We have observed a number of blazars at wavebands ranging from radio to γ-ray. We find bright γ-ray emission to be associated with strong synchrotron flares observed at lower frequencies. The X-ray flux and entire radio spectrum of 4C 39.25 have each increased in strength by 30% over a 2-year period, in agreement with the prediction of the bent relativistic jet model.

The nature of the successful reverberation campaigns on variable active galaxies is summarized. A general summary of the completed campaigns is given. A discussion of the results of the first order (CCF) analysis is given and some comments are made on the directions needed to obtain more physcally meaningful solutions to the inversion problem presented by such observations of the sources of activity in the centers of active galaxies

The various methods used to infer the physical conditions and location of the material responsible for broad line emission in AGN are reviewed. Recent efforts have focussed on reverberation mapping, whose basic concepts and experimental constraints are discussed. A new method for analyzing the results of monitoring experiments, regularized linear inversion, is presented. This method is then applied to published data from the 1989 IUE campaign on NGC 5548, and the results found contrasted with those obtained by the previous standard method, maximum entropy.

The amplitude–time lag (“ΔA-Δt”) relation is considered in order to describe behaviour of the emission-line spectrum of an active galactic nucleus during a separate active event. Here ΔA, called the amplitude, is the maximum relative increment of the flux in a line, and Δt is the time lag between the maximum of the ionizing continuum flare and the maximum of the flare in a line. As suggested by Shevchenko (1988), the construction and analysis of such relations can be used to discriminate between broad-line region models. Comparison of theoretical “ΔA-Δt” relations with the observed one composed by data for flares in various lines during a separate active event, is proved to be a useful tool for investigating the geometry of a broad-line region, for studies of the form of phase functions of a typical line-emitting cloud in various lines, as well as for clearing up the duration and amplitude of the initial flare in the ionizing continuum. The advantage of this method is that it utilizes the most general observed characteristics of the emission-line flares and nevertheless provides basic information on the allowed BLR models before the detailed modelling of emission-line light curves is performed.

We present preliminary results on a combined HST/IUE/ground-based monitoring campaign on the Seyfert 1 galaxy NGC 5548 undertaken by the International AGN Watch in order to answer questions that require both high temporal resolution (one day) and high signal-to-noise ratios. Our preliminary conclusions are (1) the ultraviolet and optical continuum variations are simultaneous to within a day (2) the He II λ1640 variations lag behind the continuum by about 1.7 days, and (3) the velocity field of the C IV-emitting region is not characterized by bulk by radial motion, but the higher velocity gas seems to originate closer to the continuum source than the lower velocity gas.

This contribution discusses the connection between variability in radio and optical with structural variations observed with VLBI. Structural changes do not have to start in the core, and intensity variations may be caused by components in the jet outside the core. The scenario is probably more complicated than present day theories assume.

Millimeter-VLBI provides an angular resolution of up to a few tens of microarcseconds and allows imaging of compact radio sources, self-absorbed at longer wavelengths, with unsurpassed angular resolution. At 43 GHz the participation of the VLBA and the 30 m-MRT at Pico Veleta (e.g. Krichbaum et al., 1993 a&b), and at 86 GHz the addition of the 100 m-RT at Effelsberg and the 30 m-MRT (Schalinski et al., 1993, and this volume) have improved the imaging capabilities of mm-VLBI observations.

Results: The increased sensitivity of mm-VLBI observations allows the investigation of fainter objects, previously not accessible. As one example we show in Fig.1 the first detection of the compact radio source Sgr A∗ in the Galactic Center with VLBI at 43 GHz in May 1992 (Krichbaum et al., 1993d) and at 86 GHz in April 1993 (Krichbaum et al., 1994). In both observations the size of Sgr A∗ appeared to be larger than its expected scattering size, indicative of intrinsic source structure showing up at mm-wavelengths. Future monitoring with mm-VLBI is necessary to search for (not unexpected) structural variability.

Monitoring of AGN with mm-VLBI reveals in all cases observed in sufficient detail jet curvatures of increasing amplitude towards the self-absorbed VLBI-cores (e.g. in 18034-784: Krichbaum, 1990, OJ 287: Krichbaum et al., 1993c), and sub- or superluminal motion along ‘quasi-helically’ bent trajectories (e.g. 3C 84: Krichbaum et al., 1993b; 3C 273: Krichbaum et al., 1993c), which differ sometimes for adjacent jet components (e.g. 3C 345: Krichbaum & Witzel, 1992, Krichbaum et al., 1992&1993a). In 3C 84, 3C 273 and 3C 345 the apparent velocity of jet components varies systematically along the jet axis, in 4C 39.25 (Alberdi et al., 1993) a moving component decelerates and brightens, all of this suggesting differential Doppler boosting and motion along three-dimensionally curved trajectories. In 3C 345 the complex kinematics of C4 and C5 (Zensus, this volume) has been geometrically modeled by motion along a helical path on the surface of a conical jet (Qian et al., 1992, Steffen et al., 1993, and this volume; see also Camenzind, this volume). As a new example, the oscillations of the inner jet and its velocity variations βapp(r) are shown for the BL Lac object 1803+784 in Fig. 2 (see the maps in: Krichbaum et al., 1993b). The frequent occurence of ‘quasi-sinusoidal’ bends in the inner jets of very different classes of AGN (QSO's, BL Lac's, Seyfert's) suggests that this effect is common in a large fraction of AGN and that the underlying jet-physical process may be fundamental for the understanding of the creation of jets.

The central regions of many nearby active galaxies are often heavily obscured by dust and gas and are visible at only the far infrared (FIR) and radio wavelengths. High-resolution radio observations provide an invaluable tool for clarifying the dominant power source in an active galaxy which could be due to either an intense burst of star formation or an active galactic nucleus (AGN). An AGN is normally assumed to be powered by a supermassive black hole with an accretion disk, where the black hole forms as the end product of stellar evolution. There have been suggestions linking both these forms of activity to galaxy interactions, but the conditions that might lead to the formation of an AGN and how it is fuelled are not well understood.

We present the results of X-ray and millimetre monitoring of the blazar 3C273 at 1–2 day intervals over the period 12 December 1992 to 24 January 1993. No large flares are seen in this period but variations in both wavebands of ∼ 30% on few day timescales are apparent. The ROSAT PSPC X-ray spectrum consists of 2 power-law components with the harder component dominating above 0.5 keV. There is very little correlation between the variability of the soft and hard components. The soft component does not correlate with the millimetre variations, but the hard component correlates reasonably well and leads the millimetre variations by about 10 days. These results show that the hard X-ray component cannot be a simple extrapolation of the millimetre/IR synchrotron component but may be explained as a self-Compton component in a shocked jet.

The quasar PG 1116+215 (Schmidt and Green, 1983) was selected because it is an intrinsically bright low redshift quasar. With z = 0.177 the soft X-ray emission is not significantly redshifted outside the ROSAT band. Its apparent magnitude mv = 15.04 (Véron and Véron, 1989) corresponds to Mv = −25.0 for H0 = 50 km s−1 Mpc−1 and q0 = 0. An additional selection criterion was low NH. Radio observations give NH = 1.44 × 1020 cm−2 (Elvis, Lockman and Wilkes, 1989).

The continuum emission components of Seyfert galaxies and quasars are reviewed with a particular emphasis on the correlations observed between these components. It is shown that the blue bump emission which is observed in Seyfert galaxies and quasars and not in BL Lac type objects is not hidden in the latter objects by a strong beaming of the jet emission. The shape of the blue bump is described and some consequences of these observations for the interpretation of this component are given. The relationship between the X-ray emission of quasars and the radio beaming properties of the object should be re-visited when the shape of the soft excess emission can be better established.

X-ray spectra of many AGNs are dominated by strong absorption-like features at low (0.5–1 keV) energies. Recent progress in modeling AGNs indicate that pure X-ray absorption is unlikely to be observed and the contribution of emission and reflection must be significant. Several such models are shown and compared with what is likely to be observed by two X-ray experiments, ROSAT and ASCA. It is argued that X-ray colors may be useful diagnostic tools in analyzing such spectra.

We examine the surface density of quasars expected in the ROSAT PSPC surveys, using quasar optical luminosity functions and model continuum spectra including a soft X-ray excess. We show that the surface density is a sensitive function of the characteristic energy of the soft excess, and the absorption within the quasar as well as the interstellar medium. Comparision of the predicted surface density with the results of various surveys allows useful constraints to be put on the nature of the soft excess.

The broad band energy distributions of blazars are revisited with particular emphasis on the sources detected in γ-rays by the Compton Observatory (GRO). The observed distributions can be broken down into two main components, corresponding to two broad peaks in the vFv representation. The first occurs in the FIR-optical range, the second in the MeV-GeV region. In the case of MKN 421, which may be representative of X-ray selected BL Lacs, the first peak is shifted to higher frequency (≃ 1016 Hz) and the γ-ray spectrum extends to TeV energies. There is general agreement that the first spectral component is due to synchrotron radiation from a relativistic jet, although some problems remain in deriving the spectrum and location of the emitting relativistic electrons. The second component, which in most objects extends from the X-ray to the γ-ray range, can be naturally interpreted as inverse Compton scattering by the same electrons producing the synchrotron photons, either on the synchrotron photons themselves (SSC) or on photons external to the jet. It is argued that multifrequency studies of these sources including γ-rays will allow to test Inverse Compton models and to distinguish between different sources of photons.

We present a model for curved parsec-scale jets that includes most of the key radiation physics and relativity, but which is simple enough to allow an exploration of diverse geometries and flow parameters.

In this talk I shall address three different processes relevant to continuum variability in AGNs. The first two refer to the physical conditions in the regions responsible for the non-thermal emission, and the implications of high brightness temperatures. The third is the distinctive type of flare that results when a star is tidally disrupted by a massive black hole; this process, which merits much further study, it likely to be specially important as a diagnostic of physical conditions in low-luminosity nearby nuclei.

In this brief communication, we present some progress in the investigation of a most promising model that was designed to combine ejection with accretion. In this model, a bipolar configuration of opened magnetic field lines that thread the accretion disk, allows the extraction of angular momentum, the acceleration of matter up to super Alfvénic velocities and the self collimation of the jet. However, important issues have remained unsolved. First, a systematic method for solving the jet MHD equations with their critical surfaces was lacking. Second, the capability of accretion disks to generate super Alfvénic jets was unknown. Third, the back-reaction of the ejection on the accretion disk dynamics and its energetics remained to be done. Solving these three points led us to draw some noteworthy consequences for the understanding of AGNs.

Many compact galactic and extra-galactic sources have a luminosity in the slim accretion disk regime, i.e. L/LE ∼ 1, where LE is the Eddington luminosity. The correspondingly large accretion rates diminish the relevance of variability interpretations based on the thin disk model. This paper explores the possible connection between variability in AGN and local instabilities in slim disks, and points out some relevant areas of future research.

The moving knots observed in VLBI of compact quasars are due to off-axis substructures in the parsec-scale jets which are dragged along by the underlying plasma flow. This is suggested by detailed MHD models for the structure of relativistic jets on the parsec–scale that are based on exact solutions of the nonlinear Grad–Schlüter–Shafranov equation in the asymptotic domain. These jet models are characterized by a current–carrying core and a current–free envelope. The corresponding core–radius Rc = upRL is related to the poloidal jet velocity up and the light cylinder radius RL generated by the rotation of the magnetic surfaces. The Poynting flux, which has an inhomogeneous distribution in the jet, provides an off–axis energy reservoir that could be tapped and converted into particle acceleration by non-axisymmetric instabilities in the core–jet structure. Evidence is presented that quasi-periodic synchrotron emission (in the IR–optical) and inverse Compton emission (in X– and γ–rays) from freshly accelerated electrons are due to a lighthouse effect of the rotating knots.

Accretion disks surrounding massive black hole is an attractive scenario of nuclear activity. A number of arguments support it although there is no unquestionable proof of the existence of accretion disks in active galactic nuclei. Meaningful comparison of the disk model prediction with the data can only be made if emission of accretion disks is calculated taking into account the existence of optically thin parts responsible for the emission of x-ray radiation. Nonlocal reprocessing phenomena have to be also included. Since we have no real understanding of the viscous processes operating in accretion disks some ad hoc parameterization of these processes has to be used and its applicability should be checked by broad band comparison of predictions for continuum emission and spectral features with available data.