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.

Abstract

A set of 11 contemporaneous IUE and GINGA observations of NGC 5548 reveal the existence of correlated variations of its hard X-ray (2–10 keV) and Ultraviolet (1200−3300 Å) flux over time scales of 2 days to one year. This is best explained in the framework of a model where the X-rays irradiate a cold (T ∼ 105K) accretion disk. Only a tiny fraction of the irradiating flux is compton reflected back to the observer in the form of a hard X-ray tail while the bulk of the X-rays are absorbed in the disk and eventually re-emitted as thermal radiation in the Ultraviolet. The absence of a detectable phase delay between the two bands together with the absence of rapid (∼ hours) fluctuations of the UV flux further constrain the X-ray source to lie between 200 and 1400 Schwarzschild radii above the disk. The thermal reprocessing model provides a natural explanation for the simultaneity of the optical and UV variations in NGC 5548 and may solve most of the problems facing the accretion disk model.

Introduction

The presence of a strong Iron Kα line near 6.4 keV and a “hard tail” above 10 keV is a common property of the X-ray spectrum of Seyfert 1 galaxies [1]. This has been interpreted as evidence for reprocessing of the X-rays by a “cold” (T ∼ 105 K) accretion disk [2]. In this model, less than 10% of the irradiating photons are compton scattered back to the observer forming a spectral hump centred near 15keV while the bulk of the X-rays are absorbed in the disk.

Introduction

BL Lacertae objects have been characterized by rapid and large amplitude optical variability, by a highly variable and polarized optical continuum which is featureless, or one in which any discrete features are found only in low contrast to the continuum. In the present investigation, the BL Lacertae object with the most significant galaxy component detected to date, PKS 2201+044, has been studied. The purpose of the investigation is to present the results of eighteen years of photometric monitoring of this BL Lacertae galaxy.

Observations

The observations of PKS 2201+044 were obtained with the 0.9 m and 1.3 m telescopes at KPNO and the 42-in. telescope at Lowell Observatory Observatory, all of which were equipped with a direct CCD camera. The details of the observations, data reduction, and analysis are the same as those described by Noble and Miller (this volume).

Discussion

Multiple aperture photoelectric and CCD observations were obtained on several nights. These were used to derive an aperture correction using the method outlined by Sandage (1973), which was applied to all the observations to derive V*, the V-magnitude in a standard aperture of 15.42 arcseconds. Over the eighteen-year period of the observations, we see a general increase in brightness reaching a maximum in 1987, followed by a decline until the fall, 1991. A major exception to this is the observation of 1981 June 7 when the object was observed at V = 16.31. PKS 2201+044 was also observed at a similar brightness of V = 16.43 on 1987 November 9.

Abstract

We present early results from the UK ROSAT Deep and Extended Deep Surveys. A total of 240 faint X-ray sources have been detected, most of which are expected to be QSOs and Seyfert galaxies at redshifts z < 3, although normal galaxies and starburst galaxies are also present. We will use these surveys, together with our parallel VLA 20cm & 6cm radio surveys and multicolour optical CCD surveys, to determine the evolution of the faint end of the X-ray and optical luminosity functions (LF) of QSOs, study the multiwaveband emission mechanisms of QSOs, map their distribution over a ‘wedge’ of high redshift sky, and investigate the X-ray evolution of distant clusters of galaxies.

The Multiwaveband Surveys.

The ROSAT survey was performed in a region of high-latitude sky of very low, and uniform, Galactic column density (71019 cm−2), as determined by our 21cm and IRAS 100µm measurements. The deep survey reaches a limiting X-ray flux of 410−15 erg cm−2s−1 (0.5–2keV) over a 40 arcmin diameter region of sky and contains 96 faint X-ray sources. The extended survey stretches over a 4° × 40 arcmin strip starting from the position of the deep survey, with a limiting flux of 10−14 erg cm−2s−1 (0.5–2 keV).

Deep VLA radio maps at 20cm (and at 6cm in the deep survey area only) have been constructed to flux limits of 0.5 mJy on the deep survey field and 2mJy on the extended survey.

INTRODUCTION

In the last few years there has been a renewed interest in dynamos in rapidly rotating systems, of which the Geodynamo is the most important example. In these systems the inertial and viscous terms in the fluid momentum equations can be considered asymptotically small and one is led to consider the role played by Taylor's constraint (Jones 1991; Soward 1992). Numerical calculations of such dynamos have been carried out by solving the axisymmetric magnetic induction equations for the toroidal and poloidal magnetic field components under prescribed α and ω effects. A variety of models and geometries have been explored. The studies which are more closely related to the present work are the calculations of Abdel-Aziz & Jones (1988) and Jones & Wallace (1992) in planar geometry, of Hollerbach h Ierley (1991) and Hollerbach, Barenghi & Jones (1992) in a sphere, and of Barenghi & Jones (1991) and Barenghi (1992a,b) in a spherical shell.

The observed westward drift of some patches of the Earth's magnetic field suggests that in order to model the Geodynamo one should study the magnetic induction equation in the ato limit (Roberts 1988).