bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic Fields in Thin Accretion Disks Around Black Holes
Authors: Khanna R. , Camenzind M.
Published: July 2016

We study magnetic field topologies and magnetic field strength in a thin accretion disk around a rotating black hole. The magnetic field is assumed to enter the disk at the outer edge and is amplified in the accretion process by differential rotation. This scenario seems likely for AGN, where magnetized plasma from a molecular torus flows into an inner accretion disk.

In nonideal Newtonian magnetohydrodynamics the presence of a rotating central black hole is taken into account by using the form of the Keplerian rotation law valid for Kerr geometry outside the marginally stable orbit and a boundary layer, caused by the frame dragging effect, within. In the unstable region close to the hole the turbulent timescale is much larger than the accretion timescale so that the effective magnetic diffusivity, which is large in the disk due to turbulence, is low. As a consequence the poloidal magnetic field lines cross the horizon almost radially in agreement with [1].

We present stationary axisymmetric solutions of the induction equation for vanishing α-effect. Dipolar field structures are most favourable for the generation of fast jets and can effectively contribute to the heating of a corona or some X-ray source. Quadrupolar field structures may also drive jets, however the field strength is considerably lower and therefore also the energy that can be supplied into a corona or a jet.
Kinematic Dynamo in Turbulent Circumstellar Disks
Authors: Stepinski T.
Published: July 2016

Many circumstellar disks associated with objects ranging from protoplanetary nebulae to accretion disks around compact stars allow for the generation of magnetic fields by an αω dynamo. We have applied kinematic dynamo formalism to geometrically thin accretion disks. We calculate, in the framework of an adiabatic approximation, the normal mode solutions for dynamos operating in disks around compact stars. We then describe the criteria for a viable dynamo in protoplanetary nebulae, and discuss the particular features that make accretion disk dynamos different from planetary, stellar, and galactic dynamos.
The Internal Wave Driven Dynamo in Accretion Disks
Authors: Vishniac E.T.
Published: July 2016

We present the results of recent work on a model of angular momentum transport in thin, ionized, accretion disks. In particular, we consider three physical effects, each of which can be represented in terms of a local MHD mode in such a disk. First, we discuss the generation and propagation of internal modes within accretion disks, pointing out certain features which make them particularly promising as the driving force behind a strong, fast dynamo in accretion disks. Second, we point ou that the magnetic shearing instability (MSI) first discussed by Velikhov, and more recently by Balbus and Hawley in the specific context of these disks, provides a natural saturation mechanism for any disk dynamo, leading to an approximate equality between the dimensionless viscosity and the ratio of the dynamo growth rate to the local shear. Third, we argue that magnetic buoyancy is largely suppressed by the turbulence generated by the shearing instability. This prevents it from removing magnetic flux from the disk any faster than random turbulent diffusion. We find that the dimensionless viscosity α scales as (H/r)4/3, where H is the disk height and r is its radius.
The Magnetic Field Structures of a Class of Fast Dynamos
Authors: Lau Y.-T. , Finn J.M.
Published: July 2016

We study the magnetic field structures of fast kinematic dynamos for a class of steady chaotic flows with stagnation points. We find that the dynamos are generated by a stretch-fold-shear mechanism, which is effective only when the chaotic flow region is large enough and overlaps significantly with the rotating flux tubes.
On the Gyrotropic Magnetohydrodynamic Turbulence
Authors: Hnatich M. , Stehlik M.
Published: July 2016

1. The reflective non-symmetric (gyrotropic) stochastic magnetic hydrodynamics (MHD) turbulence is investigated by a quantum-field (QF) theory method including a renormalization group (RG), that has been successfully applied to the fully developed HD turbulence [1,2] and to the ordinary (reflective symmetric) MHD turbulence [3].
The Non-Linear Equations for the Green Function and Calculation of the Magnetic Field Turbulent Diffusivities and α-Effect
Authors: Silant'ev N.A.
Published: July 2016

The exact numerical solution of the simplest non-linear equation from the hierarchy of non-linear equations for the averaged Green function shows that such solution allows to calculate the diffusivity and α-effect coefficient with a good accuracy for an arbitrary spectra of turbulence for all values of the characteristic parameter. It is derived also the improved equation describing the evolution of admixture fluctuations in a turbulent medium which takes into account the non-linear equation for the averaged Green function.
A Torsion-Induced Coupling Between Electric and Magnetic Fields in the Solar Convection Zone
Authors: Saniga Metod
Published: July 2016

A striking formal analogy between sunspots and the type II superconductor magnetic vortex has been pointed out by Saniga (1990). On the basis of this similarity, it was shown that, by postulating the existence of a complex-valued scalar field on the Sun (a “Higgs-like field”), many features of sunspots can be reproduced. Here this formal analogy is exploited further to show that the Abelian-Higgs sunspot acquires a non-zero electric charge in the background of a curved space-time endowed with a constant, completely antisymmetric torsion tensor. The space-time torsion is also shown to relate the spot's electric and magnetic fileds.
High Resolution Observations of the Magnetic Field in IC 342
Authors: Krause Marita
Published: July 2016

The face-on spiral galaxy IC 342 is known to possess an axisymmetric spiral magnetic field (ASS) with magnetic field lines being orientated inwards everywhere at least in the analyzed radial range between 5 and 13 kpc1 (Krause et al., 1989a; Sokoloff et al., 1992). The ASS field configuration seems to be even with respect to the galactic plane.
Galactic Dynamos and Dynamics
Authors: Donner Karl Johan , Brandenburg Axel , Thomasson Magnus
Published: July 2016

We discuss some aspects of the interrelationship between the dynamo problem for galaxies and their dynamics. First, we consider the generation of magnetic fields in the presence of fountain flows and galactic winds. Next, we discuss the distortion of a steady magnetic field by tidal effects and other transient spiral features. Finally, we give an expression for the amplitude of density waves generated by large-scale non-axisymmetric fields.
Nonlinear Dynamo in a Disk Galaxy
Authors: Poezd A. , Shukurov A. , Sokoloff D.D.
Published: July 2016

A nonlinear thin-disk galactic dynamo model based on α-quenching is proposed. Assuming that the mean helicity depends on the magnetic field strength averaged across the disk, we derive a universal form of nonlinearity in the radial dynamo equation. We discuss the evolution of the regular magnetic field in the Milky Way and the Andromeda Nebula. It is argued that the reversals of the regular magnetic field in the Galaxy are a relic inherited from the structure of the seed field. We also briefly discuss the role of the turbulent diamagnetism and the effects of galactic evolution on the dynamo.
Magnetic Field Generation by Galactic Winds
Authors: Spencer S.J. , Cram L.E.
Published: July 2016

A new mechanism is presented for the local amplification and possible global dynamo maintenance of non-axisymmetric large-scale magnetic fields in disk galaxies. Shear in a galactic wind or large-scale flow of ionised gas with components axial and radial to the disk plane may regenerate large-scale magnetic fields.

Numerical results are presented from kinematic mathematical models based on a local (thin disk) approximation and an exact three-dimensional formulation. The one-dimensional thin-disk model illustrates the possibility of exponential amplification and the resulting local axial spatial structure of large-scale galactic magnetic fields. Three-dimensional results support the possibility of global wind dynamo action.
The Full “Alpha-Tensor” Due to Supernovae and Superbubbles in the Galactic Disk
Authors: Ferrière K. M.
Published: July 2016

We provide a simple and realistic expression for the alpha-tensor in the Galactic disk, when turbulence is driven by supernova explosions and superbubbles.
The Alpha-Effect by Supernova Explosions
Authors: Kaisig M. , Rüdiger G. , Yorke H.W.
Published: July 2016

The turbulence in galaxies is assumed to be driven by a sample of stochastically occurring supernova explosions. For simplicity, we replace the complicated real flow by the velocity field of the explosions themselves. The resulting turbulence model can be used to determine the turbulent electromotive force, < u′ × B′ >, a basic quantity in the dynamo theory.

If the distribution function of the SN explosion rate is known, it is sufficient to compute the α-effect for a single explosion in a rotating, density stratified galaxy. We use an axisymmetric simulation code to determine the helical velocity field of the explosion, and consider only explosions located on the rotation axis. The (homogeneous) magnetic field is assumed to be purely vertical. Therefore, only the αzz-component of the α-tensor can be obtained with this method.

The induced magnetic field fluctuations must be correlated with the velocity field in order to get the turbulent electromagnetic force. The resulting α-effect for a sample of explosions starting in the galactic midplane possesses a z-profile with an amplitude of about 60m/s. It is always negative in the northern hemisphere and positive in the southern hermisphere. The scale of the α-profile is about 60pc. The influence of the density distribution proved to be small. We consider the extreme case of uniform explosion rate and find α-values of only few meters per second for a density scale height of 100pc.
Parker Instability and the Galactic Dynamo
Authors: Hanasz M. , Lesch H.
Published: July 2016

We imagine the interstellar medium as a system of molecular clouds and isolated magnetic flux tubes. We assume that: the flux tubes appear as a result of disruption of molecular clouds containing magnetic field, their length is comparable to the mean intercloud distance of about 100 pc and diameter comparable to the diameter of the clouds ∼ 10 pc.
On the Predictive Power of the Minimum Energy Condition
Authors: Pohl Martin
Published: July 2016

We reexamine the minimum energy method to determine the magnetic field strength in spiral galaxies from the cosmic ray standpoint of view. It is shown that for example in M51 the estimated field strength is about a factor of 2 lower than obtained with the standard method. As a by-product the corresponding γ-ray flux from the galaxies can be calculated, which will allow further improvement of the method provided reliable γ-ray spectra are at hand.
Mean Electromotive Force Due to Magnetoconvection in Rotating Horizontal Layer in Dependence on Boundary Conditions
Authors: Brestenský J. , Ševčík S. , Rosenberg L.
Published: July 2016

The instability due to a vertical uniform temperature gradient is studied in a rapidly rotating horizontal layer of an electrically conducting fluid permeated by an azimuthal magnetic field linearly growing with the distance from the vertical axis of rotation. In addition to the boundary conditions used in So ward's study (1979), that is, force-free surface and perfect electrical and thermal conductivity outside, also other conditions more realistic for the Earth's core are considered, that is, rigid surface and electrically insulating walls. Using the concept of meanfield mlid mean electromotive and ponderomotive forces (E.M.F. and P.M.F.) are calculated and compared for various boundary conditions. The dependence of the E.M.F. and P.M.F. on the electromagnetic boundary conditions is strong (slight) if the boundaries are free (rigid).
The Cosmic Dynamo: Fromt= −∞ to Cowling's Theorem: A Review on History
Authors: Krause F.
Published: July 2016

An attempt is made to present a review as well on the historical development of knowledge on cosmic magnetic fields as on the development of our understanding of this phenomenon.
Thermal Or Non–Thermal X–Rays From AGNs?
Authors: Ghisellini Gabriele , Haardt Francesco
Published: July 2016

Recent data from OSSE on CGRO and SIGMA on GRANAT challenge the non-thermal interpretation of the origin of the high energy emission of AGNs, showing that the hard X-ray spectra of several Seyfert AGN are steep like those of Galactic black hole candidates. Thermal models are therefore favoured. Two–phase models, in which a hot corona is placed above a relatively cold accretion disk can account for the observed X–ray spectra and the correlated variability in the UV and X–ray bands. Cold matter, both in the vicinity of the nucleous, and located further away in the torus surrounding the nucleous, may modify substantially the spectrum with important consequences on the expected variability and spectral shape.
GINGA Observations of Active Galactic Nuclei
Authors: Inoue H.
Published: July 2016

Ginga observed 116 AGNs during its operational life from 1987 to 1991: Among them, there were 55 Seyfert galaxies, 42 quasars, 9 BL Lac objects and 10 other AGNs, although the classification is not strict. From these AGN observations, a number of fruitful results were obtained. In this review, I briefly summarize the Ginga observations of AGNs and show several similarities between the Seyfert-type AGNs and the galactic black hole candidates. I also discuss two inverse correlations between the break energy of the power law spectrum and the flux observed from the black hole candidate GS2023+338 and between the equivalent width of the iron fluorescent line and the continuum flux observed from NGC4151.
Observations of AGN with the Extreme Ultraviolet Explorer
Authors: Marshall Herman L.
Published: July 2016

The first results from surveys performed in the extreme ultraviolet (EUV) will be described in the context of studies of active galaxies and BL Lac objects. About a dozen extra-galactic sources are known so far to emit sufficient EUV radiation that they are detectable even through the Galactic interstellar medium. These results are interpreted in the context of a model of EUV or soft X-ray excesses in the case of AGN. In the case of BL Lac objects, the detections indicate that the steep soft X-ray power law spectra continue into the EUV and that there is little intrinsic gas. Finally, there now exists EUV spectra from the Extreme Ultraviolet Explorer for one BL Lac, PKS 2155-304 and two AGN: Mk 478 and NGC 5548. The spectra show no significant spectral features; for AGN, it indicates that optically thin and emission line models may have a difficult time explaining the EUV and soft X-ray bumps.
Multifrequency Variability of AGN's: Continuum Variations from the Near IR to the X-rays
Authors: Clavel J.
Published: July 2016

Because they emit copiously over more than 10 decades in frequency, Active Galactic Nuclei (AGN) cannot be understood without the help of multiwavelength observations. On the other hand, variability monitoring has also proven to be invaluable in understanding the continuum and line emission process as well as the geometry of the innermost regions in these objects. Indeed, at the heart of AGN's lies an object which is so compact that the only way to probe its structure is the study of the temporal evolution of its spectrum. The equivalent resolution which can be achieved in this way is of the order of 10 microarcsecs, far beyond the capability of any UV or optical telescope.
The Amplitude–Time Lag Relation for Emission-Line Flares of AGN
Authors: Shevchenko Ivan I.
Published: July 2016

The amplitude–time lag (“ΔA-Δt”) relation is considered in order to describe behaviour of the emission-line spectrum of an active galactic nucleus during a separate active event. Here ΔA, called the amplitude, is the maximum relative increment of the flux in a line, and Δt is the time lag between the maximum of the ionizing continuum flare and the maximum of the flare in a line. As suggested by Shevchenko (1988), the construction and analysis of such relations can be used to discriminate between broad-line region models. Comparison of theoretical “ΔA-Δt” relations with the observed one composed by data for flares in various lines during a separate active event, is proved to be a useful tool for investigating the geometry of a broad-line region, for studies of the form of phase functions of a typical line-emitting cloud in various lines, as well as for clearing up the duration and amplitude of the initial flare in the ionizing continuum. The advantage of this method is that it utilizes the most general observed characteristics of the emission-line flares and nevertheless provides basic information on the allowed BLR models before the detailed modelling of emission-line light curves is performed.
Intensive Spectroscopic Monitoring of NGC 5548 with HST and IUE
Authors: Peterson B.M. , Korista K.T.
Published: July 2016

We present preliminary results on a combined HST/IUE/ground-based monitoring campaign on the Seyfert 1 galaxy NGC 5548 undertaken by the International AGN Watch in order to answer questions that require both high temporal resolution (one day) and high signal-to-noise ratios. Our preliminary conclusions are (1) the ultraviolet and optical continuum variations are simultaneous to within a day (2) the He II λ1640 variations lag behind the continuum by about 1.7 days, and (3) the velocity field of the C IV-emitting region is not characterized by bulk by radial motion, but the higher velocity gas seems to originate closer to the continuum source than the lower velocity gas.
High-resolution radio observations of nearby galaxies
Authors: Saikia D. J. , Pedlar A.
Published: July 2016

The central regions of many nearby active galaxies are often heavily obscured by dust and gas and are visible at only the far infrared (FIR) and radio wavelengths. High-resolution radio observations provide an invaluable tool for clarifying the dominant power source in an active galaxy which could be due to either an intense burst of star formation or an active galactic nucleus (AGN). An AGN is normally assumed to be powered by a supermassive black hole with an accretion disk, where the black hole forms as the end product of stellar evolution. There have been suggestions linking both these forms of activity to galaxy interactions, but the conditions that might lead to the formation of an AGN and how it is fuelled are not well understood.
Correlated X-ray and Millimetre Variability of the Blazar 3C273
Authors: McHardy I M , Papadakis I , Leach C M , Robson E I , Junor W , Staubert R , Steppe H , Gear W K
Published: July 2016

We present the results of X-ray and millimetre monitoring of the blazar 3C273 at 1–2 day intervals over the period 12 December 1992 to 24 January 1993. No large flares are seen in this period but variations in both wavebands of ∼ 30% on few day timescales are apparent. The ROSAT PSPC X-ray spectrum consists of 2 power-law components with the harder component dominating above 0.5 keV. There is very little correlation between the variability of the soft and hard components. The soft component does not correlate with the millimetre variations, but the hard component correlates reasonably well and leads the millimetre variations by about 10 days. These results show that the hard X-ray component cannot be a simple extrapolation of the millimetre/IR synchrotron component but may be explained as a self-Compton component in a shocked jet.
High S/N ROSAT-PSPC Observations of the Quasar PG 1116+215: Power Law Shape of the Soft X-ray Spectrum
Authors: Ulrich M.-H , Molendi S.
Published: July 2016

The quasar PG 1116+215 (Schmidt and Green, 1983) was selected because it is an intrinsically bright low redshift quasar. With z = 0.177 the soft X-ray emission is not significantly redshifted outside the ROSAT band. Its apparent magnitude mv = 15.04 (Véron and Véron, 1989) corresponds to Mv = −25.0 for H0 = 50 km s−1 Mpc−1 and q0 = 0. An additional selection criterion was low NH. Radio observations give NH = 1.44 × 1020 cm−2 (Elvis, Lockman and Wilkes, 1989).
The Soft X-ray Excess in Quasars and Deep X-ray Surveys
Authors: Kembhavi Ajit , Fabian A. C.
Published: July 2016

We examine the surface density of quasars expected in the ROSAT PSPC surveys, using quasar optical luminosity functions and model continuum spectra including a soft X-ray excess. We show that the surface density is a sensitive function of the characteristic energy of the soft excess, and the absorption within the quasar as well as the interstellar medium. Comparision of the predicted surface density with the results of various surveys allows useful constraints to be put on the nature of the soft excess.
Magnetized Accretion Disks Driving Jets
Authors: Pelletier G. , Ferreira J. , Rosso F.
Published: July 2016

In this brief communication, we present some progress in the investigation of a most promising model that was designed to combine ejection with accretion. In this model, a bipolar configuration of opened magnetic field lines that thread the accretion disk, allows the extraction of angular momentum, the acceleration of matter up to super Alfvénic velocities and the self collimation of the jet. However, important issues have remained unsolved. First, a systematic method for solving the jet MHD equations with their critical surfaces was lacking. Second, the capability of accretion disks to generate super Alfvénic jets was unknown. Third, the back-reaction of the ejection on the accretion disk dynamics and its energetics remained to be done. Solving these three points led us to draw some noteworthy consequences for the understanding of AGNs.
On the Difference Between Radio Loud and Radio Quiet AGN
Authors: Mannheim Karl
Published: July 2016

Nuclear jets containing relativistic “hot” particles close to the central engine cool dramatically by producing high energy radiation. The radiative dissipation is similar to the famous Compton drag acting upon “cold” thermal particles in a relativistic bulk flow. Highly relativistic protons induce anisotropic showers raining electromagnetic power down onto the putative accretion disk. Thus, the radiative signature of hot hadronic jets is x-ray irradiation of cold thermal matter. The synchrotron radio emission of the accelerated electrons is self-absorbed due to the strong magnetic fields close to the magnetic nozzle.
Modelling the Soft X-ray Excess in E1615+061
Authors: Bałucińska-Church M. , Piro L. , Fink H. , Fiore F. , Matsuoka M. , Perola G. C. , Soffitta P.
Published: July 2016

We report results of an international UV – X-ray campaign in 1990–1992 involving the IUE, Rosat and Ginga satellites to observe E1615+061, a Seyfert 1 galaxy with peculiar spectral and intensity behaviour over the last 20 years. The source has been found to be stable in its medium state during the observations. The Ginga (1–20 keV) spectrum of E1615+061 is adequately represented by a simple power law with a photon index α = 1.8 ± 0.1. However, α ∼ 2, as expected for the intrinsic power law component in a reflection model, cannot be ruled out statistically. The Rosat PSPC (0.1–2 keV) spectra collected during the All Sky Survey and the AO-1 phase can be well-described by a simple power law (α = 2.2 ± 0.1) with cold absorber (NH = 3.5 ± 0.3 · 10λ20 H/cmλ2). Both the photon index being significantly different than that obtained from the Ginga spectrum and the column density being smaller than the galactic column (NH ∼ 4.2 · 10λ20 H/cmλ2) give an indication of a soft excess over and above the hard component seen in the Ginga spectrum. E1615+061 has been observed with IUE in 1990 and in 1992. The source was stable and the colour excess E(B-V) derived from the data = 0.1 is in good agreement with that expected from the galactic absorption.

To parameterise the soft excess we fitted the Rosat data with a two-component model consisting of a power law, and a blackbody or thermal bremsstrahlung, with a single galactic absorption term. The column density and the slope of the power law were kept constant. The blackbody temperature was 80 ± 6 eV and 63 ± 12 eV for photon index equal to 1.8 and 2.0, respectively, whereas the bremsstrahlung temperature was 220 ± 40 eV and 115 ± 30 eV for the two cases.

An attempt to model the soft excess seen in the Rosat PSPC spectrum has been made assuming that the soft excess is the high energy tail of a disc spectrum which peaks in the UV part of the spectrum. Additionally it was assumed that there is a hard component contributing to the spectrum from UV to X-rays with parameters as described by the Ginga spectrum. The best fit parameters: the mass of the central source and the mass accretion rate were around 5 ± 1 · 10λ6 M⊙ and 0.2 ± 0.04 M⊙/yr, respectively.

Our modelling shows that the soft X-ray excess can be described (χredλ2 < 1.2) as the high energy tail of an accretion disk spectrum if the intrinsic power law is quite steep (α = 2). The main contribution to the residuals in the Rosat PSPC range comes from 0.3–0.6 keV, with a tendency for these residuals to increase when the slope gets flatter. The accretion luminosity is ∼ 6.5 · 10λ44 erg/s for the best fit parameters, i.e. about the Eddington luminosity.
UV Variability of a Large Sample of AGN
Authors: Paltani S. , Courvoisier T. J.-L.
Published: July 2016

The IUE-ULDA database (Version 3.0, complete until the end of 1988) contains about 3500 spectra of more than 500 AGN of different classes (see Courvoisier and Paltani, IUE-ULDA Access Guide No. 4 A & B, ESA SP 1153 A & B 1992). We selected 67 objects for which the variability properties could be investigated. For each object we estimated the amplitude of the variability by means of the standard deviation σ(λ) of the flux at different frequencies divided by the mean flux.
Spectra of Distant Quasars and Verification of Possible Variation of Fundamental Constants over Cosmological Time-Scales
Authors: Varshalovich D.A. , Potekhin A.Y.
Published: July 2016

Constraints on possible variation rate of the fine-structure constant, , and the electron-proton mass ratio , over cosmological time scales are obtained from analyses of quasar spectroscopic data.
ROSAT Observations of Bright BL Lacertae Objects
Authors: Lamer G. , Brunner H. , Staubert R.
Published: July 2016

We have compiled a sample of 23 X-ray and radio selected BL Lacertae objects which have been observed with the Position Sensitive Proportional Counter (PSPC) on board of the ROSAT Satellite. The sample consists of three parts:

In Table I results from 4 objects observed for their known rapid X-ray variability are presented. 5 objects are the BL Lac subset of a complete sample of flat spectrum radio sources with 5 GHz flux densities > 1 Jy. Detailed results from this sample will be published in Brunner et al. 1993. The data of the 14 remainig objects were collected from the ROSAT data archive to supplement the sample. The whole sample contains 7 X-ray selected objects (XBLs,αOX < 1.2) and 16 radio selected objects (RBLs).

The X-ray spectra of the sources are well described by single power laws with galactic absorption. The X-ray energy indices αX are widely dispersed around a mean of 1.34. Significant X-ray flux variability and correlated spectral variability was detected on timescales down to hours. The object H 1218+304 was found to be rapidly variable within each of three observations. Its spectral hardness is correlated with the flux level (see Table I).

We calculated the intrinsic distributions of the spectral indices αX for the XBL and RBL samples and of the differences between ROSAT and EXOSAT ME spectral indices αPSPC – αME (only XBL sample) using a maximum likelihood fit. There is no significant difference in the mean spectral indices between the X-ray and radio selected subsamples. The mean values < αX > are 1.34 for XBLs and 1.33 for RBLs. The spectra of the X-ray selected objects slightly steepen at higher X-ray energies (< αPSPC – αME > = −0.11). This supports the view that the X-ray emission of XBLs is supplied by synchrotron radiation. The steepening of the X-ray spectrum is then due to a cutoff in the energy distribution of the electrons.
New X-ray Results on Radio Galaxies
Authors: Worrall D.M. , Birkinshaw M.
Published: July 2016

Prior to ROSAT, separation of X-ray components in radio galaxies has been limited to a few well-known sources, e.g., M 87 and Cen A. Now, from ROSAT PSPC measurements of the first six objects in our study of low-power radio galaxies, we find that both resolved (thermal) and unresolved X-ray emission in a single source is typical (Worrall & Birkinshaw 1993; ApJ, submitted). The angular size of, and fraction of luminosity in, the resolved X-ray emission varies between objects. There is evidence to relate the unresolved X-ray emission with the inner radio jet.
Optical Microvariability in Complete Samples of BL Lac Objects
Authors: Heidt Jochen
Published: July 2016

A radio-selected (1 Jy, Stickel et al., 1991) and a x-ray selected (EXOSAT, Giommi et al., 1991) sample of BL Lac objects was monitored by performing relative CCD photometry in order to examine the duty cycle, the dominant time-scales and the typical amplitudes of the variability. The samples consist of 34 and 11 objects, respectively.
Structure Function Analysis Applied to the Metsähovi Sample
Authors: Lainela Markku , Valtaoja Esko
Published: July 2016

Our monitoring program in Metsähovi, Finland, consists of a complete sample of bright, flat-spectrum extragalactic compact radio sources. For the structure function analysis we have selected 42 best observed objects from this sample. The structure function analysis gives us two parameters, the slope value α and the maximum variability timescale Tmax.
How Large are Quasar Broad-Line-Regions? Results from a Program to Monitor the PG Quasars
Authors: Maoz Dan , Jannuzi Buell T. , Smith Paul S. , Kaspi Shai , Netzer Hagai
Published: July 2016

We have monitored spectrophotometrically a subsample (28) of the Palomar-Green Bright Quasar Sample for two years in order to measure the sizes of the broad-line regions of high-luminosity AGNs. Half of the quasars showed optical continuum variations with amplitudes in the range 20–75%. In most objects with continuum variations, we detect correlated variations in the broad Hα and Hβ emission lines. The amplitude of the line variations is usually 2–4 times smaller than the optical continuum fluctuations. The lines respond to the continuum variations with a lag that is smaller than or comparable to our typical sampling interval (a few months). This suggests that the quasars have broad-line regions smaller than about 1 lt-year. The figures below show spectra and light curves for one of the quasars. Two of the quasars monitored show no detectable line variations despite relatively large-amplitude continuum changes. This could be a stronger manifestation of the low-amplitude line-response phenomenon we observe in the other quasars. Further details appear in Maoz et al. (1994, ApJ, Jan 20, in press).
Long Term X-ray Variability of NGC 4151
Authors: Papadakis I. E. , McHardy I. M.
Published: July 2016

Short time scale X-ray power spectra of AGN are in general well fitted by a power law with slopes between −1 and −2 but we expect these slopes to flatten at low frequencies (indication of such a flattening has already been seen in NGC 5506). We have searched for such a low-frequency break in the power spectrum of NGC 4151 by investigating its long term X-ray light curve (2–10 keV). To construct this light curve we used Ariel V SSI, OSO-8, HEAO-1, Ariel VI, EXOSAT ME and GINGA LAC data.
Optically Quiet Quasars – Radio and Optical Investigations
Authors: Akujor Chidi E. , Porcas R.W. , Patnaik A.R. , Ardeberg A.
Published: July 2016

The activity in the nuclei of bright galaxies could arise from a number of sources: they may contain compact variable radio sources, they may be strong IR/optical/X–ray continuum sources, or they may have strong emission lines. Usually but not always these properties go together as in radio loud quasars. However, there appears to exist a class of objects which resemble quasars in radio structure and brightness – have strong flat spectrum cores but appear fainter than would be suggested by their radio brightness – unidentified on sky surveys (Zensus & Porcas, 1985, in J. Dyson, ed, AGN). These are called ‘optically quiet quasars’ (OQQs).
Radio Source Structure and Unified Models
Authors: Hutchings J. B. , Neff S. G.
Published: July 2016

The results are presented of a program to study the radio structure of radio galaxies, and to compare them with quasars. The samples are matched in redshift and luminosity in the redshift range 0.2 to 1.0, and mapped with the same VLA configuration. The quasar results have been published in earlier papers and the radio galaxy results are in press. We compare 100 quasars with 80 radio galaxies. The comparison shows the following radio galaxies differ from quasars in the following ways:

1)

The overall sizes are larger for radio galaxies by 2 to 3 times.

2)

There are few large bend angles in radio galaxies.

3)

The lobe length ratio distribution differs.

4)

The radio galaxies have no population of one-sided sources.

5)

The lobe flux ratio has less range in the radio galaxies.

6)

Radio galaxies have few core-dominated sources and many with no detectable core: few with comparable core and lobe flux.

7)

The most luminous sources are lobe-dominated and more powerful than the quasars. The most luminous quasars are unresolved cores.

Points 1,2,3 can be fit with a single ‘unified’ model which has a distribution of sizes and bend angles up to 25°, with small expansion velocities. The quasars are viewed within a cone angle of 50° and the radio galaxies outside the cone. However, the model does incorporate a source growth scenario and points 4 through 7 imply that there are instrinsic differences between the source types as well as orientation. We also discuss the limitations on beaming and bulk motions implied by the results.
Variation of Broad Optical Emission Lines in AGN's
Authors: Bochkarev N.G. , Shapovalova A.I. , Nazarova L.S.
Published: July 2016

AGN optical spectra (4000–5000Å) have been obtained with the TV scanner of the 6-m telescope at Special Astrophysical Observatory (N. Arkhiz-North Caucasus) from 1986 to now. Most spectra were obtained with dispersion of 1 Å/channel and with 3–4Å spectral resolution and signal-to-noise ratio S/N=10–25 for individual spectra. NGC 4151, NGC 3516, NGC 5548 and NGC 7469 are the main sources of the programme and are also being used in international AGN watch programmes for diagnosis, estimation of the size and structure of the BLR.

NGC4151:

In 1987 the broad components of the Hβ and Hγ lines decreased strongly (on about 65% as compared with 1986) and fell to a level comparable to that of 1984. In 1988 they increased up approximately to the level of 1986. During the photometrically low state of NGC 4151 satellites of the strong Hβ, Hγ, Hδ and He II4686Å lines were suspected at radial velocities of +9400 km/s and −7500 km/s. The data are could be interprite a two-jet model with jet velocity of 8400–24000 km/s. (Bochkarev et al., 1989). Analysis of the spectra shows that the broad component of HeII λ 4686Å changed its asymmetry over 6 years. Bochkarev et al. (1992), Nazarova et al. (1993) discussed a possible explanation of profile HeII λ 4686Å behavior by involving a precession of cones of ionizing radiation.

NGC5548:

Cross-correction of the optical emission line light curves with the UV continuum light curve reveals that the lines respond to continuum variations with a time delay from 7 days for HeII 4686Å to 18 days for Hβ. Hγ responds more rapidly than the lower order Balmer lines and there is a general trend towards higher amplitude variations in higher-order lines. The time lags for the line variations are consistent with the general pattern of radial ionization stratification of the BLR. During 1988–89 Fe II in blend 4130Å varies with an amplitude of aproximately 20% about the mean, similar the amplitude of the Hβ variations and Balmer continuum.

NGC3516:

Variations of Hβ during 1986–88 were small (20%), the shape of the profile (including the assymetry) did not change, but Hγ - to Hβ ratios of the profile integrated intensities of broad component increased from 0.43 in 1986 to 0.8 in 1988. The 30% variations of continuum on characteristic time scale 10–15 days during the spring of 1988 were not accompanied by any noticeable changes in the line profile (Bochkarev et al., 1990).
Infrared Coronal Emission Lines and the Possibility of their Laser Emission in Seyfert Nuclei
Authors: Greenhouse Matthew A. , Feldman Uri , Smith Howard A. , Klapisch Marcel , Bhatia Anand K. , Bar-Shalom Avi
Published: July 2016

Infrared coronal emission lines are providing a new window for observation and analysis of highly ionized gas in Galactic and extragalactic sources such as Seyfert nuclei and classical novae shells. These lines are expected to be primary coolants in colliding galaxies, galaxy cluster cooling flows, cometary-compact HII regions, and supernova remnants. In this poster, we summarize results discussed in detail by Greenhouse et al. 1993, ApJS, 88, 23. We discuss approximately 74 infrared (1 < λ μm < 280) transitions within the ground configurations 2s22pk and 3s23pk (k = 1 to 5) or the first excited configurations 2s2p and 3s3p of highly ionized (χ ≥ 100 eV) O, Ne, Na, Mg, Al, Si, S, Ar, Ca, Fe, and Ni. We present results from detailed balance calculations, critical densities for collisional de-excitation, intrinsic photon rates, branching ratios, and excitation temperatures for the transitions. The temperature and density parameter space for dominant cooling via infrared coronal lines is presented, and the relationship of infrared and optical coronal lines is discussed.
Wolf-Rayet Stars as Tracers of the Recent History of the Star Formation Rate
Authors: Meynet G.
Published: July 2016

Recently Vacca & Conti (1992) have measured the ratio of the luminosity in the broad He II λ4686 emission feature to that in the Hβ emission line in fourteen starburst galaxies. They related these luminosity ratios to the relative numbers of Wolf-Rayet (type WNL) to O-type stars in these galaxies (higher is the ratio L(λ4686)/L(Hβ), higher is NWNL/NO). They found that in general the number ratios are an order of magnitude larger than those expected in region of constant star formation rate. On Fig. 1 the predicted line ratios of our starbursts models (instantaneous burst of star formation at time t = 0; initial mass function dN/dM = CM−2; stellar models from Meynet et al. 1993; conversion formula between the line ratios and the number ratios of WNL to O-type stars given by Vacca & Conti 1992, with η = 1) are compared with the observed values given by these authors. This figure shows that a starburst taking place about 2–3 millions years ago can account for the high observed values of L(λ4686)/L(Hβ). One sees that the effects of the age of the burst (i.e. the time elapsed since the burst) and of the metallicity are quite important. It is the hope that in a next future, it will be possible on the base of this kind of luminosity ratios to disentangle the various effects influencing the WR population resulting from a starburst.
Time Dependent BLR Photoionization Models
Authors: O'Brien P.T. , Goad M.R.
Published: July 2016

Recent intensive campaigns to monitor the variable broad emission lines of AGN have provided a wealth of observational data. To reliably derive the BLR structure from such data requires a detailed comparison with physically realistic models. To provide such models we have combined photoionization calculations with a BLR modelling code. We find that the line emission is significantly anisotropic for the majority of lines at some radii in our models, particularly where the ionization parameter is large. As both anisotropic line emission and negative responsivity can have a substantial impact on the form of response functions, they must be taken into account when comparing BLR models to monitoring data. Differentiating between the effects of anisotropy and negative responsivity requires comparison of the variability behaviour of lines covering a wide range in ionization state with detailed photoionization models similar to those presented here. Failure to take such physical effects into account will lead to incorrect conclusions regarding the BLR structure.

Based on the form of the anisotropic response functions for our models, we argue that in NGC5548 the high ionization lines (including CIII] λ1909) can be explained by a spherical BLR component which has a significant radial ionization parameter gradient. The clouds in this component may be optically thin at the Lyman limit in the inner part of the BLR due to the high ionization parameter. The bulk of the low ionization lines, and in particular almost all of the Balmer line emission, arises from a non-spherical BLR component, possibly an accretion disc.

We have also investigated the accuracy of the linear response approximation by implicitly calculating the emissivity as a function of continuum level. For several lines the response is significantly non-linear over the typical continuum variability range of AGN. The form of the response functions is therefore luminosity dependent. Work is under way to incorporate this effect into the maximum-entropy-method used to recover response functions from monitoring data.
Infrared Line Shapes in Active Galactic Nuclei
Authors: Thompson Rodger I.
Published: July 2016

This paper presents the complete spectrum of NGC 4151 from 0.87 to 2.5 μm as well as detail of the Paschen α profile in 3C273 at a resolution of about 5000. Analysis of the several observed Fe II lines yields a most probable electron density of 104 and a most likely temperature of 104. The Fe II to H ratio is 2+/− 1 × 10−6 which is 5–7% of the available iron if the Fe/H ratio is solar.
Optical and VLBI Polarization Measurements of AGN
Authors: Gabuzda D. C. , Sitko M. L.
Published: July 2016

One of the most promising approaches to unravelling the relationship between the optical and radio emission in AGN is to obtain nearly simultaneous optical and VLBI polarization data. We have obtained such data for five AGN (0735+178, OJ287, 1219+285, 3C 279, and BL Lac). These data suggest that there is a direct link between the optical and radio polarized emission, and that frequently the optical polarization of AGN is associated with the emergence of new VLBI components.
Spectrum of a Magnetized Accretion Disk AGN Model
Authors: Wang Yi
Published: July 2016

Using the magnetized accretion disk AGN model of Field and Rogers (1993, ApJ, 403, 94), the spectral emission from far infrared to γ-ray is calculated. The resulting spectrum closely resembles that of Seyfert 1 galaxies. The effect of optically thin dust above the accretion disk is investigated.
On Illumination Effect in AGN Disks
Authors: Yi Insu
Published: July 2016

We consider illumination effects in a weakly self–gravitating (4πGΣ ∼ Ω2z) outer disk region (R ∼ 1017cm) around a supermassive blackhole of M8 = M/108Msun = 1 with relatively high mass accretion rates (Ṁ25 = Ṁ/1025g/s = 1). (e.g., Shlosman and Begelman 1989, APJ, 341, 685; Shore and White 1982, APJ, 256, 390).
Cm-Wavelength Flux Variability: Constraints on AGN Modeling
Authors: Aller M.F. , Aller H.D. , Hughes P.A.
Published: July 2016

To study whether the radio properties of BL Lacertae type objects and QSOs differ, we initiated a program in 1979 to monitor the total flux density and linear polarization at 14.5, 8.0 and 4.8 GHz of the strongest then known BL Lac objects which met the Hewitt-Burbidge criteria (Ledden, private communications) plus 3 subsequently identified high declination BL Lacs (Biermann et al. 1981). Results based on the behavior of the 45 brightest sample members are compared here with the properties of the QSOs in the flux-limited Pearson-Readhead sample (Pearson and Readhead 1988).
Principal Component Analysis of Multiwavelength Properties of Seyfert Galaxies
Authors: Dultzin–Hacyan Deborah , Ruano Carlos
Published: July 2016

A multidimensional statistical analysis of observed properties of Seyfert galaxies has been carried out using Principal Component Analysis (PCA) applied to X-ray, optical, near and far IR and radio data for all the Seyfert galaxies types 1 and 2 for the catalog by Lipovtsky et al. (1987).
Multifrequency Spectra of EXOSAT Blazars
Authors: Ghosh K. K. , Soundararajaperumal S.
Published: July 2016

BL Lacs, Highly Polarized Quasars (HPQs) and Optically Violent Variables (OVVs) are classed as blazars. Based on the multifrequency spectral properties of these objects we have classified the blazars into two groups which are as follows:

– Group I - BL Lacs

BL Lac objects are soft X–ray sources (Hardness ratio is less than 5) with steep and flat spectral indices in the X–ray and ultraviolet bands, respectively. They are relatively less polarized (in the optical band) and low luminous objects (Mabs < −23). Radio to X–ray spectra of BL Lacs can be fitted with a single parabolic curve and these objects radiate most of their energies in the X–ray band (radio through X–ray frequencies).

– Group II - HPQs/OVVs

HPQs and OVVs are hard X–ray sources (Hardness ratio is greater than 5) with flat and steep spectral indices in the X–ray and ultraviolet bands, respectively. These are highly polarized and highly luminous (Mabs > −23) objects. Radio to X–ray continuum emission spectra of HPQs/OVVs can be fitted with two parabolic curves with the spectral break between UV and X–ray bands and these objects radiate most of their energies in the infrared band.
Optical Properties of Soft X-ray Selected Bright New ROSAT AGN
Authors: Grupe D. , Beuermann K. , Reinsch K. , Thomas H.-C. , Fink H.H.
Published: July 2016

Optical identification of bright sources from the ROSAT All-Sky Survey has led to the discovery of 40 new Seyfert galaxies with very soft X-ray spectra. Nearly half of these are narrow-line Seyfert 1 (Hβ-FWHM < 2000 km/s) with strong optical FeII emission, suggesting the presence of an unseen hard X-ray component. The remainder are Sy1.5 and a few broad-line Sy1.
The Redshift Distribution of Ly α Forest Lines in Spectra of QSOs
Authors: Keliang Huang , Hongnan Zhou
Published: July 2016

Numerous narrow absorption lines in the region of wavelength shorter than 1216(1+zem) (zem is the emission redshift), i.e. so-called Ly α forest lines, detected in QSO spectra are usually thought to be produced in intervening primeval clouds. The study of Ly α clouds may reveal how matter distributes in space and how it evolves with time at the early universe and provide valuable information about the large scale structure of the universe and its evolution. Based on intermediate resolution (1 ∼ 2 A) spectra, many authors (e.g. Lu et al. 1991) deduced that the evolutionary index γ ∼ 2, (dN/dz ∼ (1 + z)γ, dN/dz is the number of clouds per unit redshift interval at redshift z). It means that Ly α clouds have strong cosmological evolution. In recent years, there appear high-resolution (< 30km/sec) spectra of QSOs. High resolution spectra may provide more information than medium resolution spectra. Hence, it is necessary to study the evolution of Ly α clouds, using the spectra with higher resolution. Carswell et al. (1987) found γ=1 in the redshift interval 1.9–3.8. But Rauch et al. (1992) found γ=2.1 for the line sample with logN(HI) ≥ 13.75. It is more interesting that Giallongo (1991) found a differential evolution: γ is depended on the equivalent width W of line and no evolution for the strong line sample with 0.5 > W > 0.3. However, these studies involved very few QSOs (three or four). In this paper, we use a larger sample of QSOs to study the evolution of Ly α clouds.
Activity as the Result of Merging
Authors: Kontorovich V. M. , Kats A. V. , Krivitsky D. S.
Published: July 2016

The observed correlation between activity and mergers of galaxies may be explained by the compensation of the angular momentum by merging. This leads to accretion on the galaxy centre.