Evidence of TeV γ-ray emission has been found for only a handful of active galactic nuclei, with detailed investigations limited to the blazars Mrk 421 and Mrk 501. TeV γ-ray astronomy, as the highest energy band, provides important information that is hard to obtain from longer wavelength electromagnetic radiation. The current status of TeV γ-ray studies of active galactic nuclei is summarized and our understanding of the high energy phenomena taking place in active galactic nuclei is outlined, with the prospects for future TeV γ-ray observations also considered.

Precision optical astrometry of quasars and active galaxies can provide important insight into the spatial distribution and variability of emission in compact nuclei. SIM — the Space Interferometry Mission — will be the first optical interferometer capable of precision astrometry on quasars. Although it is not expected to resolve the emission, it will be very sensitive to astrometric shifts, for objects as faint as R magnitude 20. In its wide-angle mode, SIM will yield 4 microarcsecond absolute positions, and proper motions to about 2 microarcsecond/yr. A variety of AGN phenomena are expected to be visible to SIM on these scales, including time and spectral dependence in position offsets between accretion disk and jet emission. SIM should be able to answer the following questions. Does the most compact optical emission from an AGN come from an accretion disk or from a relativistic jet? Do the relative positions of the radio core and optical photocentre of quasars used for the reference frame tie change on the timescales of their photometric variability? Do the cores of galaxies harbour binary supermassive black holes remaining from galaxy mergers? In this paper we briefly describe the operation of SIM and the quasar measurements it will make. We estimate the size of the astrometric signatures which may be expected, and we discuss prospects for using astrometry as a fundamental tool for understanding quasar nuclei.

Extreme scattering events (ESEs) are dramatic variations of the flux density at gigahertz frequencies caused by ray path distortions within an isolated inhomogeneity (‘plasma lens’) in the interstellar medium. These events are characterised by a deep flux density minimum in the light curve with, in some cases, surrounding maxima. The variability time scales range from weeks to months. These phenomena show a strong frequency dependence, in which the variability amplitudes increase with wavelength. During an intraday variability (IDV) monitoring project (March 2000), a feature resembling an ESE-like event appeared in the variable light curve of 0954+658, however with a time scale of less than two days. We will discuss this effect and its implications for a better description of the interstellar medium.

We summarise results from flux density monitoring campaigns performed with the 100 m radio telescope at Effelsberg and the VLA during the past 15 yrs. We briefly discuss some of the statistical properties from now more than 40 high declination sources (δ ≥ 30°), which show intraday variability (IDV). In general, IDV is more pronounced for sources with flat radio spectra and compact VLBI structures. For 0917+62, we present new VLBI images which suggest that the variability pattern is modified by the occurrence of new jet components. For 0716+71, we show the first detection of IDV at millimetre wavelengths (32 GHz). For the physical interpretation of the IDV phenomenon, a complex source and frequency dependent superposition of interstellar scintillation and source intrinsic variability should be considered.

We present results from preliminary analysis of radio interferometric data taken from the US Naval Observatory Radio Reference Frame Image Database. The results presented here reveal valuable insight into the structural variability of the two extragalactic radio sources 0552+398 and 1308+326. We are actively pursuing analysis of additional objects.

A simple model of cosmic ray electron acceleration at the jet boundary yields a power law particle energy distribution of ultra-relativistic electrons with an energy cut-off growing with time, and, finally, a growing particle bump at the energy where energy gains equal radiation losses. For such electron distribution, in tens-of-kpc scale jets, we derived the observed time-varying spectra of synchrotron and inverse Compton radiation, including Comptonisation of synchrotron and cosmic microwave background photons. Slowly varying spectral index along the jet in the ‘low frequency’ spectral range is a natural consequence of boundary layer acceleration. Variations of the high energy bump of the electron distribution can give rise to anomalous behaviour in the X-ray band in comparison to the lower frequencies.

We have observed eight AGN since 1993 in the energy region above several hundred GeV using the CANGAROO telescopes. We observed Mrk 421 for ten nights with the CANGAROO-II 10 m telescope during its active state in early 2001 using the very large zenith angle technique. Our preliminary result implies the detection of gamma-ray emission from Mrk 421 in the energy range above 9.3 TeV. The high energy peaked BL Lacs (HBLs) PKS 2005–489 and PKS 2155–304 have been also observed. No statistically significant signals are found for both HBLs and flux upper limits are obtained. A summary of results of observations of AGN is presented in this paper.

We are undertaking an observational program using the ATCA to monitor the intraday variability (IDV) of a sample of sources at 4.8 and 8.6 GHz. The sources were selected to include the known strong southern IDV sources plus a number of sources whose IDV was recently discovered. The present monitoring program will extend over a full year in order to search for any annual cycle that may be present in the long-term IDV characteristics of these sources. In this paper we discuss the observing strategy and data analysis, and present the first results from our observations.

The inverse Compton (IC) limit, TB1012 K, on synchrotron sources places a severe limit on models for intraday variables. The conventional limit is relaxed for proton synchrotron emission, or when acceleration balances IC losses. Coherent emission avoids the limit entirely but introduces other difficulties that have been inadequately discussed.

In blazar models both protons and electrons may be efficiently accelerated in jets and produce γ-rays. Here we discuss the interactions of these γ-rays with different radiation fields. The external radiation fields within a few parsecs from the black hole involved in such interactions could be the direct radiation from the accretion disk coupled with the jet, the infrared radiation from a dusty torus, and the emission line radiation from the broad line region surrounding the accretion disk. The optical thickness for absorption of γ-ray photons in the external radiation fields is analysed for blazars and quasars.

Based on the unification theory of active galactic nuclei we briefly review the evidence for the existence of small scale dust tori in blazars/FR I. We propose that the existing jet–accretion disk symbiosis extrapolates to a large scale symbiosis between other important dusty constituents of the blazar/FR I family.

We discuss the constraints that recent observations place on circular polarisation in AGN. In many sources the circular polarisation is variable on short timescales, indicating that it originates in compact regions of the sources. The best prospects for gleaning further information about circular polarisation are high resolution VLBI and scintillation ‘imaging’ which probe source structure on milliarcsecond and microarcsecond scales respectively.

Critical progress in our understanding of high energy emission from AGN has been determined in the last 10 years by X-ray monitoring campaigns with many space missions, notably ROSAT, ASCA, RXTE, BeppoSAX, and XMM, often in conjunction with observations at other frequencies. The emphasis of the present review is on recent findings about X-ray variability of blazars. Among AGN, these exhibit the largest amplitude variations of the X-ray emission, often well correlated with variations at higher energies (GeV and TeV radiation). The accurate sampling of the X-ray spectra over more than three decades in energy, made possible by the wide energy range of BeppoSAX, has also shown strong spectral variability in blazar active states, suggesting extreme electron energies and leading to the identification of a class of ‘extreme synchrotron’ sources.

Radio variability on timescales from a few hours to several days in extragalactic flat spectrum radio sources is generally classified as intraday variability (IDV). The origin of this short term variability is still controversial and both extrinsic and intrinsic mechanisms must be considered and may both contribute to the observed variations. The measured linear and circular polarisation of IDV sources constrains the low energy end of the electron population. Any population of cold electrons within sources at or above the equipartition temperature of 1011 K depolarises the emission and can be ruled out. Intrinsic shock models are shown to either violate the large fraction of sources displaying IDV or they do not relax the light travel time argument for intrinsic variations. From structure function analysis, we further conclude that interstellar scintillation also leads to tight size estimates unless a very local cloud in the ISM is responsible for IDV.

Synchrotron self-Compton (SSC), external Compton, and hadronic models of blazar emission all invoke particle acceleration at relativistic shocks as the dissipation mechanism seeding their non-thermal X-ray and gamma-ray emission. Studies of diffusive acceleration at such relativistic shocks are more sparse than those pertaining to their non-relativistic counterparts. This paper presents acceleration time results from the theory of relativistic shock acceleration that are pertinent to AGN observations. This temporal information interfaces critically with the observed rapid variability of blazars. Very recent theoretical results are presented, where it is determined that acceleration times can never become arbitrarily short in relativistic shocks, but are dominated by diffusion in the downstream region and couple to the particle's gyroperiod. This fundamental bound links to the variability timescale to generate a firm lower bound to the environmental magnetic field of blazars such as Mrk 421. Consistency of such a bound with SSC spectral models and flare decay times is discussed.

There is new evidence which identifies seasonal changes of the variability time scale in intraday variable (IDV) sources with refractive interstellar scintillation effects. Such a RISS model takes the annual change of the Earth's velocity relative to the scattering medium into account. In September 1998 we found a remarkable prolongation of the variability time scale in the IDV source 0917+624 with only small variations in flux density during a period of 5 days. This was explained as a seasonal effect, in which the velocity vector of the Earth and the interstellar medium nearly cancelled. In order to further investigate the applicability of the model for 0917+624, we performed an Effelsberg 6 cm-flux monitoring program over the course of one year. Since September 2000, the source appears to be remarkably inactive and yet (May 2001) no return to its normal, faster and stronger variability pattern is observed. Here, our observational results and a possible explanation for the current quiescence are presented.

We present the kpc-scale behaviour of the powerful extragalactic radio source Hercules A and the behaviour of the intracluster gas in which the radio source is situated. We have found that Hercules A exhibits a strong Laing-Garrington effect. The X-ray observations have revealed an extended X-ray emission elongated along the radio galaxy axis. The estimated temperature of the cluster is kT=2.45 keV and the central electron density is n○≃7.8×10−3 cm−3 which reveals a hot, dense environment in which Hercules A is situated. From the combined study of the radio and X-ray data we have estimated a central value of 3 ≲ B○(μG)≲9.

We also present the most recent results from the analysis of the radio data on the pc-scale structure of the radio galaxy, observed at 18 cm by the EVN–MERLIN array. A faint but compact radio source, coincident with the optical centre of Hercules A, was detected by the EVN at 18 mas resolution. The total flux density of the EVN core is 14.6 mJy. Its angular size is 18×7 mas with a position angle of ≃139°. There is also evidence for extended emission in the NW–SE direction, most probably from the eastern pc-scale jet. If this is true then there is a misalignment between the direction of the pc-eastern and the aligned kpc-scale jets of ≃35°.

III Zw2 was observed with XMM-Newton in July 2000. Its X-ray spectrum can be described by a power law of photon index Γ≈1.7 with a Gaussian line at 6.7 KeV. There is no significant evidence of intrinsic absorption within the source or of a soft X-ray excess. Multi-wavelength light curves over a period of 25 years show related variations from the radio to X-rays. We interpret the radio to optical emission as synchrotron radiation, self-absorbed in the radio/millimetre region, and the X-rays as mainly due to Compton up-scattering of low energy photons by the population of high energy electrons that give rise to the synchrotron radiation.

We discuss the observational and theoretical constraints on the brightness temperature of compact opaque synchrotron sources. We consider the implication of observed apparent velocities on the amount of Doppler boosting and compare this with values deduced from directly measured brightness temperatures from ground and space based VLBI observations, as well as the implications of intraday variability. We also discuss the maximum rest frame brightness temperature expected under conditions of both inverse Compton cooling and equipartition conditions.

Results are presented of long-term monitoring at the radio telescope RATAN–600 and a model interpretation of instantaneous 1–22 GHz spectra at six frequencies for a sample of 550 compact extragalactic radio sources. The sources are selected from the Preston et al. (1985) VLBI survey and are located in the declination range from −30° to +43°. Dependence of the index of variability on frequency is analysed for different subsamples. Factors of flux density variations range from 1 to 15. A typical behaviour of spectra evolution during strong radio outbursts in various objects is recognised suggesting a common physical nature of the variability. Invoking VLBI observations in spectra–structure analysis, we find that usually more than 70% of the total emission is coming from the mas scale at frequencies higher than 5 GHz. The observed variability can be explained in the model with a relativistic jet of parsec scale in the longitudinal magnetic field or in the shock-in-jet model.

Powerful water masers have been detected in 23 active galactic nuclei (AGN) to date. Most of the sources that have been monitored show evidence for variability, which may be either intrinsic (response of the masers to a time-varying pump or background source) or extrinsic (the effects of scintillation in the interstellar medium of the Galaxy). I briefly review the examples in which these mechanisms may be operating (interstellar scintillation in the Circinus galaxy, a time-varying background source in Mrk 348, and a time-varying AGN luminosity in NGC 1068).