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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

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

The theory of emission-line regions in active galactic nuclei

The inability of photoionization models to predict the observed amount of energy in the Fe II lines and the Balmer continuum has led to the hypothesis that an additional, nonphotoionized region is present to supplement the intensities of low ionization lines. The question of reddening is addressed in light of its influence on the interpretation of the lines and continuum. It was found that X-rays can raise the gas temperature enough for collisional excitation to affect the Balmer decrement, and that narrow line emission from AGN (ranging in ionization level from Seyfert galaxies to low-ionization nuclei) can be attributed to photoionization by a nonthermal continuum for appropriate ionization-parameter values.

Ferland, G. J.↗
NASA NTRS1980

X-ray astronomical spectroscopy

The current status of the X-ray spectroscopy of celestial X-ray sources, ranging from nearby stars to distant quasars, is reviewed. Particular emphasis is placed on the role of such spectroscopy as a useful and unique tool in the elucidation of the physical parameters of the sources. The spectroscopic analysis of degenerate and nondegenerate stellar systems, galactic clusters and active galactic nuclei, and supernova remnants is discussed.

Holt, S. S.↗
NASA NTRS1983

The Einstein observatory medium sensitivity survey - The quasars sample

A complete sample of X-ray-selected quasars and Seyfert galaxies extracted from the Einstein Observatory Medium Sensitivity Survey is described, and their physical characteristics are discussed. A Hubble constant of 50 km/s per Mpc and a Friedmann universe with a deceleration parameter of q(0)=0 are assumed throughout the survey, and the sample of X-ray sources was selected according to the following criteria: the significance of the detection exceeds a confidence level of 5 beta; the source is physically unrelated to the target of the IPC observation; and that the source is at a galactic latitude higher than 20 deg. The redshift and optical luminosity distributions of 55 Active Galactic Nuclei (AGNs) detected in the survey are discussed. A number flux relation for the different classes of extragalactic objects are analyzed in detail, using a Maximum Likelihood method and the assumption that the relation can be represented as a power law.

Gioia, I. M.↗
NASA NTRS1984

The line continuum luminosity ratio in active galactic nuclei - or, on the 'Baldwin effect'

The luminosity dependence of the equivalent width of CIV in active galaxies, the 'Baldwin' effect, is shown to be a consequence of a luminosity dependent ionization parameter. This law also agrees with the lack of a 'Baldwin' effect in Ly alpha or other hydrogen lines. A fit to the available data gives a weak indication that the mean covering factor decreases with increasing luminosity, consistent with the inference from X-ray observations. The effects of continuum shape and density on various line ratios of interest are discussed. Previously announced in STAR as N83-35975

Mushotzky, R.↗
NASA NTRS1981

A soft X-ray study of the Large Magellanic Cloud

It is estimated on statistical grounds that of the 97 sources detected in a survey of the Large Magellanic Cloud (LMC) by means of the Einstein Laboratory's imaging X-ray telescope, less than 25 are likely to be either foreground stars or background quasars. The largest class of identified sources discovered is that of supernova remnants (SNRs). It is confirmed by means of high resolution maps that six more sources are also SNRs, and an additional 20 to 25 unidentified sources are suspected, in light of the available temporal, spatial and spectral information, to be SNRs. Of the remaining approximately 40 sources, about half are expected to be either background active galactic nuclei or foreground stars. The unexpectedly large number of SNRs found in the survey suggests that present knowledge of the galactic remnant population may be incomplete.

Long, K. S.↗
NASA NTRS1981

NGC 7714 - The prototype star-burst galactic nucleus

New X-ray, ultraviolet (IUE), optical, and radio (VLA) observations are presented for the bright nucleus galaxy NGC 7714. All data are explainable as resulting from intense star-formation activity in this nucleus - a star burst. The X-ray and radio luminosities are explained by supernova remnants arising from the massive stars, and approximately 10,000 supernova remnants are required in a volume of 280 pc radius. It is thought that NGC 7714 is typical of a significant class of emission-line galaxies. It is pointed out that individual supernovae should be observable in this, or similar nuclei, because they are expected to occur at rates approaching one per year.

Weedman, D. W.↗
NASA NTRS1980

X-ray astronomical spectroscopy

The elementary spectral signatures of fundamental physical processes that might be operable in galactic and extragalactic X-ray sources are reviewed. Some of the instrument limitations of experimental techniques used in X-ray astronomical spectroscopy are described. X-ray spectroscopic measurements are examined for various types of X-ray-emitting objects, including degenerate and nondegenerate stellar systems, supernova remnants, clusters of galaxies, active galactic nuclei, and quasars.

Holt, S. S.↗
NASA NTRS1982

A complete X-ray sample of the high-latitude /absolute value of b greater than 20 deg/ sky from HEAO 1 A-2 - Log N-log S and luminosity functions

An all-sky survey of X-ray sources was performed, complete to a limiting sensitivity of 3.1 x 10 to the -11 ergs/sq cm/s in the 2-10 keV band. The complete sample has allowed construction of luminosity functions based on a flux-limited sample for clusters of galaxies and active galactic nuclei. Integration of the best-fit luminosity functions indicates that clusters of galaxies contribute about 4% of the 2-10 keV DXRB, and active galactic nuclei about 20%. It is predicted that many of the objects seen in the deep survey should be local, relatively low luminosity active galactic nuclei and clusters of galaxies.

Piccinotti, G.↗
NASA NTRS1982

A medium sensitivity X-ray survey using the Einstein Observatory - The log N-log S relation for extragalactic X-ray sources

The paper presents results of an X-ray survey of about 50 sq deg of the high galactic latitude sky at sensitivities in the range of 7 x 10 to the -14 to 5 x 10 to the -12 ergs/sq cm/s. The number-flux relation is derived for the extragalactic population to yield a best-fit power-law slope of 1.53 + or - 0.16, and the content of the sample is analyzed in terms of types of sources, appearing to be significantly different from the content of similar samples selected at higher fluxes. The medium sensitivity sample of extragalactic sources is dominated by active galactic nuclei, while samples selected at higher fluxes and higher energies are dominated by clusters of galaxies. Thus, the number-flux relation for extragalactic sources may be interpreted to a first approximation as the sum of the two different distributions with flatter and steeper slopes describing clusters and AGNs, respectively.

Maccacaro, T.↗
NASA NTRS1983

Active galaxies and the diffuse Gamma-Ray background

A model for the origin of relativistic particles and gamma rays in active galactic nuclei and quasars, together with recent HEAO-1 observations of the spectra of active galaxies from 2 to 165 keV, provide the basis for a reexamination of the nature of the extragalactic gamma ray background. Active galaxies account for the observed background if their X-ray spectra steepen to E.021 above 100 keV, as observed in Cen-A, together with a further steepening to E.021 as a result of absorption of gamma rays by photon-photon pair production interactions with X-ray photons. The compactness of active galaxies required to give this steepening is consistent with estimates of their typical luminosity and radius.

Kazanas, D.↗
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

Gamma-rays from active galactic nuclei

Gamma-ray observations of active galactic nuclei (AGNs) are reviewed and the best candidate mechanisms for gamma ray production in AGNs are examined. Both nonthermal and thermal mechanisms are considered. Hot accretion disks around massive black holes in the centers of AGNs could provide the required thermal electrons, pions, and relativistic lectrons and positrons to explain both the X-ray and gamma emission.

Kafatos, M.↗
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 NTRS1985

The central engine of quasars and AGNs: A relativistic proton radiative shock

Active galactic nuclei (AGNs) and quasars (QSOs) appear to emit roughly equal energy per decade from radio to gamma-ray energies (e.g. Ramaty and Ligenfelter 1982). This argues strongly for a nonthermal radiation mechanism (see Rees 1984). In addition, statistical studies have indicated that the spectra of these objects in the IR-UV and 2 to 50 keV X-ray band, can be fitted very well with power laws of specific indices. These spectral indices do not seem to depend on the luminosity or morphology of the objects (Rothschild et al. 1983; Malkan 1984), and any theory should account for them in a basic and model independent way. If shocks accelerate relativistic protons via the first-order Fermi mechanism (e.g. Axfor 1981), the radiating electrons can be produced as secondaries throughout the source by proton-proton (p-p) collisions and pion decay, thus eliminating Compton losses (Protheroe and Kazanas 1983). As shown by Kazanas (1984), if relativistic electrons are injected at high energies, e+-e- pair production results in a steady state electron distribution that is very similar to that observed in AGNs, independent of the details of injection and the dynamics of the source. The conditions required by this mechanism are met in the shock model of Eichler (1984) and Ellison and Eichler (1984) which allows the self-consistent calculation of the shock acceleration efficiency.

Kazanas, D.↗