ray background cannot have X-ray spectra similar to the power laws measured for bright active galactic nuclei. The optical consequences of this disparity are pursued by considering a standard model for the photoexcitation and heating of the line-emitting gas surrounding a central source (e.g., such as a quasar). The optical line emission to be associated with compact young quasar sources having the same X-ray spectrum as the X-ray background is found to be substantially different from that characteristic of typical quasars. Implications on quasar source counts and the identification of such new objects are discussed.

Boldt, E.↗
NASA NTRS1984

A wind-type model for the generation of astrophysical jets

Wind-type solutions for the generation of astrophysical jets from active galactic nuclei and stellar sources, such as those associated with SS 433 and protostellar objects, are discussed. Acceleration, collimation, and morphology are consistently interpreted in terms of a flow starting from the galactic or stellar core inside the 'throat' of a thick accretion disk.

Ferrari, A.↗
NASA NTRS1984

X-ray spectral signatures of accreting black holes

The X-ray properties of the binary black hole candidates include distinctive bimodal spectral behavior where the X-ray spectra switch between being very soft (alpha greater than 3) and hard (alpha about 0.7). It is pointed out that this spectral transition occurs at about 1 percent L(edd) (for accretion onto a 10 solar masses black hole), and that this corresponds to the luminosity where an optically thick electron-positron pair plasma will form. Active galactic nuclei (AGN) are often considered to be accreting black holes. An X-ray spectral similarity between the above binary sources and both the Seyfert I and BL Lac classes of AGN is demonstrated. As many of these objects radiate most of their luminosity as X-rays, such a similarity may be expected if the underlying mechanisms are similar.

White, N. E.↗
NASA NTRS1984

The Euclidean slope of the X-ray source counts - A 'cosmic conspiracy'

The selection criteria adopted for the Einstein Observatory Medium Sensitivity survey are delineated and the results of an analysis of the source-count relation for extragalactic X ray sources is presented. Only IPC fields at least 20 deg outside of the galactic plane, and then only those images in the center of the fields, were included. A total of 345 objects were sighted in the 0.3-3.5 keV energy interval, and 77 percent were concluded to be extragalactic. Most were associated with active galactic nuclei (AGN), including Seyfert galaxies and quasars, and galactic clusters. A slope of about 1.7 was calculated for the power law of the energy spectra of AGNs and close to 1.0 for clusters.

Gioia, I. M.↗
NASA NTRS1984

X-ray selected quasars and Seyfert galaxies - Cosmological evolution, luminosity function, and contribution to the X-ray background

The cosmological evolution and the X-ray luminosity function of quasars and Seyfert galaxies (active galactic nuclei /AGNs/) are derived and discussed. The sample used consists of 56 objects extracted from the expanded Einstein Observatory Medium Sensitivity Survey, and it is exclusively defined by its X-ray properties. The distribution in space of X-ray selected AGNs is confirmed to be strongly nonuniform; the amount of cosmological evolution required by the data is in agreement with a previous determination based on a smaller sample of objects. The X-ray luminosity function (XLF) is derived. The high-luminosity part of the XLF is satisfactorily described by a power law of slope gamma approximately 3.6. A significant flattening is observed at low luminosities. The simultaneous determination of the cosmological evolution and of the X-ray luminosity function of AGNs is then used to estimate the contribution to the extragalactic diffuse X-ray background. Using the best fit values for the evolution of AGNs and for their volume density, it is found that they contribute approximately 80 percent of the 2 keV diffuse X-ray background. Uncertainties in this estimate are still rather large; however, it seems difficult to reconcile the data with a contribution much less than 50 percent.

Maccacaro, T.↗