Introduction

X-ray emission provides ∼ 10% of the bolometric luminosity of a typical Seyfert galaxy and as X-ray photons of energy > 2 keV can penetrate column densities of > 1 × 1022 atoms cm−2, such observations provide the best observational probe available (with current instrumentation) of the active nucleus, and its immediate environment.

Variations cannot be observed faster than the light travel time, thus δt ∼ r/c gives an upper limit to the size of the emitting region. A variability timescale may be associated with the dynamical (orbital) timescale of the inner accretion disk. If the X-rays arise from r ∼ 5rs and rs = 2GM/c2 then t ∼ 50M6S (M6 are units of 106 solar mass). This is reasonable if a persistent period were found, but could be misleading otherwise.

Two other parameters of interest are efficiency η (of mass to energy conversion) and compactness ℓ. Efficiency, η ∼ 5 × 10−43dL/dt, if η > 0.1 an exotic mechanism is required (such as relativistic beaming). Compactness, ℓ = Lστ/Rmec3 (Svensson 1986; Guilbert, Fabian & Rees 1983). When ℓ > 10 the source becomes optically thick to γ-rays and pair production becomes important (assuming the spectrum extends to ∼> 1MeV), affecting the X-ray spectrum and the temporal behaviour (e.g., Mosalik & Sikora 1986; Fabian et al. 1986).

Historical

Ariel V and HEAO-1 established long term (days to years) variability as a property of AGN (e.g., Marshall, Warwick & Pounds 1981), with variations in amplitude of factors of ∼ 7 or so being common.

The high sensitivity and improved spectral resolution (ΔE/E = 0.4 at 1 keV) of ROSAT has enabled high quality spectra to be obtained in the soft X-ray (0.1−2 keV) band. When added to simultaneous Ginga data, spectra extending up to 20 keV are achieved.

3C273 was observed by ROSAT in a pointed observation in June 1990 and again in December 1990 during the survey, when it was also observed by Ginga. The December observation yielded a 3σ detection in the SI filter of the Wide Field Camera. E1821+643, benefits from being at a high galactic latitude and was observed for a total of 40 days during the ROSAT survey, with an effective exposure time of 9ks. Ginga observed the source during this period. It was also observed serendipitously during the pointed phase in February 1991.

Both sources need a two component model to fit their combined 0.1−20 keV spectrum (see Tables 1 & 2). The spectrum of 3C273 has a significant soft excess which exists below 1.2 keV (source rest frame). This may be adequately parameterised by a 200eV Bremsstrahlung or a power law of α = 3.4, however, a single temperature Blackbody or Raymond-Smith model is not a good fit to the data, neither are elementary disc models.

E1821+643 has a strong, steep soft excess which rises below 0.65 keV (source rest frame) in both ROSAT observations.

Abstract

Recent work has revealed that a number of Seyfert 2 galaxies exhibit conicallyshaped regions of gas apparently illuminated by a collimated, nuclear ionizing source. In this work, we test one model for this collimation, namely that the cones result from shadowing of a compact nuclear continuum source by a thick, dusty torus. From the emission-line ratios measured for gas within the cones, we have calculated the number of ionizing photons emitted by the compact nucleus. Then, on the assumption that the nuclear source radiates isotropically, we have found the power incident on the torus, which is expected to be reradiated in the infrared. Given the uncertainties in the calculation, and the fact that the torus may be somewhat anisotropic in the infrared, we find the observed IRAS luminosities are consistent with the torus model in 9 objects with sufficient data to perform the calculation.

Introduction

There is now considerable evidence that the ionizing photons escape from the nuclear source anisotropically in many Seyfert 2 galaxies (see Wilson 1992 for a recent review). For some objects, this anisotropy becomes evident in the elongated morphologies detected in images taken through narrow-band filters centred in high excitation lines (e.g. [OIII]λ5007) or through “ionization maps” (Pogge 1988a,b) – ratios between the continuum subtracted images in [OIII]λ5007 and in Hα+ [NII]λλ6548,6583.

In this work, we combine new with existing data for a sample of nine Seyfert 2 galaxies with known “ionization cones” in order to test whether the collimation is the result of shadowing of radiation from a small, isotropic, nuclear source by a thick dusty torus, as suggested in the “Unified Scheme” for Seyfert galaxies (Antonucci & Miller 1985; Krolik & Begelman 1986).

Abstract

The structure and emission properties of accretion discs are briefly reviewed, with special attention given to geometrically thin “standard” accretion discs. Different approximations used to compute their emission spectrum are summarized, and it is shown that the bulk of the luminosity is emitted in the EUV range (the “UV bump”). However, according to recent optical, UV and X–ray monitoring campaigns of Seyfert galaxies, it appears that a large fraction of the observed optical–UV and even X–ray continuum is reprocessed by the disc from a primary source, probably of hard X–ray continuum, sitting near the black hole and heating the farther regions of the disc. A first consequence is that the disc must also contribute some fraction of the line emission. A second consequence is that standard accretion discs drawing their energy from viscosity and radiating it locally are ruled out in AGN, and another type of model must be built.

Generalities on accretion discs

For the reader who wishes to know more than just the basic properties that are recalled below, several reviews on accretion discs can be recommended: Pringle (1981), Frank, King & Raine (1992; revised version), Begelman (1985), Shields (1990), Treves, Maraschi & Abramovicz (1988).

If the matter accreted onto the black hole possesses angular momentum, it will settle in an accretion disc, in which matter is transported inward and angular momentum outward. This may be achieved through viscous friction, or through other stresses such as produced by a magnetic field.

Abstract

We have previously reported evidence for radio-luminosity dependence of the strength of the near-infrared ‘alignment effect’ in radio galaxies (Dunlop & Peacock 1991). Here we present evidence for an associated radio-luminosity dependence in the level of optical activity found in radio galaxies at z ≃ 1. We find that this correlation, the strength of which is maximised by considering a combination of radio power and spectral index, is very similar and probably closely related to the correlation between radio-jet power and LNLR found by Rawlings & Saunders (1991); the available evidence suggests that both correlations in fact arise from an underlying correlation with environment.

These correlations along with (i) the universal shape of the supposedly stellar UV-continuum in powerful high-redshift radio galaxies; (ii) the detections of significant optical/UV polarisation in several 3CR radio galaxies; (iii) the inaccuracy of the optical-radio alignments; (iv) the close spatial correspondence between the extended UV continuum and line emission; and (v) the correlation between radio-lobe depolarisation and extended optical emission, indicate that a large fraction of the optical/UV activity and the optical alignment effect in the 3CR sample is the result of Thomson scattering of a ‘flat’ (fv ∝ v−0.2) quasar continuum emitted within a broad cone centred on the radio axis.

To investigate the relationship between the optical/UV activity in high-z radio galaxies and their radio properties, we have considered a composite sample of radio galaxies with z ≃ 1 which spans a decade in radio luminosity.

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.