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).

INTRODUCTION

Although kinematic dynamo models reveal some of the basic features of Solar and stellar magnetic fields, a fully satisfactory model must allow the dynamics to emerge as part of the solution of the governing system of equations. This has been attempted in a number of mean-field studies starting with Proctor (1977). It has been shown that solutions exist in which axisymmetric fields become saturated at a finite energy by the action of the macroscopic Lorentz force acting on the fluid.

The mean-field formalism is used in turbulent convection zones to parametrise the effects of the small scale dynamics on the magnetic field (Steenbeck et al. 1966). The influence of the small-scale turbulence on the macroscopic motions can be similarly modelled by the ‘Λ-effect’, representing the Reynolds stresses of anisotropic turbulence induced in a rotating, stratified medium (Rudiger 1989). The resulting mean-field equations describe the evolution of quantities averaged over time or length scales greater than those of the turbulence.

INTRODUCTION

Our galaxy and others are permeated by magnetic fields. They play an important role in star formation, in the support of molecular clouds against collapse, and in cosmic ray confinement. With a field strength of a few microgauss, they are comparable in in energy density to thermal energy, radiation, and cosmic rays. These fields are widely assumed to be the result of a dynamo operating on an initial seed field.

Dynamos work by folding magnetic field lines back on themselves constructively more often than destructively. Mean field theory assumes that the many folds in the field with no net contribution are destroyed, usually by resistivity. What would happen if these small disordered fields were not destroyed? They would obscure the growing large-scale field and might dominate the total magnetic energy. This is indeed a concern for galactic dynamo theory as magnetic loops 0.1 pc across need 1022 years to decay ohmically.

THE DYNAMO IN THE CONVECTION ZONE

In 1969, Steenbeck & Krause presented results of the first hydrodynamic dynamo model acting in the turbulent convection zone (CZ) and based on the idea of mean field electrodynamics. They introduced two spherical shells for the induction effects: in the inner, there is the differential rotation (Ω ∼ r) and in the outer, one has the turbulent rotating matter (α ∼ Cos v). This simple model is in agreement with most of the observed magnetic patterns, such as the butterfly diagram (Figure 1), H ale's polarity rule and the 22 year period of the Solar cycle.

During half a cycle, i.e. eleven years, the activity belts, as a measure of the toroidal field, move from about ±30° latitude towards the equator. In the vicinity of the pole, there are no active regions. But observations of torsional oscillations (Howard & LaBonte 1982) and Solar wind (Legrand & Simon 1991) suggest that the toroidal field starts the reversal of its polarity there (Schussler 1981).

INTRODUCTION

This paper is concerned with the effect of an imposed magnetic field on thermal convection. Although this is not directly relevant to dynamo theory, the question of the interaction of convection with magnetic fields is important for a full understanding of a convectively driven dynamo. The motivation for this work is to understand some aspects of convection in the Sun, particularly in regions of high magnetic field, such as sunspots, where the strong, predominantly vertical field inhibits thermal convection, causing the spot to appear dark. This work is part of an ongoing collaboration with Michael Proctor and Nigel Weiss. This brief report summarises some of the main results of this work; further details will be published in a future paper (Matthews, Proctor & Weiss 1993).

Linear theory for the onset of magnetoconvection in an incompressible fluid was discussed extensively by Chandrasekhar (1961).

INTRODUCTION

My objective in these five lectures was to introduce the fundamental physical ideas underlying cosmological models of an isotropic hot Big-Bang and the development of large-scale structure in such an expanding universe. My instructions from the Organizing Committee were to assume that the audience had not studied cosmology before, and that is what I have tried to do. My final lecture reviewed a number of results in infrared observational cosmology, but constraints of time and space have not permitted inclusion of that material here.

Much of the material on isotropic cosmological models is developed very well in any number of existing sources (although this is less so for the theory of galaxy formation) and it is with some diffidence that I offer my version. The references list a number of other developments of these topics, and I have borrowed from most of them. Perhaps the best recent text which covers these and many other topics in modern cosmology very thoroughly, and yet very readably, is that by Kolb and Turner (1990). My own research in infrared observational cosmology received an important stimulus from a series of similar lectures presented by Malcolm Longair (1977) over 15 years ago. I hope these lectures might, in a similar way, give young astronomers an introduction which will generate enthusiasm for inventing new infrared observing programs bearing on fundamental cosmological problems.

THE ISOTROPIC UNIVERSE

Introduction

We begin by taking a very large-scale view of the universe, and make a simplicity approximation that the universe is smooth, with no structure. In this approximation we can think of the universe as a fluid (of galaxies) of density ρ, pressure p.

A hotly debated topic is the origin of the large scale galactic magnetic field. Originally, it was supposed by Fermi and others that the field had a primordial origin and was maintained against Ohmic decay by the large inductance of the galactic disk. (The time scale for Ohmic decay by ordinary Spitzer resistivity is extremely long, of order 1026 years.) However, there have been several objections to the primordial theory (Parker 1979). One objection is that turbulent resistivity is sufficiently large to destroy the field in a Hubble time. A second objection is that if it is not destroyed by turbulent resistivity, it can escape from the galactic disc by ambipolar diffusion. Probably the strongest objection has been that there seems no known way to produce a magnetic field in the early universe on a large enough scale and of sufficient strength to provide a primordial origin.

Fast dynamo instabilities are the subject of intense research (reviewed in Childress 1992), through numerical simulations and analytical studies of simple models. However little is known about how a fast dynamo instability might saturate and what the resulting spatial structure and temporal behaviour of the field might be. Does a fast dynamo saturate by suppressing the flow field until the effective magnetic Reynolds number is reduced to a value of order unity or by modifying transport effects of the flow (Vainshtein et al. 1993)? Is the saturated magnetic energy in equipartition with the kinetic energy and how is the magnetic energy distributed; in particular how much energy is stored in large-scale field components (Vainshtein & Cattaneo 1992)? Does the field contain the fine structure typical of kinematic fast dynamo instabilities and is it intermittent in time? The difficulty in answering these questions is that dynamo action only allows growth of 3-d magnetic fields and through the Lorentz force this leads to all the complexities of 3-d MHD turbulence. Numerical studies are computationally expensive and only moderate values of Rm have been achieved (see, for example, Gilman 1983, Glatzmaier 1985, Meneguzzi & Pouquet 1989, Nordlund et al 1991 and Galanti et al 1992).