Abstract

Recent observations with ROSAT have revealed an absorption feature in the spectra of some active galaxies, which is associated with the K–edge of highly ionized oxygen. This confirms that a large column of partially ionized material, the so–called warm absorber, which had been inferred from previous X–ray observations at higher energies, is present in the line–of–sight.

Introduction

X–ray evidence for highly ionized material in the line–of–sight to AGN has been accumulating over the past few years. Halpern (1984) originally suggested that highly ionized gas might be responsible for the soft X–ray absorption observed in some AGN, and cause spectral variations, as the opacity of the material changes with the ionizing flux. Further evidence was found using EXOSAT with many workers presenting evidence for flux–correlated changes in the soft X–ray absorption (e.g., Pan et al. 1990). However, given the widely recognized complexity of Seyfert spectra and the many candidate processes for producing the spectral changes, such observations were inconclusive. Subsequently, analysis of the Ginga spectra showed evidence for absorption at the iron–K edge, at an energy apparently higher than that expected from cold iron at 7.1 keV, in ∼ 50% of a sample of Seyfert galaxies. Nandra (1991) found a mean energy ∼ 8 – 8.5 keV, depending on the assumed continuum model. Again, however, it was difficult to establish the ionization state, or even the significance of the edge feature, in many individual cases, given the modest resolution of the LAC detector.

Abstract

Various observational tests of unified schemes for radio sources are reviewed critically. It is shown that, although some of the results might raise doubts, there exists no definitive evidence against these schemes.

Introduction

There is now considerable evidence to support the idea that the radiation from active galactic nuclei (AGN) is emitted anisotropically. For radio-loud objects much of the evidence has been gathered at radio wavelengths, and includes the measurements of superluminal motions in radio cores (e.g. Zensus 1988), the statistics of core and lobe dominated radio sources (e.g. Orr & Browne 1982), and the polarization asymmetry in the extended radio structures (e.g. Garrington et al. 1988, Laing 1988). For the radio-quiet objects the beautiful emission line images of radiation cones (e.g. Pogge 1989, Tadhunter & Tsvetanov 1989), and the detection of scattered broad lines within the extended regions (e.g. Miller et al. 1991), are also very strong evidence in favour of anisotropy.

These observations are comforting for the proponents of unified schemes who use anisotropy to explain the relationships between certain classes of AGN in terms of orientation effects. However, not all of the evidence is so positive, and in this paper I review some recent results which are less supportive of unified schemes.

Types of unified schemes

There is not just one unified scheme based on anisotropy/orientation effects, but several, and it is important to be clear about what we are discussing.

Abstract

The observational properties of knots in jets of two high-z quasars are presented and discussed. The knots along 3C9 and 3C309.1 jets have similar characteristics, suggesting that a common process may be responsible for knot formation. The knot's spectral indices tend to be flat at regions of disturbed flow and may be explained by particle re-acceleration due to small-scale shocks.

Introduction

Unresolved or partially resolved regions of enhanced synchrotron emission are often found in jets of extragalactic radio sources.

The VLA has provided maps of kiloparsec-scale jets with multiple knots of arcsecond dimensions in nearby FR I sources like M87 (z=0.0043) [1] and NGC6251 (z=0.023) [2] and provided valuable information about the physics of energy transports in jets.

A considerable amount of theoretical work has been done to explain the nature of such compact structures. However, most of the published data of high-z objects show unresolved knots along their jets. Spectral index information is difficult to obtain for sources with small angular sizes (< 30″), due to practical limitations. This is most unfortunate for distant sources since only knots are bright enough to be detected within low-brightness collimated flows.

This work is an attempt to give an interpretation of MERLIN, VLA and EVN observational data at 6 and 18 cm for two high-redshift quasars with bent and knotty jets, and to describe their radio properties. In calculating distances Ho = 75 km s−1Mpc−1 and qo = 0.5 are adopted.

Abstract

We present spectropolarimetric observations of the quasars 3C273, CTA102 and 3C345. A synchrotron origin is the most likely explanation for the polarization of 3C273. The quasar CTA102 may have strongly polarized FeII lines and, along with 3C345, some minima in the polarization are alongside the red wing of the MgII and CIII] lines.

Introduction

An often debated problem is that of the variability and polarization of the emission lines in AGNs. Despite the work done on Seyfert-2 and radio galaxies (e.g., McLean et al. 1983; Antonucci & Miller 1985; Antonucci 1992; Jackson in this meeting) and on quasars (Goodrich & Miller 1988), which demonstrates that the lines are usually polarized, the lack of appropriate instruments has severely limited the number of research groups dedicated to this important task.

The aims of our observations were, first, to get a spectropolarimetric sample of quasars. The objects are chosen for a) their brightness, brighter than 17 mag, b) their polarization, larger than 3%, and c) their known variability. Besides these, some other objects, like 3C273, of which polarization has never been reported to be larger than 2.5%, were also included in the sample. The second aim is to see if the emission lines are polarized and search for possible variability. Comparing lines and their continuum polarization and variability we should find new and powerful parameters for studying the actual models for quasars and for the origin of their polarized radiation.

Abstract

The correlation observed in NGC4151 between the O, UV and X-ray fluxes is explained in terms of reprocessing of hard X-rays by a thick disk that reradiates the incoming energy into O and UV photons. The flatness of the UV spectrum and the upper limits on X-ray reprocessed components (high energy bump, variable part of iron line) demand tight limits on the mass of the central object, the luminosity (absolute and relative to the Eddington one) and the extension of the spectrum in the γ-ray region.

Introduction

Reprocessing of X-rays by a thick medium has been called for to explain two X-ray features observed in several Seyfert galaxies: the iron line and the high energy bump. The fact that reprocessing of hard X-rays may play an important role also in the optical and UV, an idea firstly advanced in, has been recently proposed to account for the short time-scale correlation of optical and UV light curves, too short to be explained by processes directly connected to accretion by a disk.

Similar considerations apply to the case of NGC4151, where the optical, UV and X-ray fluxes are correlated down to a time scale of 1 l.d. - although the correlation between UV and X-ray breaks at higher UV luminosities (we will comment on this behaviour in the following). In this object, however, the absence of an high energy bump as well as of a broad and variable iron line apparently argues against the presence of a thick reprocessor near the central source.

The Compton reflection of X-rays by low temperature electrons has received much attention in the recent past. It is a useful framework for the explanation of some features of the spectra of AGNs and a basic ingredient for understanding the X-ray background spectrum, together with the cosmological evolution of AGN. Since the basic works of Illarionov et al. (1979) and of Lightman et al. (1981), principally based on a semi-analytical approach to the problem, many authors have developed careful Monte Carlo methods to include the effects of the Klein-Nishina cross-section and to extend the study up to the hard X- and γ-ray regions. However, the semianalytical approach has some useful advantages. Firstly it allows us to obtain directly the reflected spectrum in the energy region and at the inclination angle of interest; in addition it may be versatile enough to link with other methods for the computation of the spectrum in the UV and IR regions, where other physical processes are important, but where the X-rays reprocessed by thermal matter may be an important part of the overall luminosity.

The basic assumption of this approach is the separability of the spatial and energy transport problems and of the absorption due to bound-free transitions.

Abstract

One zone chemical evolution models are developed for application to QSO broad emission line regions. The elemental abundances derived from the broad line ratios imply that the gas is highly evolved, with metallicities ranging from ∼1 to ≳ 10 times solar. The short timescales (i.e. ≲ 1 Gyr if qo ≈ ½) and relatively flat initial mass functions (compared to the solar neighbourhood) needed to fit most of the high redshift line ratios are almost identical to the parameters used in one zone models of elliptical galaxies. We conclude that the QSO phenomenon generally follows an episode of rapid star formation, exactly like that expected in massive, young galactic nuclei.

An observed trend in the emission line data suggesting higher metallicities at high redshifts could result from a mass–metallicity–redshift relation among the QSOs. Thus the highest mass QSOs (and/or host galaxies) might form only at high redshifts (e.g. z > 2).

Introduction

The broad emission line spectra of QSOs show that heavy elements are present at redshifts up to nearly z ∼ 5. Therefore some amount of star formation must have occurred before the QSOs ‘turned on’. Unfortunately, the line strengths are not indicative of the overall metal content of the gas, but some of the line ratios are sensitive to the relative abundances (see Ferland & Hamann this volume). The relative abundances can in turn be used to constrain both the metallicity and the past evolution because the elements form by different processes and on different timescales; cf. [2].

Abstract

Two Seyfert 1 galaxies of comparable intrinsic luminosity, NGC 5548 and Markarian 279, have varied by a factor 2 in the optical band within a period of 5 years (1985–1990) during which both have been frequently observed. The large amplitude of the long-term variations reveals evidence that the profile of the broad emission lines depends on the luminosity of the line itself, in the sense that the asymmetry of the lines is stronger when the objects are brighter. This indicates that the structure of the broad line region in these two sources is stable on time scales of 5 years, and that their transfer functions are illumination-dependent in at least part of the radial velocity space. The effect, however, is absent in other, lower luminosity Seyfert 1 galaxies.

Introduction

The technique of reverberation mapping described by Blandford & McKee (1982) has been applied successfully in recent years to derive the transfer function (TF) of the broad line region (BLR) of well-monitored Seyfert 1 galaxies (e.g. Krolik et al. 1991). Until now the common assumption has been that the shape of the TF does not depend on the illumination of the BLR: this implies that if the continuum of a source were constant, the resulting broad lines would have the same profile irrespective of their intensity. The data presented in this paper, however, indicate that the broad lines of two Seyfert 1 galaxies, Markarian (Mkn) 279 and NGC 5548, display different characteristic profiles at different intensities, and therefore that the respective TFs have illumination-dependent shapes in at least part of the radial velocity range.

Abstract

We present results from an analysis of Hα, [OIII] images and long slit spectroscopy of the Seyfert 2 galaxy NGC5953. These show that the nucleus is extended in the northeast direction and surrounded by a vigorous burst of recent star formation.

Introduction

Some interacting Seyfert galaxies show strong circumnuclear emission associated with recent star formation, and one good example of these objects is NGC 5953. This Sa galaxy has a Seyfert 2 nucleus and is in interaction with the late type Scd galaxy NGC 5954 which has a LINER type nucleus, shows distorted spiral arms and is located 44″ to the northeast of NGC 5953.

The radio continuum map at 1.4 GHz shows a diffuse structure over the main body of the galaxy and an enhanced extended component NE of the nucleus. There is a local maximum at p.a. 90°, 5” from the nucleus (Jenkins 1984); this component has been identified with a supergiant HII region (Rafanelli et al., 1990)

We present narrow band images in Hα, [OIII]λ5007 and the nearby continuum, that were obtained with the 4.2m William Herschel Telescope, using the Taurus II box in imaging mode with the f/4 camera. The spatial resolution was 0.27″/pixel. In addition we obtained long-slit spectroscopy with the IDS spectrograph at the 2.5m Isaac Newton Telescope. We used an EEV CCD detector with a spatial resolution of 0.6″/pixel, covering the spectral regions 3300–5200 Å and 5000–6900 Å. A slit of width 1.5″ was oriented in the position angle 44°.

Abstract

We apply the binary black hole model to explain the wiggles in the milliarcsec radio jet of the superluminal quasar 1928+738 (4C73.18) observed with VLBI at 1.3 cm wavelength by Hummel et al. (1992). The period and amplitude of the wiggles can be explained as due to the orbital motion of a binary black hole with mass of order 108 solar masses, mass ratio larger than 0.1 and orbital radius ∼ 1016 cm. The jet's inclination to the line of sight should be small, confirming the standard interpretation of superluminal motion and one-sidedness as due to relativistic motion in a direction close to the line of sight. The small orbital radius suggests that the binary has been losing a significant amount of orbital energy during the last 107 years, possibly by interaction with the matter which is flowing through the active galactic nucleus.

Introduction

Galaxy mergers must have been a common phenomenon especially during the collapse and virialisation of rich groups and clusters of galaxies. These mergers lead to the formation of massive binary black holes in galactic nuclei if black holes of 107−9M⊙ are formed in the nuclei of most bright galaxies at redshifts of about 2. A massive binary black hole (MBBH) may manifest itself by Lens-Thirring precession of a jet emitted along the spin axis of one of the holes (Begelman et al. 1980, hereafter BBR).

Abstract

We present evidence for changes in the transfer function of the broad line region (BLR) in NGC 3516 on time scales of a couple of months. If this occurs in most AGN, it would mean a serious complicating factor for mapping the BLR by reverberation methods. Furthermore, the Hβ profile in this object shows a time-variable dip at the same velocity shift as the strong absorption in CIV λ1550. A corresponding feature is not present in Hα. The question addressed is whether the Hβ dip is absorption or a dip between two emission peaks. These results are part of the LAG project: during the first five months in 1990, a sample of 6 Seyfert-1 galaxies and 2 QSO's were monitored spectroscopically and photometrically. Significant line-profile changes were found in the lower luminosity objects in the sample. The higher luminosity objects displayed continuum variations only.

The time variable transfer function in NGC 3516

In this section we will show that the transfer function of the BLR in NGC 3516 changed significantly over a time scale of several months. This observation could have a profound impact on the use of reverberation mapping as a means to map the structure of the BLR, because it is one of the basic assumptions of reverberation mapping that the transfer function does not change during the experiment.

Abstract

We review the evidence that ejected radio material plays a fundamental role in the formation and kinematics of the Narrow Line region and the extended emission line regions associated with radio jets in radio galaxies and QSO's. In Seyfert galaxies, the key observation is the existence of high-velocity (several hundred km s−1 from the systemic velocity of the galaxy) emission line components which are found systematically closer to the nucleus that the radio emission peaks. We describe how this result can be explained with a high speed bowshock model. In radio galaxies, the strong shock created by the jet results in a surrounding hot cocoon of gas expanding away from the jet axis. These expanding cocoons are visible in the form of double velocity structure in high resolution optical spectra and have now been detected in 3C120, 3C 171, 3C405 and 3C265. The velocity separation between the components can be as high as several thousand km s−1 We briefly discuss how these cocoons can be used to verify the relativisic beaming hypothesis in systems with strong one-sided jets.

Introduction

Extended emission line regions (EELR) closely aligned with the radio structure have been found in Seyferts [20] and radio galaxies at both low [3] and high redshift [10]. The physical conditions and kinematics of these EELR provide a probe of both the radiation field of the AGN and the role played by ejected material in exciting the emission [13].

Abstract

Magnetic forces seem likely to play the dominant role in both the acceleration and initial collimation of relativistic jets in AGNs. I describe recent developments in the theory of hydromagnetic jets and winds.

Introduction

Hydromagnetic propulsion as a mechanism for accelerating jets has become attractive largely through a process of elimination. Other mechanisms, such as acceleration by gas or radiation pressure, have been examined and found inadequate. The observational case for relativistic flow velocities, on both pc and kpc scales, continues to mount (see, e.g., and for recent reviews). Models involving acceleration by radiation pressure would have to be stretched to extremes in order to account for the Lorentz factors ∼ 2 – 10 which are needed to explain most instances of onesidedness and superluminal motion. Losses due to catastrophic cooling place even more severe constraints on acceleration by gas pressure. Recent observations of “intra-day” radio variability, may require Lorentz factors as high as ∼ 100 if catastrophic Compton losses are to be avoided.

Magmetic propulsion has other attractive features besides the ability to produce the high speeds indicated by observations. Chief among these is the tendency of magnetically driven flows to “self-collimate” due to the development of a predominantly toroidal magnetic field. Thus, it may not be necessary to invoke a “funnel” or external confining medium to explain the collimation of jets.