Abstract

Much effort has gone into analysing the now well-known luminosity evolution of the quasar population. Theoretical models, such as the accretion disk model, however, predict a spectral evolution which is more complicated than a simple overall luminosity shift [3], and more detailed spectral information is necessary to compare physical models with the data. We investigate the distribution and evolution of quasars on the color-flux plane in order to test for consistency with the thin accretion disk model.

Introduction

Much effort has gone into analysing the now well-known luminosity evolution of the quasar population. Theoretical models, such as the accretion disk model, however, predict a spectral evolution which is more complicated than a simple overall luminosity shift [3], and more detailed spectral information is necessary to compare physical models with the data. We investigate the distribution and evolution of quasars on the colour-flux plane in order to test for consistency with the thin accretion disk model.

Quasar Colours

It has been known since the first quasar surveys that active galactic nuclei (AGN) occupy a specific region of the two colour diagram, separate from most other galactic and extragalactic sources. Consequently, UBV photometry has been used to select AGN in numerous surveys up to a redshift z ∼ 2.2 where the Lα emission line and absorption edge are redshifted into the observed bands.

Abstract

A technique is presented to deconvolve variable Seyfert 1 profiles that consist of components that differ in radial velocity coverage and time scale of variability.

Introduction

Variable structure in the emission-line profiles of AGN can in principle be used to map out the kinematics of the broad-line region (BLR). However, profile variations can also occur if the profile consists of more than one component and each component is variable in flux on a different time scale (Peterson et al. 1990). In order to use profile variations as a probe of the BLR, it is important to devise techniques to deconvolve components that may represent physically distinct emission-line regions. One possible technique will be outlined using data obtained from the 1989 campaign to monitor the Seyfert 1 galaxy NGC 5548 with the International Ultraviolet Explorer (IUE), as well as IUE spectra obtained by Webb and Crenshaw in April, 1990, when NGC 5548 was in a particularly low state.

Figure 1 shows that the profile of C IV λ1550 is much narrower in the low state, suggesting that there is a strong narrow component that has not varied as much as the rest of the profile. In addition, it is clear that excess emission in the blue wing, which Peterson et al. (1990) claim is a distinct component that varies on a longer timescale than the majority of the broad-line flux, has nearly disappeared.

Recently some interesting spectral features, such as an Fe line-edge system and a hard X-ray bump, have been observed in many Seyfert nuclei (e.g. Nandra et al., 1991, hereafter NPS91). These observed characteristics are naturally explained by the ‘two-component’ model which consists of a non-thermal hot plasma and a ‘cold’ plasma which reprocesses and reflects the non-thermal radiation. The presence of such cold matter in the vicinity of the central region is a natural consequence of the accretion model of active galactic nuclei (AGN) (e.g. Guilbert and Rees 1988, hereafter GR88). It is also supported by X-ray observations of many Seyfert nuclei by the EXOSAT and Ginga missions (e.g. NPS91, and references therein), for instance, the simultaneous rapid variations of the X-ray continuum and Fe line fluxes (Kunieda et al., 1990). The cold component is envisioned to be in a form of either a thin disc (or slab) or an assembly of clouds distributed over a three-dimensional configuration (GR88; Celotti, Fabian and Rees 1992, hereafter CFR92). For convenience, we shall call the former kind ‘the disc model’ and the latter ‘the cloud model’.

Here a model is presented which assumes the existence of ‘cold’ dense clouds near the central engine of AGN. In order for the cold clouds to exist near the central engine they must be sufficiently dense, and some physical pressure is required to confine them. This pressure may be supplied by equipartition magnetic fields (Rees 1987, CFR92).

Introduction

The Autocorrelation Function (ACF) of the intensity fluctuations of the X-ray background (XRB) is an integrated view of the clustering properties of the source populations contributing to the XRB, so studies of the ACF provide information on the origin of the XRB as well as on clustering evolution of the underlying sources.

Recently ROSAT Deep Surveys have shown that about 40% of the soft XRB (E ≤ 3keV, henceforth SXRB) is contributed by sources brighter than S(0.5–2keV) ≥ 7 × 10−15 erg s−1 cm−2 (Hasinger et al. 1991; Shanks et al. 1991). On the other hand only a few percent of the hard XRB (3 < E < 60keV, henceforth HXRB) has already been resolved into sources. ROSAT observations also show that the SXRB is actually very smooth on arcminute scales with a current upper limit on the ACF in the 0.9–2.4keV band W(9′) ≤ 2 × 10−3 (Hasinger 1992). As regards the HXRB, the results of the ACF analyses on sub-degree angular scales have been used to put significant constraints on clustering of AGNs and galaxy clusters (Martín-Mirones et al. 1991; Carrera et al. 1992). Here we will discuss data on ACFs to obtain limits on clustering, clustering evolution and volume emissivity of X-ray sources contributing to the SXRB and HXRB.

Constraints on the clustering scale and the clustering evolution of AGNs and on their contribution to the XRB

Recent analyses of large samples of optically selected QSOs have produced consistent values of the clustering scale, 12 ≲ r0 ≲ 20 Mpc (H0 = 50kms−1 Mpc−1), and of the evolution of the correlation function with ∈ ≥ −1.2, a constant comoving clustering scale being slightly favoured (Boyle 1991; Andreani & Cristiani 1992).

We present the results from a detailed analysis of the X-ray spectral variability of Mkn 841 based on EXOSAT, Ginga and ROSAT observations over the period 1984 June to 1990 July.

Variability is apparent in both the soft (0.1–1.0 keV) and medium (1–20 keV) energy bands (Fig. 1). Above 1keV, the spectra are adequately modelled by a power-law with a strong emission line (of equivalent width ∼ 450 eV). The energy of the line (∼ 6.4 keV) is indicative of K-shell fluorescence from neutral iron, leading to the interpretation that the line arises via X-ray illumination of cold material within the accretion flow. In addition to the flux variability, the continuum shape also changes in a dramatic fashion, with variations in the apparent photon index ΔГ ∼ 0.6.

The large equivalent width of the emission line suggests an enhanced reflection component in this source, compared to other Seyferts observed with Ginga. The spectral changes are interpreted in terms of variable power-law continuum superimposed on the flatter refection component. For one Ginga observation, the reflected flux appears to dominate the medium energy X-ray emission, resulting in an unusually flat slope (Г ∼ 1.0).

The soft X-ray excess reported by Arnaud et al. (1987), is found to be highly variable by a factor ∼ 10. No evidence is found for an intrinsic column density nH ≲ few 1020 cm−2.

Abstract

We present the results from truly simultaneous UBVRI photopolarimetric observations on “normal” quasars at the Nordic Optical Telescope. Some of the observed quasars show no measurable polarization, this being consistent with previous measurements of these objects. On the other hand in PG 1008+133 and PG 1351+64 we detected low (1.5%) polarization. In PG1351+64 this polarization shows also wavelength dependence and in PG 1008+133 the position angle show wavelength dependence. These are the first such observations in these quasars.

Observations

We have selected quasars from the Veron-Cetty & Veron (1991) catalogue. The only selection criteria being: the object is bright enough to be observed in reasonable integration time. In practice this means that Vmag < 16. And that the object can be observed from La Palma, using the Nordic Optical Telescope (Declination > −30°).

The observations were made at the Nordic Optical Telescope during February 1991 and July 1992. We used the Turku photopolarimeter. This instrument gives truly simultaneous results in five (UBVRI) colour bands. A detailed description of the instrument and the observing method can be found from Takalo et al. (1992, and references therein). The total integration time depended on the object brightness, ranging from one to four hours.

Results

The main results are listed in Table 1., where we list the observed objects according to the measured polarization.

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).