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NASA NTRS1991

Variable soft X-ray excesses in active galactic nuclei from nonthermal electron-positron pair cascades

In the present study of the formation of steep soft X-ray excesses that are superposed on flatter, hard X-ray power-law spectra in nonthermal electron-positron pair cascade sources, the soft excess in pair-cascade AGN models appears as a steep power law superposed on the tail of the UV bump and the flat nonthermal (hard X-ray) power law. The model-parameter space in which an excess in soft X-rays is visible is ascertained, and the time-variability of soft excesses in pair cascade models is examined. It is established that the parameter space in which soft excesses appear encompasses the range of preferred input parameters for a recently development Compton reflection model of UV and X-ray emission from the central engine of an AGN.

Zdziarski, Andrzej A.↗
NASA NTRS1990

The all-sky X-ray foreground - Outstanding issues

An all-sky X-ray foreground survey can be used to determine how supermassive black holes trace the underlying mass distribution responsible for the particular velocity of the local group. The dipole moment for the cosmic X-ray background is compared to that for the all-sky extragalactic foreground of discrete sources, and the survey of AGNs is determined to be an effective method for discerning pronounced large-scale foreground anisotropies. The required sensitivity for an all-sky survey of this type is found to be 3.3 x 10 to the -13th ergs/sq cm/s which would allow an unbiased AGN survey.

Boldt, Elihu↗
NASA NTRS1991

IUE observations of NGC 1068 - The extended high-ionization emission region

The emissions of the highly ionized species of N V, C IV, and He II have been used to delineate the extent of the high-ionization emission region (HIER) of NGC 1068. The observed fluxes from these features are quite large and are equal to those seen in IUE exposure of the central AGN. The HIER is extremely anisotropic about the nucleus with emission seen only in the NE quadrant within 2.5 kpc of the AGN. The correlation of strong emission in the HIER with the UV starburst knots strongly suggests that much of the observed emission is produced in the galactic plane of NGC 1068. A speculative, qualitative model is presented which supports the so-called dusty torus model and suggests that there has been preferential clearing of attenuating gas and dust both very near the nucleus and in the inner galactic disk.

Truong, K. Q.↗
NASA NTRS1991

Hybrid accretion disks in active galactic nuclei. I - Structure and spectra

A unified treatment is presented of the two distinct states of vertically thin AGN accretion disks: a cool (about 10 to the 6th K) optically thick solution, and a hot (about 10 to the 9th K) optically thin solution. A generalized formalism and a new radiative cooling equation valid in both regimes are introduced. A new luminosity limit is found at which the hot and cool alpha solutions merge into a single solution of intermediate optical depth. Analytic solutions for the disk structure are given, and output spectra are computed numerically. This is used to demonstrate the prospect of fitting AGN broadband spectra containing both the UV bump as well as the hard X-ray and gamma-ray tail, using a single accretion disk model. Such models are found to make definite predictions about the observed spectrum, such as the relation between the hard X-ray spectral index, the UV-to-X-ray luminosity ratio, and a feature of about 1 MeV.

Wandel, Amri↗
NASA NTRS1991

The discovery of a young radio galaxy at z = 2.390 - Probing initial star formation at z less than approximately 3.0

The discovery of a weak radio galaxy from the Leiden Berkeley Deep Survey at a redshift of 2.390 is presented, as well as nine-band photometry for the galaxy and for surrounding objects. The source 53W002 is not variable on the time scales of years. Its rest-frame UV continuum is compared with IUE spectra of various nearby galaxies with relatively recent starbursts, and with nearby AGNs. It is inferred from the C IV/Ly-alpha and N V/Ly-alpha ratios that 53W002 has a Seyfert 1-like AGN, and that the ratios constrain the nonthermal component to about 35 percent of the observed UV continuum. Several independent age estimates yield a consistent value of 0.25-0.32 Gyr. The available data are consistent with 53W002 being a genuinely young galaxy seen at a redshift of 2.390 during its first major starburst. It likely started forming most of its current stars at redshifts between 2.5 and 3.0, suggesting that radio galaxies do not form the bulk of their stars coevally, but start doing so over a lengthy period of cosmic time.

Windhorst, Rogier A.↗
NASA NTRS1991

The ultraviolet spectra of nearby radio galaxies

New and archival IUE SWP spectra are reported for nine nearby radio galaxies (V is less than 15 mag), together with optical emissionlike data for these galaxies as well as a number of candidates with weaker line emission. Both their UV line and continuum properties, as well as their UV and UV-optical line ratios, are examined. Ly-alpha emission is found to be common among local radio galaxies, at modest luminosities (typically 10 exp 41-42 erg/s). No apparent relation is found between L(Ly-alpha) and radio power for the nearby radio galaxies alone. The Ly-alpha/H-alpha ratio in low power nearby radio galaxies is 2-5 times lower than the prediction for case B recombination. The destruction of Ly-alpha photons by grains during resonant scattering can explain the observed deficiency for reasonable metallicities. The nearby radio galaxies have in general a small C IV/Ly-alpha ratio (less than 0.1). Comparison of the C IV and Ly-alpha strengths with those in luminous AGN suggests that most of the UV continuum comes from the stellar population, and not from the AGN.

Keel, William C.↗
NASA NTRS1992

Anisotropic line emission and the geometry of the broad-line region in active galactic nuclei

Attention is given to an escape probability-based formalism, including the interaction with a background opacity, which is used to predict line-beaming factors for broad-line region (BLR) clouds of AGN. Several computational details necessary to model gas exposed to an intense nonthermal continuum are outlined. It is found that all hydrogen lines are strongly inwardly beamed, and that C IV 1549 is moderately beamed. The implications of this for the geometry of the emission-line regions of AGN are discussed. Beaming is found to cause the source-cloud separation to be overestimated by variability studies (so that the radiation field at the BLR is even more intense than previously thought); it can even cause spherical geometries to have little instantaneous response to continuum changes.

Ferland, G. J.↗
NASA NTRS1992

Spectroscopy of emission-line nebulae in powerful radio galaxies - Interpretation

Long-slit optical spectra of the emission-line nebulae associated with 21 low-redshift (less than 0.2) radio galaxies are analyzed. Nebulae are classified kinematically into three types: rotators, calm nonrotators, and violent nonrotators; these types are characterized. It is proposed that the rotators have dynamically young disks of gas recently acquired by the radio galaxy in an interaction or merger with a gas-rich galaxy. This is consistent with the data on the morphologies, colors, and stellar dynamics of radio galaxies with strong emission lines. It is inferred from the association of the large-scale gas kinematics with the radio and optical properties of an active galaxy that the angular momentum of the gas which fuels the AGN may be an important parameter in the determinant of how activity is manifest in an AGN.

Baum, S. A.↗
NASA NTRS1991

Neutron stars and black holes

The radiation of neutron stars is powered by accretion, rotation, or internal heat; accreting black holes are thought to be the central engines of AGNs and of a handful of binary X-ray sources in the Galaxy. The evolution of a neutron star depends on the coupling between the rotating neutron and proton fluids in the interior, and between these fluids and the crust; it also depends on the magnetic and thermal properties of the star. Significant progress has been made in recent years in the understanding of radial and disk accretion by black holes. Radiation from pair plasmas may make an important contribution to the X- and gamma-ray spectra of AGNs and black holes in binary systems.

Lamb, F. K.↗
NASA NTRS1992

Deep infrared galaxies

High signal-to-noise ratio optical spectra of 17 infrared-bright emission-line galaxies near the north ecliptic pole are presented. Reddening-corrected line ratios forbidden O III 5007/H-beta, N II 6583/H-alpha, S II (6716 + 6731)/H-alpha, and O I 6300/H-alpha are used to discriminate between candidate energy generation mechanisms in each galaxy. These criteria have frequently been applied to optically selected samples of galaxies in the past, but this is the first time they have been applied to a set of faint flux-limited infrared-selected objects. The analysis indicates the sample contains seven starburst galaxies and three (AGN). However, seven galaxies in the present sample elude the classification scheme based on these line ratios. It is concluded that a two-component (starburst plus AGN) model for energy generation is inadequate for infrared galaxies.

Ashby, Matthew↗
NASA NTRS1992

ROSAT all-sky survey on the Einstein EMSS sample

The cosmological evolution and the luminosity function (XLF) of X ray selected Active Galactic Nuclei (AGN's) are discussed. The sample used is extracted from the Einstein Observatory Extended Medium Sensitivity Surveys (EMSS) and consists of more than 420 objects. Preliminary results from the ROSAT All-Sky Survey data confirm the correctness of the optical identification of the EMSS sources, thus giving confidence to the results obtained from the analysis of the AGN's sample. The XLF observed at different redshifts (up to z approx. 2) gives direct evidence of cosmological evolution. Data have been analyzed within the framework of luminosity evolution models and the two most common evolutionary forms, L sub x(Z) = L sub x(0) x e(sup Cr) and L sub x(Z) = L sub x(0) x (1 + z)(exp C), have been considered. Luminosity dependent evolution is required if the evolution function has the exponential form, whereas the simpler pure luminosity evolution model is still acceptable if the evolution function has the power law form. Using the whole sample of objects the number-counts and the de-evolved (z = 0) XLF have been derived. A comparison of the EMSS data with preliminary ROSAT results presented at this meeting indicates an overall agreement.

Maccacaro, Tomasso↗
NASA NTRS1992

ROSAT-IUE All Sky Survey (RIASS): First results on active galaxies

To optimize the activity of the RIASS collaboration was established between IUE and Rosat projects. Some of the results obtained are given and the importance of such coordinated observations for the Spectral Energy Distribution (SED) definition of Active Galactic Nuclei is illustrated. All the participants in the AGN part of the RIASS program are listed. The RIASS observations allow for a comprehensive study of the overall spectral energy distribution of AGN's. For the first time, data from different instruments can be used to determine the SED, since the observations were simultaneous. A preliminary analysis shows that the energies radiated in the UV and the soft X-rays domains are closely related. The spectra of individual objects as well as the correlation found between the intensity of CIV lambda 1550 emission line and the soft X-rays luminosity, suggest that the excess detected in this band is the high energy tail of the big UV bump.

Wamsteker, W.↗
NASA NTRS1991

Can the cosmic x ray and gamma ray background be due to reflection of a steep power law spectrum and Compton scattering by relativistic electrons?

We reconsider the recent model for the origin in the cosmic X-ray and gamma-ray background by Rogers and Field. The background in the model is due to an unresolved population of AGNs. An individual AGN spectrum contains three components: a power law with the energy index of alpha = 1.1, an enhanced reflection component, and a component from Compton scattering by relativistic electrons with a low energy cutoff at some minimum Lorentz factor, gamma(sub min) much greater than 1. The MeV bump seen in the gamma-ray background is then explained by inverse Compton emission by the electrons. We show that the model does not reproduce the shape of the observed X-ray and gamma-ray background below 10 MeV and that it overproduces the background at larger energies. Furthermore, we find the assumptions made for the Compton component to be physically inconsistent. Relaxing the inconsistent assumptions leads to model spectra even more different from that of the observed cosmic background. Thus, we can reject the hypothesis that the high-energy cosmic background is due to the described model.

Zycki, Piotr T.↗
NASA NTRS1993

Quasars in rich galaxy clusters

The evolution of AGN activity in rich clusters of galaxies is found to be approximately 5 times more rapid than that in poor clusters. This rapid evolution may be driven by evolution in the dynamics of galaxy cluster cores. Results from our spectroscopic studies of galaxies associated with quasars are consistent with this scenario, in that bright AGN are preferentially found in regions of lower velocity dispersion. Alternately, the evolution may be driven by formation of a dense intra-cluster medium (ICM). Galaxies close to quasars in rich cluster cores are much bluer (presumably gas rich) than galaxies in the cores of other rich clusters, in support of this model.

Ellingson, Erica↗
NASA NTRS1993

IUEAGN: A database of ultraviolet spectra of active galactic nuclei

In 13 years of operation, IUE has gathered approximately 5000 spectra of almost 600 Active Galactic Nuclei (AGN). In order to undertake AGN studies which require large amounts of data, we are consistently reducing this entire archive and creating a homogeneous, easy-to-use database. First, the spectra are extracted using the Optimal extraction algorithm. Continuum fluxes are then measured across predefined bands, and line fluxes are measured with a multi-component fit. These results, along with source information such as redshifts and positions, are placed in the IUEAGN relational database. Analysis algorithms, statistical tests, and plotting packages run within the structure, and this flexible database can accommodate future data when they are released. This archival approach has already been used to survey line and continuum variability in six bright Seyfert 1s and rapid continuum variability in 14 blazars. Among the results that could only be obtained using a large archival study is evidence that blazars show a positive correlation between degree of variability and apparent luminosity, while Seyfert 1s show an anti-correlation. This suggests that beaming dominates the ultraviolet properties for blazars, while thermal emission from an accretion disk dominates for Seyfert 1s. Our future plans include a survey of line ratios in Seyfert 1s, to be fitted with photoionization models to test the models and determine the range of temperatures, densities and ionization parameters. We will also include data from IRAS, Einstein, EXOSAT, and ground-based telescopes to measure multi-wavelength correlations and broadband spectral energy distributions.

Pike, G.↗
NASA NTRS1992

The Colorado IUE active galaxy survey. I - Blazars

This paper presents consistently reduced UV continuum flux densities amd spectra indices derived from all 499 usable archived IUE observations of blazars made through mid-1991. The extraction of 1D spectra and the measurement of spectral parameters are discussed. A detailed error analysis is presented, including a discussion of the difference in the calibration between the IUE long- and short-wavelength cameras. The resulting data are presented in tabular and graphical form and are available by electronic mail as well. These results will be most useful for variability studies, but can also be used to characterize the broad-band energy distributions and to compare blazar properties with those of other AGN. This is the first in a series of papers, in which extracted spectral parameters will be published for the over 4500 spectra of AGN in the IUE archives.

Edelson, Rick↗
NASA NTRS1992

The near-ultraviolet spectrum of Markarian 205

We report measurements of the absorption and of the emission lines between 1600 and 3200 A in the spectrum of the nearby AGN Markarian 205 (z = 0.071), which lies at a projected distance of 3 kpc (H0 = 100 km/s) from the nucleus of the nearby barred spiral galaxy, NGC 4319 (z = 0.0047). The results were obtained using high-resolution (R = 1300) observations with the Faint Object Spectrograph of the HST. A total of 15 absorption lines, 13 of which are produced by Galactic gas, and four AGN emission lines are detected. Two of the absorption lines, the Mg II resonant doublet, are produced by gas in the intervening galaxy NGC 4319. This is the first detection of absorption due to intervening gas in this famous quasar-galaxy pair.

Bahcall, John N.↗
NASA NTRS1992

X-ray flares from runaway pair production in active galactic nuclei

The hard X-ray spectrum of AGNs is nonthermal, probably arising from an electron-positron pair cascade, with some emission reflected off relatively cold matter. There has been interest in models on which protons are accelerated and create relativistic electrons on interaction with a local radiation field. It is shown here that a sufficient column density of protons can lead to runaway pair production: photons generated by the relativistic pairs are the targets for the protons to produce more pairs. This can produce X-ray fluxes with the characteristics observed in AGN. The model predicts the maximum ratio of luminosity to source size as well as their spectrum in the early phases. The same mechanism may also be able to create the knots of synchrotron-radiating pair plasma seen in sources such as 3C273.

Kirk, J. G.↗