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NASA NTRS1993

Cross section parameterizations for cosmic ray nuclei. I - Single nucleon removal

Parameterizations of single nucleon removal from electromagnetic and strong interactions of cosmic rays with nuclei are presented. These parameterizations are based upon the most accurate theoretical calculations available to date. They should be very suitable for use in cosmic ray propagation through interstellar space, the Earth's atmosphere, lunar samples, meteorites, spacecraft walls and lunar and Martian habitats.

Norbury, John W.↗
NASA NTRS1993

Star trapping and metallicity enrichment in quasars and active galactic nuclei

Recent observational evidence suggests that the metallicity in quasars within a wide range of redshifts, in particular in gas flowing out of the nuclear regions, may be approximately redshift-independent and comparable with or larger than solar. It is plausible that the nuclear metallicity can be internally generated and maintained at approximately time-stationary values in quasars. We identify and estimate efficiency of a mechanism for rapid metallicity enrichment of quasar nuclear gas (in general, in active galactic nuclei) based on star-gas interactions and equivalent to an unusual mode of massive star formation. The mechanism involves capture of low-mass stars from the host galaxy's nucleus by the assemblages of clouds or by accretion disks orbiting the central massive objects (e.g., black holes). Trapping of stars within gaseous disks/clouds occurs through resonant density and bending wave excitation, as well as by hydrodynamical drag. The time scale for trapping stars with total mass equal to that of disk fragment/cloud is of order Hubble time and is remarkably model-independent. Our results show that the described mechanism can produce features suggested by observations, for example, the (super) solar gas metallicity in the nucleus. Thus the observed metallicities in high-redshift quasars do not necessarily imply that global star formation and efficient chemical changes have occurred in their host galaxies at very early cosmological epochs.

Artymowicz, Pawel↗
NASA NTRS1993

MAST - A mass spectrometer telescope for studies of the isotopic composition of solar, anomalous, and galactic cosmic ray nuclei

The Mass Spectrometer Telescope (MAST) on SAMPEX is designed to provide high resolution measurements of the isotopic composition of energetic nuclei from He to Ni (Z = 2 to 28) over the energy range from about 10 to several hundred MeV/nuc. During large solar flares MAST will measure the isotopic abundances of solar energetic particles to determine directly the composition of the solar corona, while during solar quiet times MAST will study the isotopic composition of galactic cosmic rays. In addition, MAST will measure the isotopic composition of both interplanetary and trapped fluxes of anomalous cosmic rays, believed to be a sample of the nearby interstellar medium.

Cook, Walter R.↗
NASA NTRS1993

Active galactic nuclei make the cosmic X-ray background

While it has long been known that active galactic nuclei (AGNs) make up a significant part of the X-ray background near 1 keV, it has been generally thought that the typical AGN X-ray spectrum did not fit the higher energy background spectrum. More recent data (from Ginga, SIGMA, and OSSE) show that the X-ray spectra of radio-quiet AGNs do in fact resemble the X-ray background spectrum. These data are well described (from 2 to 100 keV) by a model in which the X-ray photons are made by thermal Comptonization, and a fraction (around a half) of the X-rays are reflected by a cool surface. Using the observed evolution of X-ray-bright AGNs, we show that the known AGN population, radiating the spectrum observed at low redshift and integrated in redshift back to z about 4, is capable of producing the entire X-ray background from 2 to 100 keV.

Zdziarski, Andrzej A.↗
NASA NTRS1991

Related investigations on the physics of high energy emission from active galactic nuclei

The Final Technical Report on a number of related investigations on the physics of high energy emission from active galactic nuclei, such as Seyfert galaxies and quasi-stellar objects is presented. The chief conclusions of the work are briefly described, and citations to the papers supported by this grant and published in the refereed scientific literature are provided. Areas of research included: 'warm' galaxies observed in x rays; x ray/infrared correlations in galaxies; the contribution of active galaxies to the cosmic x ray background radiation; and an unusual x ray emitting starburst galaxy.

Margon, Bruce↗
NASA NTRS1994

Time-dependent behavior of active galactic nuclei with pair production

We study the properties of coupled partial differential equations describing the time-dependent behavior of the photon and electron occupation numbers for conditions likely to be found near active galactic nuclei (AGN). The processes governing electron acceleration are modeled by a stochastic accelerator, and we include acceleration by Alfvenic and whistler turbulence. The acceleration of electrons is limited by Compton and synchrotron losses, and the number density of electrons depends on pair production and annihilation processes. We also treat particle escape from the system. We examine the steady, (possibly) oscillatory, and unstable solutions that arise for various choices of parameters. We examine instabilities related to pair production and trapping and consider the formation of pair jets.

Li, H.↗
NASA NTRS1994

Gamma-ray properties of active galactic nuclei

Recent observations by the Compton Observatory (CGRO) have increased our knowledge about the gamma-ray emission from Active Galactic Nuclei (AGN) considerably. The three most important findings of CGRO with respect to AGNs are: first, no Seyfert 1 galaxy has been found to show emission above 500 keV. The by far strongest Seyfert 1 galaxy NGC 4151 shows a spectrum which falls off exponentially with an e-folding energy of 39 keV between 65 and 500 keV. OSSE so far has detected or has indications of detections for seven additional Seyfert 1 galaxies, which, however, all show very weak hard X-ray emission compared to NGC 4151. No annihilation feature has been seen from any Seyfert galaxy to this date. Second, the radio galaxy Cen A shows a power-law energy spectrum from hard X-ray energies of about 150 keV to at least 3 MeV. It has not been seen at EGRET-energies. Third, a new class of AGN was discovered at energies above 100 MeV by EGRET. The power of these objects in gamma-rays can dominate the luminosity in other spectral ranges. These objects are associated with extragalactic sources that have blazar properties. The high-energy gamma-ray emission is probably produced in relativistically outflowing jets. At hard X-ray energies the objects are rather weak. Spectral breaks at MeV energies were found by COMPTEL for three of these objects (3C 273, 3C 279, and PKS 0528+134).

Schoenfelder, V.↗
NASA NTRS1994

Interaction of ultraviolet and X-ray radiation with gamma rays produced by a jet in active galactic nuclei

Recent observations by the Compton Gamma-Ray Observatory give evidence for the existence of a type of blazar with strong gamma-ray emission. Data obtained by EGRET for the quasar 3C 279 show a spectrum between 100 MeV and 10 GeV. Photons of such energies should interact with the X-rays and produce positron/electron pairs. If the optical depth against pair production for the gamma rays is large (tau(gamma gamma) greater than 1), the gamma-ray spectrum should be affected. The importance of this process has been pointed out by, e.g., Maraschi, Ghisellini, & Celotti (1992). Several works (e.g., Dermer 1993; Zbyszewska 1993; Sikora, Begelman, & Rees 1993) concerning gamma-ray radiation from quasar 3C 279 have proposed a model in which the gamma rays are produced via interaction between a moving cloud of relativistic electrons and external soft photons. The presence of gamma rays in active galactic nuclei spectra gives constraints on the localization and the luminosity of the medium which produces ultraviolet/X-ray photons. We investigate what conditions should be fulfilled in the above model to avoid the absorption of the gamma rays due to pair production.

Zbyszewska, Magda↗
NASA NTRS1994

Cross section parameterizations for cosmic-ray nuclei. 2: Double nucleon removal

Parameterizations of double nucleon removal from the electromagnetic and strong interactions of cosmic rays with nuclei are presented. These parameterizations are an extension of previous single nucleon removal parameterizations and combined they represent the dominant part of the electromagnetic dissociation encountered by a cosmic ray on its traversal through matter. Such parameterizations should be very useful in studying cosmic-ray transport through the interstellar medium, the Earth's atmosphere, spacecraft walls, and extraterrestrial matter.

Norbury, John W.↗
NASA NTRS1994

Very high energy gamma rays from active galactic nuclei: Cascading on the cosmic background radiation fields and the formation of pair halos

Recent high-energy gamma-ray observations (E(sub gamma) greater than 100 MeV) of blazar Active Galactic Nuclei (AGNs) show emission spectra with no clear upper energy cutoff. AGNs, considered to be possible sources for the observed flux of cosmic rays beyond 10(exp 19) eV, may well have emission extending into the very high energy (VHE), (E(sub gamma) greater than 100 GeV) domain. Because VHE gamma-rays are absorbed by pair production on the intergalactic background radiation fields, much of this emission may not be directly visible. The electromagnetic cascades initiated by absorbed VHE gamma-rays, however, may be observable. Since, most probably, the velocities of (e(+), e(-)) pairs produced in a cascade are quickly isotropized by an ambient random magnetic field, extended 'halos' (R greater than 1 Mpc) of pairs will be formed around AGNs with VHE emission. The cascade radiation from these pair halos is emitted isotropically and should be observable at energies below a few TeV. The halo radiation can be distinguished by its characteristic variation in spectrum and intensity with angular distance from the central source. This variation depends weakly on the details of the central source model, e.g., the orientation and beaming/opening angle of an emitting jet. Limiting or determining the intensity of the pair halo can thus serve as a model-independent bound on or measure of the VHE power of AGNs. Next-generation Cherenkov telescopes may be able to image a pair halo.

Aharonian, F. A.↗
NASA NTRS1994

High-energy spectra of active nuclei. 1: The catalog

This paper presents a catalog of high-energy spectra (E is greater than or equal to 0.01 keV) of active galactic nuclei (AGNs). The catalog contains 209 objects (140 Seyfert galaxies, 65 quasars, and 4 objects otherwise classified), for a total of 1030 spectra. Most of the data have been collected from the literature over a period spanning more than 20 yr starting from the early 1970s up to the end of 1992. For a numbner of objects (17), EXOSAT/ME data have been extracted and analyzed, and the 27 spectra obtained have been added to the database. For each object we report individual observation spectral fit parameters using a power-law model corrected for cold gas absorption along the line of sight (photon index, 1 keV intensity and hydrogen column density), plus other relevant data. It is hoped that this database can become a useful tool for the study of the AGN phenomenon in its various aspects.

Malaguti, G.↗
NASA NTRS1994

ROSAT observations of X-ray-selected active galactic nuclei

We have observed sample of six optically faint, X-ray-selected active galactic nuclei with the Roentgen Satellite (ROSAT) Position Sensitive Proportional Counter (PSPC). These objects represents a sample of soft and hard spectrum sources; all were observed previously with the Einstein X-Ray Observatory. Included in this sample is EO132.8-4111, whose soft X-ray and ultraviolet flux distributions bear a striking resemblance to that of Mrk 841, but whose relative hard X-ray emission is much less than that of Mrk 841. The spectra are all fitted satisfactorily with single power-law models and their spectral indices (with one exception) agree with those estimated from the Einstein Image Propotional Counter (IPC) observations. Tests of short-term variability are carried out, with all sources showing constant count rates over the period of their ROSAT observations. Some sources do, however, show significant changes in count rate when compared to previous Einstein data taken a decade earlier.

Thompson, R. J., Jr.↗
NASA NTRS1994

High-energy radiation from active galactic nuclei

Two recent findings concerning high-energy radiation properties of active galactic nuclei -- discovery of breaks in hard X-ray spectra of Seyfert galaxies, and discovery of huge fluxes of hard gamma rays from blazars -- seem to press us to change our standard views about radiation production in these objects. I review briefly the existing radiation models, confront them with the newest observations, and discuss newly emerging theoretical pictures which attempt to account for the discoveries.

Sikora, Marek↗
NASA NTRS1995

Elliptical accretion disks in active galactic nuclei

We present a calculation of the profiles of emission lines originating in a relativistic, eccentric disk, and show examples of the resulting model profiles. Our calculations are motivated by the fact that in about one-quarter of the double-peaked emission lines observed in radio-loud active galactic nuclei (and in the mildly active nucleus of NGC 1097), the red peak is stronger than the blue peak, which is contrary to the prediction of relativistic, circular disk models. Using the eccentric disk model we fit some of the observed profiles that cannot be fitted with a circular disk model. We propose two possible scenarios for the formation of an eccentric disk in an active galactic nucleus: (a) tidal perturbation of the disk around a supermassive black hole by a smaller binary companion, and (b) formation of an elliptical disk from the debris resulting from the tidal disruption of a star by the central black hole. In the former case we show that the eccentricity can be long-lived because of the presence of the binary companion. In the latter case, although the inner parts of the disk may circularize quickly, we estimate that the outer parts will maintain their eccentricity for times much longer than the local viscous time. We suggest that it may be possible to detect profile variability on much shorter timescales than those ranging from a decade to several centuries by comparing the evolution of the line profile with detailed model predictions. We argue that line-profile variability may also be the most promising discriminant among competing models for the origin of asymmetric, double-peaked emission lines.

Eracleous, Michael↗
NASA NTRS1994

Cosmic-ray isotopic composition of C, N, O, Ne, Mg, Si nuclei in the energy range 50-200 MeV per nucleon measured by the Voyager spacecraft during the solar minimum period

The isotopic composition of C, N, O, Ne, Mg, Si cosmic ray nuclei has been measured in the energy range 50-200 MeV per nucleon using data collected by the High-Energy Telescope of the cosmic-ray subsystem experiment on the Voyager 1 and 2 spacecraft. These data were collected during the period of minimum solar activity in 1986-1988 at an average distance of 27 AU with an effective solar modulation that was much less than at the Earth. The isotope analysis, based on the energy loss - total energy method, has a mass resolution of 0.2 amu for carbon and 0.4 amu at silicon. We find a (C-13)/(C-12) ratio slightly lower and a (O-18)/(O-16) ratio slightly enhanced over their solar system value. We also observe the previously reported enhancement of the (Ne-22)/(Ne-20) ratio relative to solar at the cosmic-ray source but only a weak, if any, enhancement of the (Mg-25)/(Mg-24), (Mg-26)/(Mg 24), and (Si-30)/(Si-28) ratios.

Lukasiak, A.↗
NASA NTRS1994

Disk-driven hydromagnetic winds as a key ingredient of active galactic nuclei unification schemes

Centrifugally driven winds from the surfaces of magnetized accretion disks have been recognized as an attractive mechanism of removing the angular momentum of the accreted matter and of producing the bipolar outflows and jets that are often associated with compact astronomical objects. As previously suggested in the context of young stellar objects, such winds have unique observational manifestations stemming from their highly stratified density and velocity structure and from their exposure to the strong continuum radiation field of the compact object. We have applied this scenario to active galactic nuclei (AGNs) and investigated the properties of hydromagnetic outflows that originate within approximately 10(M(sub 8)) pc of the central 10(exp 8)(M(sub 8)) solar mass black hole. On the basis of our results, we propose that hydromagnetic disk-driven winds may underlie the classification of broad-line and narrow-line AGNs (e.g., the Seyfert 1/Seyfert 2 dichotomy) as well as the apparent dearth of luminous Seyfert 2 galaxies. More generally, we demonstrate that such winds could strongly influence the spectral characteristics of Seyfert galaxies, QSOs, and BL Lac objects (BLOs). In our picture, the torus is identified with the outer regions of the wind where dust uplifted from the disk surfaces by gas-grain collisions is embedded in the outflow. Using an efficient radiative transfer code, we show that the infrared emission of Seyfert galaxies and QSOs can be attributed to the reprocessing of the UV/soft X-ray AGN continuum by the dust in the wind and the disk. We demonstrate that the radiation pressure force flattens the dust distribution in objects with comparatively high (but possibly sub-Eddington) bolometric luminosities, and we propose this as one likely reason for the apparent paucity of narrow-line objects among certain high-luminosity AGNs. Using the XSTAR photoionization code, we show that the inner regions of the wind could naturally account for the warm (greater than or approximately equal to 10(exp 5) K) and hot (greater than or approximately equal to 10(exp 6) K) gas components that have been inferred to exist on scales less than or approximately equal to 10(exp 2) pc in several Seyfert galaxies. We suggest that the partially ionized gas in the inner regions of the wind, rather than the dusty, neutral outflow that originates further out in the disk, could account for the bulk of the X-ray absorption in Seyferts observed at relatively small angles to their symmetry axes. Finally, we discuss the application of this model to the interpretation of the approximately 0.6 keV X-ray absorption feature reported in several BLOs.

Konigl, Arieh↗
NASA NTRS1994

Narrow-line region kinematics in Seyfert nuclei

We present results of a study of narrow-line region (NLR) kinematics in Seyfert nuclei. We draw five important conclusions from this work. The scenario in agreement with the greatest number of observational considerations consists of clouds which are accelerated outward with v varies as r(exp 1/2) (i.e., constant force) and n(sub e) varies as 1/r(exp 2)). The clouds start out at the inner NLR radius with n(sub e) approximately = 10(exp 6)/cu cm and with a very large column density (10(exp 23) - 10(exp 24)/sq cm). These clouds are uniformly accelerated from a few tens of km/s to less than or approximately equal to 1000 km/s. When the clouds reach the outer NLR radius, they have n(sub e) approximately = 10(exp 2)/cu cm and a column density of 10(exp 21) - 10(exp 22)/sq cm. The clouds maintain an ionization parameter of about 0.3 throughout the NLR.

Moore, David↗
NASA NTRS1995

The Hubble Space Telescope Quasar Absorption Line Key Project. 10: Galactic H I 21 centimeter emission toward 143 quasars and active Galactic nuclei

Sensitive H I 21 cm emission line spectra have been measured for the directions to 143 quasars and active galactic nuclei (AGNs) chosen from the observing lists for the Hubble Space Telescope (HST) Quasar Absorption Line Key Project. Narrow-band and wide-band data were obtained with the National Radio Astronomy Observatory (NRAO) 43 m radio telescope for each object. The narrow-band data have a velocity resolution of 1 km/s, extend from -220 to +170 km/s, and are corrected for stray 21 cm radiation. The wide-band data have a resolution of 4 km/s and extend from -1000 to +1000 km/s. The data are important for the interpretation of ultraviolet absorption lines near zero redshift in Key Project spectra. Twenty-two percent of the quasars lie behind Galactic high-velocity H I clouds with absolute value of V(sub LSR) greater than 100 km/s whose presence can increase the equivalent width of interstellar absorption lines significantly. This paper contains the emission spectra and measures of the H I velocities and column densities along the sight line to each quasar. We discuss how the measurements can be used to estimate the visual and ultraviolet extinction toward each quasar and to predict the approximate strength of the strong ultraviolet resonance lines of neutral gas species in the HST Key Project spectra.

Lockman, Felix J.↗