Introduction

Recent work in the theory of non-linear dynamical systems has centred on the concept of chaos, a term that applies to a great variety of situations and configurations. This relatively new subject is fascinating in its own right, and the rapidly growing body of knowledge surrounding it has uncovered a number of characteristics shared by all chaotic systems. The concept has not only achieved an extensive currency throughout the mathematical and scientific communities but has also captured the interest of many in the nonmathematical world, the latter largely due to an excellent popular discussion of the history and basic ideas by James Gleick (1988)

If the dynamics of the solar magnetic field are due to magnetoconvective dynamo action within the Sun, then the activity cycle is governed by the non-linear equations of magnetohydrodynamics, discussed in Chapter 11. A number of investigators have suggested that solar and stellar activity cycles are chaotic phenomena and have begun to explore the implications of cyclic systems which are chaotic.

If stellar activity cycles are indeed examples of chaotic systems, then they will share in the universal characteristics of such systems. In order to discuss the implications for cyclic activity, a brief outline of the relevant concepts in the theory of chaos is called for.

Introduction

The recognition that magnetic fields are an essential component not only of solar and stellar activity but also of the structure of galaxies, quasars, and pulsars has focussed considerable theoretical interest on the origin and maintenance of cosmic magnetic fields. Since the length scales associated with many cosmic magnetic fields are very large, the ohmic decay times (see §4.1 and below) are long, and there is no difficulty in explaining the continued existence of primordial or fossil fields, such as the megagauss fields found in magnetic A-type stars; but the changes observed to occur in many cosmic magnetic fields, over periods which may be short compared with the decay time, entail an interaction between the plasma motions and the existing fields which may also maintain these fields against ohmic decay. This has become known as dynamo action, and, in order to understand evolutionary changes occurring in the solar magnetic cycle, it is necessary to probe further into the underlying theory.

Parker (1970) drew attention to the curious asymmetry throughout the universe between electric and magnetic charge on the one hand, and the corresponding fields on the other.

Basic data

The historical studies traced in the previous chapter provided an introduction to our knowledge of the structure of the Sun and of cyclic activity. We now offer a brief summary of the general state of our knowledge of the physical properties of the Sun and of solar-type stars, together with some basic theory relevant to an understanding of cyclic phenomena. The interested reader who desires further information is referred to the more general accounts listed in the references (e.g. Mihalas 1978, Foukal 1990, Stix 1989, Zirin 1989).

Stars are generally classified according to their luminosity and surface temperature, a classification scheme which has been codified as the Hertzprung-Russell (H-R) diagram. In this diagram the absolute magnitude (or logarithm of the total luminosity) is plotted against the logarithm of the surface temperature. In the Harvard classification scheme the categories O, B, A, F, G, K, M, R, and S represent decreasing surface temperatures and increasingly complex spectra, and the Sun (of type G2) sits squarely in the middle. It is about 4.5 x 109 years old, less than half the age of the oldest stars in our galaxy, and in these, as in many other respects, it is a very ‘ordinary’ star.

Abstract

Balmer emission line variations of the integrated flux as well as profile variations for the Seyfert galaxy NGC4593 are presented. This active galaxy was observed in an international monitoring campaign. The broad emission line profiles consist of at least three components varying in an independent way. The FWZI of the broad line profiles of the Hα line remained constant during the intensity variation indicating a turbulent velocity field of the BLR.

Introduction

It has been shown by simple kinematical model calculations that the response of the broad line profiles to an outburst of the central continuum source of an active galactic nucleus (AGN) is different for radial, rotational, and turbulent motions (Pérez, Robinson, de la Fuente 1992, Welsh, Home 1991). The detailed study of variations of the broad emission line profiles provides a powerful tool to investigate the structure and the kinematics of the central broad-line region (BLR), which cannot be directly resolved. Therefore, the investigation of the temporal evolution of broad emission line profiles with respect to continuum variations yields information for distinguishing between these possible types of the kinematics of the BLR gas. The analysis of the variability of emission line profiles requires spectra with high spectral resolution and high signal-to-noise ratio monitored over at least several months with a sampling rate of a few days.

Abstract

Thirteen years of IUE observations of the Seyfert 1 galaxy Fairall-9 have shown a large variability in the UV continuum and UV emission lines, Lyαλ1216 and CIVλ1550. The relation between UV continuum and X-ray (no delay) is similar to that found for lower luminosity objects and suggests that X-ray reprocessing causes the UV continuum. We also use the line variability to support and refine a gaussian decomposition of the profiles of Lyα and CIV, based on a previous study of Hβ. The decomposition of Lyα and CIV shows a well identified component structure valid for both lines. The components respond very differently to the changes in the ionizing continuum brightness, confirming the different physical nature of the material associated with them.

The UV-X ray continuum relation

We show in figure 1 (1a and 1b) the UV and X-ray light curves of F-9, while fig. 1c and 1d show relations between these two quantities. Although the time resolution in the CCF (1d) is of course limited by the sparse X-ray sampling no evidence is found for delays of the size suggested by the IR (400 days). We see here also that at high UV brightness the rather tight correlation between the 2-10 keV flux and F(1338Å) breaks down. Although the details of this behaviour are currently not fully understood it has been suggested that at low levels the UV continuum is the result of reprocessing of X-rays emitted above the disk while the huge UV variations could be associated with major accretion events.

Abstract

This is a short summary of several detailed calculations of strong radiative cooling behind supernova shock waves evolving in a high density medium. These lead to definite predictions about the lag, the observed delay between sudden changes in the continuum ionizing radiation followed, after some time, by changes in the intensity of the emmision lines from the broad line region of AGNs. A full description of these results is due to appear soon in a journal.

Introduction

In the starburst model of AGNs, sometimes viewed as exotic and/or unconventional, the applied physics are in fact most conventional, as it uses the little, or the lot, that we know about real events: the physics of stars and stellar evolution and their interaction with the surrounding gas, and with these predictions are made. In this model, the observed broad emission lines and their variability, are generated by “compact”, strongly radiative supernova remnants which are expected to occur in the central regions of early type galaxies undergoing a violent nuclear burst of star formation. The activity of all stars and the frequency of supernova explosions soon establishes a high pressure region around the cluster. This high pressure, as it acts upon the slow winds from the massive stars (M ≥ 8M⊙), leads to the development of a high density circumstellar medium (n0 ≥ 107 cm−3) around each of the potential supernova stars.

Abstract

We describe constraints on the metallicity of quasar broad line region gas. The overall emission line spectrum is surprisingly insensitive to order of magnitude changes in the global metallicity Z. Indirect methods, employing photoionization models and explicit stellar chemical evolution models of the selective enrichment of the elements, must be used to infer the metal enrichment. Two line ratios, both involving NV λ1240, are developed to measure Z. The first is the ratio of NV to the collisionally excited line CIV λ1549. The second is the ratio of NV to the recombination line HeII λ1640. Both indicate nitrogen enhancements exceeding an order of magnitude above solar. These results imply high metallicities in high redshift quasars, a property they have in common with the cores of massive galaxies.

Introduction

One of the longest standing goals of AGN emission line research has been to use these lines to measure the composition of the emitting gas. This could then test models of both the quasar phenomenon and stellar nucleosynthesis (Davidson and Netzer 1979; Shields 1976). Fundamental uncertainties concerning the nature and geometry of the BLR have made this work difficult.

Variability studies have rejuvenated interest in the BLR by providing methods to directly measure quantities such as the distance between continuum source and emitting clouds. These have shown that the clouds are both denser and exposed to a far more intense radiation field than had been inferred (Peterson 1992; Ferland and Persson 1989; Ferland et al. 1992).

Abstract

The polarization properties of a two-phase model, recently proposed to explain the X-ray emission of Active Galactic Nuclei, have been calculated for different values of the model parameters. An important signature of the model is the orthogonality between the UV/soft X-ray and hard X-ray polarization.

Recently, a two-phase model in which hot, thermal electrons in an optically thin layer comptonize the soft photons coming from an underlying cold, optically thick accretion disc, has been proposed to explain the X-ray emission of Active Galactic Nuclei.

Assuming a plane-parallel geometry, and isotropic and unpolarized disc thermal radiation, we have calculated the polarization properties as a function of the energy and of the inclination angle, for different values of τ0, the optical depth of the hot phase (which, in the adopted model, is related to the electron temperature). This was done by solving the well-known equation of radiative transfer by separating the different scattering orders. The polarization of the X-rays reflected from the disc has also been taken into account. In the figure we show the degree of polarization as a function of the energy for different values of the inclination angle (at the two extremes of the energy range ∣P∣ increases with it). The assumed energy shape of the thermal radiation is a black-body with T=50eV. Note that the hard X-rays have a negative polarization (i.e. the polarization vector lies in the meridian plane), while the polarization of the UV/soft X-rays is positive (i.e. the polarization vector is perpendicular to the meridian plane).

Abstract

The strong evolution of the host object population postulated in hierarchical models for structure formation is invoked to explain the observed strong evolution of the space density of quasars. The quasar activity is interpreted as marking the advent of a new step in the hierarchic build-up of bigger and bigger dark matter halos. The Press–Schechter formalism within the CDM scenario is used to estimate the number of newly forming dark matter halos. Pronounced peaks are found in the number density of newly forming massive black holes, capable of explaining the short time scale of the evolution of the quasar population. A gratifying fit to the observed luminosity function is obtained.

Quasar evolution in the CDM scenario

Soon after the discovery of the first quasars it was noticed that quasars are a strongly evolving population of objects. With the increasing number of known intermediate redshift quasars it became possible to determine the time evolution of the luminosity function of quasars. The main feature of the luminosity function is a characteristic break luminosity which decreases with time. The quasar luminosity function is most naturally interpreted as a superposition of many generations of short-lived quasars with a life time ∼ 108yr and a characteristic mass that decreases with time as ∼ (1 + z)3.

In hierarchical models for structure formation, such as the CDM scenario, larger and larger structures build up by merging of smaller structures and the smaller structures are at least partially erased.

Abstract

Short-wavelength IUE archival data for NGC 3783 were re-extracted and analysed to constrain numerical modelling parameters in detailed photoionization analyses. The He IIλ1640, C IVλ1549, and C IIIλ1909 line intensities and trends can be reasonably well reproduced by a two cloud component model. In order to produce satisfactory line intensities or trends for other higher ionization lines and lower ionization lines, still more gas components are necessary with gas density ranging from roughly 1011 cm−3 to 109 cm−3 or less. In going from the inner to outer clouds, the optical depth increases and the gas density decreases approximately as r−2.

The amount of dust obscuration along the line of sight, as required by the models, is consistent with reddening estimates from HeII line ratios, and CNO abundance ratios derived from intercombination line intensities suggest abundances of carbon, nitrogen and oxygen lower by a factor of about 2 relative to solar. The HeII line rest equivalent widths from the models suggest a gas covering factor of order 0.25.

Introduction

Although sophisticated numerical photoionization models have been employed for two decades in investigations of the broad–line–emitting regions (BLRs) of Seyfert 1 galaxies, some of the most fundamental BLR properties such as the intrinsic ionizing radiation field, the ionization parameter, emitting gas density, chemical abundances, and the “covering factor” (the fraction of solid angle occupied by optically thick gas around the ionizing radiation source) are still open to question.

Abstract

A realistic stellar cluster potential may have an observable effect on the thermal structure of an AGN accretion disc. Optimization of model parameters has found an extremely good fit to the broad-band spectrum of 3C 273 for reasonable assumptions about the central black hole and surrounding cluster.

Introduction

Various authors have shown that compact stellar clusters around AGN Black Holes are important in determining the properties and evolution of Active Galactic nuclei, e.g.. We describe a way of directly determining the mass and size of such clusters in certain cases.

Radiation from an AGN accretion disc derives, at least in part, from the local release of binding energy through viscosity. Here we assume that accretion is the sole energy source: the spectrum of the disc is determined by the potential into which it is falling. We derive an estimate of the disc spectrum by assuming black-body emission.

In a forthcoming paper, we find the spectrum expected from this model AGN, in which a dense, young stellar cluster surrounds a massive Black Hole. Here, we show a fit of this model to the continuum of 3C 273 (in its low state).

A Model of 3C 273

In Perry & Williams, we derive cluster parameters from the parameters of the fit given by. We have optimized the fit of our model to the data, (Fig. 1). In this figure, the different symbols identify different observations; the points near 1021 Hz are upper limits.

Abstract

The properties of the iron Kα line emitted by a photoionized accretion disc have been calculated for different source geometries.

The properties of the iron Kα fluorescence line emitted by an α-viscosity accretion disc illuminated by an external X-ray source have been calculated for different values of the disc accretion rate ṁ. The vertical ionization structure of the matter has been computed by using the numerical code described in. Two different source geometries have been studied: a point source located at 20 rg (=GM/c2) above the disc on its symmetry axis, and an extended source above the innermost part (r = 6 – 50rg) of the disc. Assuming α=0.1, a Schwarzschild black hole and a hard luminosity equal to the disc luminosity, we find that for large values of ṁ (≲ 0.2, in units of the critical value) the matter can be significantly ionized, and the iron line equivalent width can reach values as high as 250 eV for the point source, and up to about 400 eV for the extended source (while for neutral matter it is ∼150eV for a face-on disc). The line centroid energy, in the emitting rest frame, is significantly higher than 6.4 keV, the value for neutral iron. A further increase of ṁ (≲ 0.5) leads to a strong decrease of the line intensity, because the iron becomes fully stripped in the inner region of the disc.

Abstract

Long time scale ultraviolet light curves of blazars and Seyfert 1s both show very strong continuum variations, but this similarity vanishes when short time scales, spectral variability and correlations between variability and luminosity are studied. For instance, blazars show much more rapid variations than Seyfert 1s. Also, the spectra of Seyfert 1s harden as the source brightens, while blazars show little spectral variability. Third, the most luminous blazars tend to be the most strongly variable, while for Seyfert 1s, the strongest variations are seen in the least luminous sources. These differences suggest that in spite of some overall similarities, the observed emission from blazars and Seyfert 1s have different physical origins. These results are consistent with models which hold that the ultraviolet emission from blazars is incoherent synchrotron emission from a jet, while that from Seyfert 1s is dominated by thermal emission from an accretion disk.

Background

In the 13 years since its launch, IUE has obtained over 5000 ultraviolet spectra of active galactic nuclei (AGN). In this paper, we use ∼2500 spectra of 16 objects to survey the ultraviolet variability properties of Seyfert 1s (defined to include quasars as well; [2]) and blazars (BL Lacs and OVV quasars; [6]).

Long and Short Time Scale Variability

Although it is a common prejudice that blazars are more strongly variable than Seyfert 1s, the long term variability properties of the two types of object are actually very difficult to distinguish.

Abstract

Gamma ray emission from extragalactic sources is interpreted as the Doppler boosted annihilation and Inverse Compton radiation from a relativistic electron-positron beam in the frame of the two-flow model. In the case of 3C279, the high luminosity and the rapid variability of gamma ray emission suggest a relativistically moving source, but even so the compactness cannot be smaller than unity at light week scale with a reasonable Doppler factor. This supports the two-flow model of extragalactic radio sources, where the small scale emission comes from a relativistic electron-positron beam, heated by a MHD jet responsible for the large-scale (kpc) radio structures.

Introduction

The GRO satellite has detected intense gamma ray emission from several Active Galactic Nuclei and quasars. Remarkably, all of them are associated with a flat spectrum radio source, whose radio spectral index αr is smaller than 0.5, and half of them exhibit known or probable superluminal motions (the others have not been observed at different epochs in VLBI). Just like the commonly invoked Doppler beaming amplification of radio emission, the high γ-ray luminosity suggests also that the emitting source is moving relativistically.

For 3C279 in particular, the spectrum reported by Hermsen & al. show a maximum emission per logarithmic energy interval around 10 MeV, with a photon spectral index of approximately 1.5 below the turn-over frequency and approximately 2 above it. A rapid flare has been observed with an increase of the luminosity by a factor 5 on a time scale of 2 days.

Abstract

We present and discuss the results of long-slit spectroscopic observations of the extended emission line regions (EELR) in NGC4388, a Seyfert 2 galaxy in which extended, off-nuclear broad Hα emission (FWZI ≈ 4000 km s−1) has been reported (Shields & Filippenko 1988). These features have been interpreted as scattered radiation from a Seyfert 1 nucleus that is obscured along our line of sight. Our spectroscopic observations cover a large fraction of the inner part of the EELR, including some of the positions where the presence of broad lines has been claimed. Broad wings in the Hα + [NII] λλ6548, 6583 complex are also present in our data but they can be explained by the superposition of several narrow components. However, we cannot, at present, exclude the possibility that an intrinsically broad component to Hα exists at some locations. The implications of our results for unified models of Seyfert galaxies are briefly discussed.

Introduction

NGC4388 is a high-inclination Seyfert 2 galaxy with bright EELR (Colina et al. 1987, Pogge 1988, Corbin et al. 1988). Radio observations reveal a double-peaked source close to the apparent optical nucleus and more extended emission aligned roughly perpendicular to the galaxy disk (Stone et al. 1988, Hummel & Saikia 1991). The axis of the cone-like, high-excitation gas is almost perpendicular to the galaxy disk and close to that of the extended radio emission (Pogge 1988, Corbin et al. 1988), suggesting that the EELR are photoionized by nuclear radiation which escapes preferentially along and around the radio axis.

Abstract

Although claimed to possess a Seyfert nucleus, NGC 1808 reveals radio properties, line widths and ratios that are consistent with a few, albeit powerful, SNRs. There is as yet no compeling evidence for Seyfert activity, and this galaxy should be reclassified as a starburst galaxy (it shows many similarities to NGC 253 and M82).

Introduction

NGC 1808 is a ‘hotspot’ or Sersic–Pastoriza galaxy at a distance of 16.4 Mpc (1″ = 80 pc). Although this galaxy shows optical hotspots, these hotspots have largely disappeared at near–infrared wavelengths. It is a highly inclined and dusty Sbc spiral, with evidence for a burst of star formation in the circumnuclear region about 5 × 107 yrs ago. Two pieces of evidence suggested that NGC 1808 habours a Seyfert nucleus:

• High resolution spectra revealed broad lines (Veron–Cetty & Veron 1985).

• The 6cm radio luminosity of the compact nucleus is 500 times that of the most luminous Galactic supernova remnant (SNR), suggesting a non–stellar nucleus (Saikia et al. 1990).

Line Ratios

We have obtained a red spectrum of NGC 1808 (see Forbes, Boisson & Ward 1992 for details) containing the [SIII] lines at 9069 and 9532Å. Fig. 1 shows a diagnostic diagram based on the sulphur line ratios. We also show the mixing curve between the location of HII regions and Galactic SNRs. This line ratio diagram (and those for [OI]6300Å, [NII]6583Å, [SII]6717+6731Å) are consistent with a high abundance HII region and a ∼ 20% contribution from SNRs, i.e. no Seyfert nucleus is required to explain the line ratios.