Abstract

The Starburst model for radio-quiet Active Galactic Nuclei (AGN) postulates that the activity seen in most AGN is powered solely by young stars and compact supernova remnants (cSNR) in a burst of star formation at the time when the metal rich core of the spheroid of normal early type galaxies was formed. In this model, the broad permitted lines characteristic of the Broad Line Region (BLR) and their variability are originated in these cSNR. Combined analytic and numerical hydrodynamic simulations, with static photoionization computations have shown that cSNR can reproduce most of the basic properties of the BLR in low luminosity AGN.

We have explored the hypothesis that QSOs are the young metal rich cores of massive elliptical galaxies forming at z ≳ 2.0. Only a small fraction (∼ 5%) of the total mass of a normal spheroid, the core mass, is needed to participate in a burst to explain the observed luminosities and luminosity function of Quasars at z ≳ 2.0. We predict that the progenitors of QSOs should look as dusty starbursts and about 4 times more luminous than QSOs themselves.

Introduction

The hypothesis that a Starburst can power the most extreme forms of nuclear activity has been proposed several times in the past (Shklovskii 1960, Field 1964, McCrea 1976), but was not favoured mainly because it failed to explain satisfactorily the observed large luminosity and variability of quasars, their radio emission, unresolved images, the presence of extremely broad permitted emission lines in the spectrum and their observed intensity ratios.

Introduction

We have heard many times during this conference that variability is a powerful tool to investigate the nature of AGN. The goal of this talk is to demonstrate that the clues derived from variability studies may be partial, disappointing and even misleading, if some characteristics of variability are overlooked and the proper statistical tools are not adopted. To convince you that such a caveat, although obvious, is important and productive, we should like to focus on a few issues, particularly important from our point of view.

The stochastic nature of the light curves

First of all, let's draw your attention to the fact that light curves of quasars have generally random behaviour, i.e., that the knowledge of the value of a time series at a given instant does not allow (in a broad sense) one to forecast the future evolution of the light curve. What does it mean from a physical point of view? It means that quasars are dynamical systems of high dimensionality, systems whose temporal evolution is described by an extremely large set of differential equations (or by partial differential equations). In other words, quasars are dynamical systems whose evolution is determined by a large number of external factors. To understand this point let us consider an example close to common sense: a pendulum. As is well known, the temporal evolution of such a system is described by a second order ordinary differential equation.

Abstract

The first ROSAT X-ray spectra of two high-redshift quasars reveal unexpectedly strong absorption when compared with similar luminosity objects at lowredshift. A third quasar shows none. A fourth, low-redshift, radio-loud quasar (3C351) with extended radio structure, shows absorption possibly due to a warm absorber with a strong OVII absorption edge.

Introduction

X-ray spectral observations of quasars have been confined to low redshift objects (z≤0.5) whose proximity makes them bright enough to study and also to those with relatively bright X-ray flux (αox≲1.5). ROSAT, with its high sensitivity, enables us to observe the spectra of high redshift (z>2) and large αox quasars for the first time. We have begun a ROSAT observing program to study the X-ray spectra of quasars selected to cover the full range of continuum properties. In particular this sample includes objects at high redshift, with relatively faint X-ray flux and with a full range of radio properties: strong, weak, extended and compact. We are also carrying out a follow-up observing program to obtain multi-wavelength (infrared – ultra-violet) data for all our ROSAT-observed quasars.

Sampling the full quasar population with ROSAT

To date we have received and analysed data for > 25 quasars. Their spectra are generally steeper than those seen at higher (e.g. Einstein IPC) energies, as observed in general with ROSAT. Our current sample includes 4 high-redshift (z>2.8) quasars with sufficient counts (> 350) to obtain spectral information (Table 1).

Abstract

The variability properties of a sample of over 300 optically–selected quasars near the South Galactic Pole (SGP) have been studied using a series of eleven UKST Bj plates at seven epochs, spanning 16 years. Quasars of high luminosity show significantly less variation than those with low luminosity. A similar, though much weaker, trend with redshift was found; lower redshift quasars varying proportionally more than high redshift quasars. The observed trends are a consequence of an intrinsic dependence of quasar variability on luminosity combined with the effects of time–dilation and have strong implications for quasar samples selected solely on variability.

Introduction

Variability provides a simple yet powerful means for investigating the physical processes at work in the inner regions of AGN. The primary diagnostics for optical variability are: the dependence on absolute magnitude and redshift, the timescale of variations in the quasar rest frame and the degree of coherence of individual quasar light curves — in our case taken as an ensemble. In addition to providing insight into quasar models an important feature of such a study is the ability to predict selection effects for quasar samples chosen purely on the basis of variability (e.g., Hawkins 1986). In this paper we summarise our method and results: a more detailed account is given in Hook et al. (1991) and Hook et al. (1992).

Data

The sample of quasars was taken principally from the catalogue of Hewitt & Burbidge (1989) with additional objects from other surveys.

Abstract

Large-scale multiwavelength spectroscopic monitoring campaigns are producing new information about the central regions of AGNs. Reverberation mapping experiments are now being undertaken, and these are providing useful constraints on the structure and kinematics of the broad-line region and leading to reinvestigation of the physics of the line-emitting gas. In this contribution, the fundamental assumptions of reverberation mapping, some of the principal results of recent monitoring campaigns, and questions that have arisen from recent work are briefly reviewed.

Introduction

The broad emission lines in the spectra of AGNs vary in response to changes in the luminosity of the central ionizing source with a time delay due to light traveltime effects within the broad-line region (BLR). It is in principle possible to make use of these light travel-time effects to map out the geometry and kinematics of the BLR through detailed comparison of the continuum and emission-line variability. This technique, known as “reverberation” or “echo” mapping, requires large amounts of high-quality data. Recent campaigns to measure continuum and emission-line variations in AGNs are for the first time providing data suitable for this purpose. These new data are leading to important new inferences about the nature of the BLR and the central source. In this contribution, I will discuss progress made in application of reverberation mapping techniques and mention some of the areas where further progress can be made in the near future. A more complete review of the subject will be provided elsewhere.

Abstract

AGN emit energy across the electromagnetic spectrum from radio waves to gamma rays. Observations in any single waveband give a very incomplete view of the relevant physical processes; higher energies generally probe smaller scales near the nucleus. AGN with compact radio cores show nonthermal emission from relativistic particles at many wavelengths. Many observed properties are altered by beamed emission from a relativistic jet. Low luminosity AGN without strong radio cores show thermal emission from cool dust in the far infrared, and from hot gas in the ultraviolet. Anisotropic obscuration leads to observed properties which are a function of orientation. A new window on AGN has been opened with the detection of high energy gamma rays.

Introduction

The greatest challenge in research on Active Galactic Nuclei (AGN) is to understand the physical mechanisms behind the prodigious energy output of these distant sources. Progress has been slow because the continuum emission extends over at least eighteen decades in frequency (from 108 to 1026 Hz). Unfortunately, we operate our narrow bandwidth detectors in a broad bandwidth universe. Until the past decade, most of our information had come from two limited windows at radio and optical frequencies. Space missions and new detector technologies have opened up a variety of new wavebands, for example in the millimetre, far infrared, X-ray and γ-ray parts of the spectrum.

Abstract

The radio properties of radio quiet active galaxies are revisited and considered under the starburst without black hole model. These radio properties are consistent with the luminosity, compactness and spectral index expected from a massive starburst process, where bright and compact radio supernovae and supernova remnants, i.e. radio hypernovœ, generate the radio emission.

Introduction

Since the discovery of quasars, theoretical and observational work has been done in order to characterize the variety of active galactic nuclei (AGNs), and to understand the physical mechanisms operating in these regions. The most popular scenario considers the presence of an accretion disc around a massive black hole. Under this scenario, different regimes of accretion and/or black hole masses plus some anisotropy in the radiation field could account for the whole variety of AGNs. Alternatively, a different model based on the evolution of a central compact star cluster, has been proposed and worked out in some detail (Terlevich, 1990 and references).

A natural way to ascertain the true nature of the compact objects in AGNs, is to look at the central regions of galaxies with the highest spatial resolution available, i.e. radio observations. High resolution VLBI and VLA observations with 5 and 250 mas resolution, respectively, allow the nuclei and cores of nearby active galaxies to be mapped with typical resolutions of a few parsecs. In this paper I briefly discuss the radio properties, absolute luminosity, spectral index, morphology and compactness of radio quiet active galaxies and compare these properties with the predictions of the starburst model.

Abstract

The properties of two complete samples of flat-spectrum radio quasars and radio-selected BL Lacs are analysed to look for any relationship between the two classes. It is shown that BL Lacs are not quasars with emission lines swamped by an enhanced optical continuum but their line luminosities are intrinsically weak. Moreover, an evolutionary connection between the two classes does not seem to be supported by the present data, while micro-lensing of quasars by stars in foreground galaxies can be ruled out as an explanation for the BL Lac phenomenon. BL Lacs and flat-spectrum radio quasars probably represent separate instances of relativistic beaming in low- and high-luminosity radio galaxies respectively.

Introduction

BL Lacertae objects are special types of active galactic nuclei, characterized by rapid variability, relatively high optical polarization, flat radio spectrum, radio coredominance, superluminal motion, and weak or absent emission lines. Flat-spectrum radio quasars (FSRQs) are quasars with spectral index α ≤ 0.5 (Fv ∝ v−α) at a few GHz, in many ways similar to BL Lacs (members of the two classes are often grouped together under the blazar category). The most striking difference between BL Lacs and FSRQs is the presence of strong emission lines in the latter objects. Although there are undoubtedly some borderline objects in which emission lines appear when the continuum is in a low state, a rest-frame equivalent width of 5 Å seems to separate quite well the two classes (see discussion in Stickel et al. 1991).

Unification of AGNs

There is now ample evidence for partial unification of (a) Seyfert 1 and Seyfert 2 galaxies, in terms of obscuring tori and orientation effects; (b) steep-spectrum (lobe dominated) radio quasars and flat-spectrum (core dominated) radio quasars, in terms of relativistic beaming and orientation effects; and (c) blazars, radio-loud quasars and broad-line radio galaxies, and narrow-line radio galaxies, in terms of obscuring tori, relativistic beaming, and orientation effects. Variants or subsets of these basic sets have also been proposed — e.g., BL Lac objects and Fanaroff-Riley (1974) Class I galaxies. Much of the observational evidence was discussed at this conference, primarily by Bob Fosbury, Clive Tadhunter, Neal Jackson, Thaisa Storchi-Bergmann, Paul Alexander, Paolo Padovani, Dave Axon, and Alec Boksenberg, as well as in several posters; it involves spectropolarimetry, superluminal motion, projected sizes, relative numbers of different objects, lobe depolarization asymmetry, the inverse-Compton limit, ionization cones, variability, and other phenomena. A reasonable conclusion is that unification schemes must be correct at least to some degree.

It is not so clear, however, that the strong forms of the unification schemes are valid. Specifically, let me focus on the unification of Seyfert 1 and Seyfert 2 galaxies (Antonucci & Miller 1985). The strong form states that these are exactly the same objects, simply viewed from different directions; no other factors (such as differences in the thickness or opening angle of the obscuring torus) are involved.

Cloud velocities in the NLR appear to be related mainly to the host galaxy (Whittle, 1992). This is compatible with evidence that the clouds are predominantly infalling (De Robertis & Shaw, 1990), with only a minor component of the line emission arising from jet induced emission. Either these infalling clouds contribute significantly to the BLR, connecting this to the NLR smoothly through an intermediate zone, or they are destroyed. We expect that the cloud geometry and dynamics will be reflected in the line emission ratios and profiles. We look here at the emission from clouds that are destroyed in an outflowing supersonic wind from the central nucleus which we assume to be in pressure balance with the ambient ISM (Smith, 1984; Mobasher & Raine, 1987)

We model the hydrodynamic evolution of two clouds each having an initial density of 104 cm−3 and a temperature of 104 K with a free-fall velocity of 2 × 107 cm s−1. The smaller cloud has a mass of 6.6 × 10−4 M⊙ (rc = 25 × 1015 cm) and the larger one a mass of 6.6 × 102 M⊙ (rc = 25 × 1017 cm). The clouds fall under gravity into a supersonic wind of Mach number 1.5 with a density that increases as r−2. The initial distance from the continuum source is 1021 cm and its luminosity is ∼ 1044 erg s−1.

For the hydrodynamic simulation we employ a code utilising a first order Godunov scheme (Godunov, 1959) developed by R. Hillier of Imperial College London and adapted for astrophysical use at Leicester University by M. Dubai and P. Foulsham.

Abstract

We present evidence that BL Lac objects are a distinct class, rather than QSOs viewed close to, or within, the ‘critical cone’ of their collimated flow: both statistical analysis of the Stokes parameters and VLB polarimetry imply that the magnetic field structures in BL Lacs and QSOs are intrinsically different.

The low degree of polarization in the quiescent state of radio variable AGN, and the occurrence of occasional “rotation” events, have provided evidence for the presence of a highly tangled magnetic field. We would anticipate the Stokes parameters (Q, U) versus time plots of BL Lac objects and QSOs to show systematic differences if the relative strengths of tangled and ordered field components differ between classes. Fig. 1 illustrates typical Q-U plots for each class, derived from single dish observations, showing that the dispersion is large compared to the offset from zero for the BL Lac (interpreted as due to the presence of a weak mean field, so that evolution is dominated by the random walk of points in the Q-U plane as ‘new’ magnetic cells are advected into the window of observation), but small compared to the offset for the QSO (interpreted as due to a stronger mean field: the random walk is now about a point displaced from the origin by virtue of this stronger, axial field). A statistical analysis of 51 sources confirms this trend.

Abstract

A preliminary analysis of radio and optical data for the low-frequency-selected Molonglo Quasar Sample provides new evidence that radio beaming, as inferred from core dominance of the radio emission, may be accompanied by an enhancement of the optical continuum.

Introduction

There is growing evidence that the optical continuum of radio quasars is not emitted isotropically (Tadhunter et al. 1987; Penston et al. 1990; Jackson & Browne 1991), implying that magnitude-limited samples could be seriously affected by an orientation bias (Kapahi & Shastri 1987). To minimise this bias we have defined a new complete sample from the 408 MHz Molonglo Reference Catalogue (MRC; Large et al. 1981), based on deep optical identifications from UK Schmidt plates. Quasars selected at low frequency are expected to be dominated by their unbeamed extended radio flux, rather than any relativistically beamed core component, and should therefore have their radio jet axes oriented randomly in the sky.

Sample Definition and Observations

Over 700 MRC radio sources, with peak flux density S408 > 0.95 Jy and falling in a 10° declination strip, −20° > δ > −30°, were mapped at 843 MHz in ‘snapshot’ mode with the Molonglo Observatory Synthesis Telescope. Optical counterparts (complete to the plate limit, BJ = 22.5) were then identified from UK Schmidt survey plates. All 82 resulting QSO candidates in two RA regions (09h–14h and 20h–06h) were subsequently imaged with the VLA at 5 GHz; 31 of these had previously published redshifts.

INTRODUCTION

A soft X-ray excess below 2 keV is a common feature in the X-ray spectra of AGN (Turner & Pounds 1989; Masnou et al. 1992). A popular interpretation is that it represents the high energy tail of the big blue bump for which the models include accretion disks and reprocessing in cold matter. ROSAT provides us with the opportunity to study the parameters of the soft X-ray excess for the first time.

In this paper, we discuss new ROSAT PSPC spectra (covering 0.1-2.4 keV) of four AGN taken from the USS survey which selected the softest sources in the Einstein IPC database (Córdova et al. 1992; Puchnarewicz et al. 1992a, hereafter C92 and P92a): these AGN are E1346+266, E0845+378, E0844+377 and E2034−228.

E1346+266 – A HIGH REDSHIFT ULTRA-SOFT X-RAY AGN

Observationally, soft X-ray AGN are generally found at low redshifts; all of the soft excess AGN in the Turner & Pounds and Masnou et al. samples have redshifts below 0.2, leading to the suggestion that a redshift of 0.5 is sufficient to make any soft X-ray component undetectable (Masnou et al. 1992; P92a). This implies an upper limit to the effective temperature of the soft component in the rest-frame, providing an observational constraint on models for the big blue bump.

E1346+266 has a redshift of 0.92, much higher than the suggested z=0.5 cut-off, yet the ROSAT PSPC spectrum confirms earlier indications from the Einstein data that this object has a strong soft X-ray excess.

Abstract

It is probable that the nuclei of all active galaxies radiate anisotropically either due to intrinsic beaming and/or due to extrinsic causes such as shadowing. Observations of scattered light and fluorescently excited extended line emission can be used to map the radiation pattern. The best established of the AGN unification schemes appears to be that for powerful radio sources. High redshift radio galaxies reveal their quasar/blazar nuclei clearly in their rest-frame ultraviolet radiation. Studies of local objects show in detail the physical process which are operating in these distant sources.

Introduction

The concept of radiation anisotropy has been remarkably successful in allowing the unification of various classes of active galaxy and quasar. Whilst the possibility of explaining the differing appearance of all the classes of active extragalactic sources simply in terms of orientation effects seems unrealistic now, it does appear that broad subclasses can be collapsed in this way. Thus, for example, the three subgroups of Seyfert 1s and 2s, Fanaroff/Riley class one (FR I) radio galaxies and BL Lac objects, and powerful radio galaxies and quasars, can be understood as entities having similar central engines and nuclear environments, but whose orientation with respect to the observer is the dominant variable determining the detailed appearance.

Fundamental differences in the nature of the central energy source may well exist and, if so, will certainly break the unification at some level.

Introduction

It is widely believed (but not proven) that accretion of matter onto a supermassive black hole is the primary source of energy for active galactic nuclei. Features commonly observed in AGN spectra such as the width of the broad emission lines and the ultraviolet to soft X-ray excess have been attributed to the accretion process. Many attempts have been made to fit accretion disk model spectra to the continuum energy distribution (CED) of AGNs to refine and/or verify these models. Thin disk models have had some success in explaining the CED of Seyfert 1 galaxies and low redshift quasars (Malkan & Sargent 1982; Sun & Malkan 1989; Laor 1990) but these fits have been made only to the optical/ultraviolet part of the CED. For the accretion rates and the black hole masses expected in AGNs the disk emission is expected to peak in the extreme ultraviolet/soft X-ray region and observations close to this peak would put tighter constraints on the thin disk models. EUV observations of four AGNs are described in this paper.

The EUV Observations

ROSAT/WFC survey images have been analysed to identify EUV detections of Seyfert 1 galaxies and quasars. Four AGNs were detected (Table 1). The image of Q1821+64 may be a blend of the quasar and a nearby white dwarf K1-16 (Grauer & Bond 1984) which is 100” from the quasar and would not be resolved by WFC.

Abstract

Virtually all accretion disk models predict that QSOs observed from nearly edge-on should show extremely high equivalent-width emission lines. These are not seen. Either accretion disks must be significantly non-planar, or most edge-on QSOs must be concealed by an obscuring torus.

Model

If the UV-optical continuum emission of QSOs comes from an accretion disk, it will be emitted anisotropically. If in addition the line radiation is either isotropic, or anisotropic in a different way from the continuum radiation, then identical QSOs observed from different orientations will show different emission-line equivalent widths.

I assume that all QSOs have the same intrinsic line-to-continuum flux ratio, and that the line radiation is isotropic. Any magnitude-limited sample is strongly biased towards face-on QSOs, and this bias is taken into account using luminosity function information. A wide variety of both thick and thin disk models have been used.

Results

A typical predicted equivalent-width distribution is compared with an observed distribution in the figure. Both are taken from. Two discrepancies are evident. Firstly, the observed distribution has a broader, smoother peak than the prediction. This can easily be explained if there is an intrinsic dispersion in QSO equivalent-widths. Secondly, the model has a tail of very high equivalent-width QSOs not seen in the observations. This tail is significant at the 99% confidence level, for most accretion disk models, and for Lyman-α, C III] and Mg II as well as C IV.

Abstract

New results from multi-frequency monitoring campaigns of variable flat-spectrum radio sources are reported. They strengthen the assumption that the intraday variability occurs in a correlated fashion throughout the radio, optical and X-ray wavebands. Various properties of the behaviour exclude propagation effects as the dominant cause of the variations. This implies excessive brightness temperatures. A large fraction of the primary synchrotron radiation may be upscattered into the Gamma ray regime. We also discuss first results from polarization studies in different frequency regimes. At least the BL Lac object S5 0716+714 exhibits variations of the polarized flux which are correlated with variations of the total flux. Neither simple two component models nor “christmas-tree” scenarios of a large number of individual emitters seem able to explain the polarization data.

Introduction

Compact, flat-spectrum radio sources are well known to be variable at optical and radio frequencies. It was generally assumed that the typical timescales increase with wavelength (from hours at x-ray energies to days/weeks at optical wavelength and months/years at radio frequencies). During the last few years we have performed several simultaneous multi-frequency campaigns with high temporal resolution to study the nature of the fastest variations which probe the smallest regions in these AGN. Variability on timescales of hours to a few days were found to be common at optical and radio frequencies in a sample of bright radio-sources selected to have flat spectra in the 3-5 GHz regime.