The central region of M87 in the K′ (2.1 μm) - band was imaged using the 2.2m telescope on Calar Alto, Spain. In addition to the jet our K′ exposure (Fig. 1) shows an arclike structure ∼ 24 arcsec southeast of the nucleus. This is the IR counterpart to the feature detected by Stiavelli et al. (1992) in the optical and to the brightness maximum of the southeastern radiolobe designated as “theta” (Hines et al. 1989). Its radio to IR spectral index is α = −0.88 ±0.01 (Sv ∞ v α), consistent with the radio spectral index of α = −0.84 ± 0.07 (Salter et al. 1989) of the whole radio lobe measured between 5 GHz and 230 GHz.

Simultaneous observations of 8 Seyfert 1 type AGN (Fairall-9, Mrk 590, NGC 4051, 3C 273, NGC 5548, Mrk 841, Q 1821+643 and 3C 390.3) obtained with ROSAT and IUE (RIASS program), and for 5 sources (Fairall-9, NGC 4051, 3C 273, Mrk 841 and Q 1821+643) with Ginga, have been analysed with the aim of describing the UV to soft X-ray spectral component in these sources.

The IUE-ULDA database (Version 3.0, complete until the end of 1988) contains about 3500 spectra of more than 500 AGN of different classes (see Courvoisier and Paltani, IUE-ULDA Access Guide No. 4 A & B, ESA SP 1153 A & B 1992). We selected 67 objects for which the variability properties could be investigated. For each object we estimated the amplitude of the variability by means of the standard deviation σ(λ) of the flux at different frequencies divided by the mean flux.

Ultraluminous IRAS Galaxies (ULG's) have luminosities comparable to quasars while their space density is much higher than that of active galaxies. Much debate has centered around the origin of the energy source for these objects, whether this is a burst of star formation or a hidden quasar. The sample studied here is the Sanders et al. (1988) sample, 10 objects with LFIR ≥ 1012 L ⊙. We discuss our new observations at X-ray and submm wavelengths together with other published data for some of the objects. Some useful ideas can be gained from comparisons of the shape of the spectral energy distributions (SED's) of the ultraluminous objects with other “archetype” objects such as typical starbursts i.e. M82 or type 2 AGN i.e. NGC1068.

Previously we have studied the jet of the quasar 3C 273 at optical and radio frequencies. In our first set of X-ray data with 17.2 ksec integration time obtained with the ROSAT HRI, the jet is easily visible extending out from the bright quasar core. The total number of counts in the jet lies in the range 200 to 300, depending on the details of the background model. This corresponds to an X-ray flux fv (2.9 × 1017 Hz) = 65 … 140 nJy (lower limit, synchrotron radiation α = −0.8 … upper limit, bremsstrahlung α = 0, NHI = 1.8 × 1020 cm−2), in good agreement with the value derived from the EINSTEIN observations.

The near infrared emission (NIR) of radio quiet active galactic nuclei is commonly attributed to dust, absorbing and re-emitting some of the nuclear UV radiation. The dust can survive only at distances larger than a few light months, and any rapid variation of the nuclear radiation, on timescales of a day or less, will be completely smeared out.

Advanced multi dimension radiative transfer calculations for an AGN source with a dust disk shows that the AGN-IR emission can be due to reradiation from heated dust in a thick disk. The models produces a weak silicate feature for a wide range of physical conditions, in agreement with the observations.

The comparison of the Byurakan classification of central parts of galaxies with the results of their radio observations showed that there is a definite correlation: the radio emission is more often observed in those galaxies which were suggested to have active nuclei.

The properties of the soft X-ray excesses of bright Seyfert 1 galaxies and Quasars are described using the observations obtained with the PSPC (0.1–2.4 keV) detector of the XRT telescope aboard ROSAT during the ROSAT all sky survey (RASS). The sample consists of 58 Seyfert 1 type AGN detected with more than 300 counts during the RASS and observed at least once with IUE.

The soft X-ray photon indices of our sample members range from 1.6 to 3.4 in a wide distribution (< Γ > = 2.50, σ = 0.48). The width of the distribution is considerably larger than the mean statistical uncertainty on the individual spectral slopes (σ = 0.33). Excepting for IC 4329A and Mrk 766, the mean contribution of absorbing cold matter intrinsic to the Seyfert galaxies of our sample to the absorbing column density is less than 1020cm −2. In IC 4329A and Mrk 766 intrinsic absorbtion is observed at soft X-ray. Both sources are also strongly reddened by dust.

An excess of soft X-ray flux is detected in 90% of the sources above the exptrapolation of the hard X-ray power law. It can be shown that the PSPC spectral slope is a measure of the strength of the soft X-ray excess. If the reddened sources are excluded, a correlation appears between the strength of the ultraviolet blue bump and the soft X-ray photon index (figure 1). The ratio of the ultraviolet to infrared fluxes and the ultraviolet spectral slope are also related to the strength of the blue bump. The observations are compatible with a model where most of the spectral variations arising among the sources studied are driven by the strength of the bump component, which varies by a factor of 100 from object to object. A bump model consisting of a power law with a high energy cutoff at 80 eV can fit most of the sources. In any case, the spectral energy distribution of the ultraviolet to soft X-ray bump is characterised by vF v(1375 Å) = (1–5) ∫ε > 150eV F εdε.

An analytic model for a stationary force-free relativistic magnetized jet is presented. In those models a poloidal current provides the tension that balances the internal pressure. Our ignorance of the detailed conditions in the collimation zone is accounted for by specifying the current distribution as an (essentially free) function of the flux surfaces. The asymptotically cylindrical Grad-Schlüter-Shafranov (GSS) equation is solved for a nonlinear current distribution, I(Ψ) = c(1 – exp(−bΨ)) which covers both diffuse (b 《 1) and sharp (b 》 1) pinches, and weak (small c) and strong (large c) currents.

CGRO observations of high energy γ-rays from active galactic nuclei (AGN) suggest that beaming may be prevalent in these sources. Anisotropic effects in AGN continuum spectra produced by the inverse Compton scattering mechanism are outlined, in particular the resulting spectral breaks and modification of spectral indices that depend strongly on the electron anisotropy and the observational viewing perspective.

Optically thin thermal emission may be a viable alternative to optically thick accretion disks for the origin of the optical/UV Big Blue Bump. In this contribution I summarize a number of points favoring the optically thin, or free-free, hypothesis. The free-free model is consistent with the most stringent quasar size and luminosity constraints provided sources are composed of a large number of very small and dense cloudlets or filaments.

The escape of relativistic neutrons from their production region can have various consequencies for the morphology of active galaxies. The phenomena of luminosity gaps and radio jet lighting in extragalactic radio sources (EGRSs) fit into the model that employs relativistic neutrons as the vector for particle and energy transport out of the central engines of AGNs. The central radio gaps reveal themselves as regions of relativistic neutron beam transport. The relativistic neutrons, which decay in flight after traveling for ∼ 103 γn s, produce secondaries which are responsible for the radio jet lighting and the associated phenomena in EGRSs.

Several points indicate that extragalactic jets can interact with dense gaseous obstacles which occur on their ways. Examples of these interactions are the knotty structure of the radio and optical jet in M 87 and in other objects. These observations have been interpreted by Blandford & Königl in terms of collision of a jet with supernova remnants. We have reanalysed this idea taking into account new observations and improvements in the theory of diffusive shock acceleration. We find that the model requires a very high supernova birthrate (∼ 1 SN/year), which is not observed. It is more probable that the “obstacles” are formed by the stellar winds from the red giants. We estimate that the value of the magnetic field is ∼ 10−5 G in the interaction region (r=1kpc) (paper in preparation).

Since its launch, CGRO has detected more than 20 γ-ray emitting AGN, most of them associated with powerful, radio-loud, flat-spectrum objects, exhibiting VLBI superluminal motions. In the case of 3C279, the huge value of the apparent luminosity (∼ 1048 erg.s −1) and the variability time-scale of a few days (Hartmann et al., 1992) gives a very large compacity lapp ≃ 200, that is, the medium should be completely thick to γ-rays. This contradiction can be explained if the γ-rays originate from a relativistic jet pointing at a small angle with respect to the line of sight (Maraschi et al., 1992). However, the still large value of compacity suggests the existence of an inner, more compact region where pair production can take place efficiently (Henri et al., 1993). This supports the so-called “two-flow” model, where the superluminal motion is attributed to the expansion of a relativistic pair plasma heated by a MHD jet from an accretion disk (Sol et al., 1989). Hence we propose to interpret the spectral break observed in many objects around a few MeV (Lichti et al., 1993) by an opacity effect due to photon-photon absorption by pair production.

We have simulated the reflection on cold matter in a variety of situations to determine which informations can actually be inferred from observations. We modelled a semi-infinite plane parallel medium of solar abundance matter, semi-isotropically illuminated by a X/γ ray source. The spectra are calculated from a Monte-Carlo method without any approximation in the cross-sections. Θ is the angle over which the reflecting matter is seen (90°=face-on), Θ=all means a spatially integrated spectrum. Fref is the ratio of the reflected over direct component.

We are studying the slow (ṁ < 1) spherical accretion of a gas onto a supermassive (M ≈ 108M ⊙) black hole in the presence of a strong tangled magnetic field. In the core with radius 2.5 rg < r < 10 rg protons are isotropized due to scattering in magnetic field, but are not thermalized since the characteristic time of pp — Coulomb scattering is less than the infall time. A proton moves in the electron gas with a friction due to pe — scattering, gradually transferring energy to electrons. The standard equations for the proton gas allow the virial regime of accretion when the kinetic energy of the proton is a function of a distance only Ek (r) = (2/5) mpc 2 (rg /r). The model is relevant to the slow subsonic settling of matter onto the black hole, as, for example, in the upstream region after the shock standing at a distance r ∼ 20 rg (Mészàros and Ostriker 1983). Electrons are thermalized and are cooling predominantly by bremsstrahlung radiation. For ṁ ≲ 0.1 the core is transparent for bremsstrahlung photons. In agreement with Park (1990) the e+e − - pair production is found to be insignificant. The equilibrium between the energy release in pe — scattering and the bremsstrahlung radiation results in the almost isothermal core with the temperature T e ≈ 4 me , which slightly increases towards the inner edge of the core. The only role of magnetic field is the isotropization of the proton gas, as the synchrotron radiation is strongly self-absorbed. Therefore the model is insensitive to the precise value of H.

The far-infrared (FIR) spectra of radioweak quasars show a steep rise from the mm to the FIR wavelengths with an spectral index often α ≥ 2.5, and a slow decrease beyond the FIR with α ≃ −1. A comparison of the FIR luminosity with both radio and X-ray emission demonstrates, that only the active nuclei can provide the energy to heat the dust. We propose that the heating is due to the central engine via relativistic particles. These particles originate from a putative source on the symmetry axis close to the central engine and diffuse through a tenuous galactic halo to heat dusty molecular clouds in a disk configuration. We demonstrate that this mechanism can produce the FIR spectra with reasonable assumptions, and compare them with observations (Chini et al. 1989). We examine the diffusion through the halo and the energy deposition in the disk for two types of source (pointsource, line source, intensity decrease with z −β) and a diffusion coefficient, dependent on the spherical radius r (D ∞ r γ). We obtain the total energy deposition in the disk as a function of the disk radius. For the calculation of the FIR spectra, one must determine the dust temperature. We construct a heat balance that takes into account the absorbed and emitted emission of energy. The calculated dust temperature distribution depends on two parameter: the radial dependence of the diffusion coefficient and the z-dependence of the line source. Then we calculate the FIR spectra of a disk in the wavelength range 10 to 1300 μm. We determine the luminosity of a finite disk, where the limits are determined by the maximal and minimal temperature (T D,max = 1500K given by the dust destruction, T D,min = 20K given by a transition to dust heating from young stars in the extended disk).

To resolve the above dilemma, two essentially different approaches are undertaken: First, a new, detailed analysis of the entire radiation spectrum of Sgr A∗, from radio band up to gamma-rays, is reviewed, which enables us to put substantial constraints on the mass of a putative black hole. The derived upper limit turns out to be too small to allow the black hole to serve as an ‘engine’ for a Seyfert galaxy. Second, analyses of recent data on the 10 KeV gas in the central 200 pc and on star formation history at the Galactic center both make a star burst the likely episode in a recent past. Taken together, the two approaches seem to indicate that the history of the central part of our Galaxy can be better described as that of a starburst, rather than a Seyfert, galaxy.

Observational evidences of dust in the nuclear region of AGNs are substantial (Rudy 1984, ApJ, 284, 33; Jones et al. 1984, PASP, 96, 692). The ionization cones observed in several Seyfert galaxies has been interpreted as shadowing effects by a dust obscuring torus which hides the broad emission line region (BLR) and the central source (Wilson 1992; Storchi-Bergmann, Mulchaey and Wilson 1992, ApJ 395, L73). A large sample of optical and far-IR data for IRAS Seyfert galaxies has been analysed together with dust emission models (Bonatto and Pastoriza 1993), where it has been concluded that the same dust emission model can be applied to both Seyfert types. In order to further study the effects of dust in the spectra of active galactic nuclei, we have obtained spectrophotometry of 21 IRAS Seyfert galaxies in the range 3500–7200 Å and analyse them in conjuction with their IRAS fluxes. The stellar population type is derived from comparisons with normal galaxy templates using dilution effects in the K CaII line as discriminator. For 55% of the sample the population is of late type. For the rest, blue continua due to recent star formation and/or power-law may amount up to 30% at 4000Å. We conclude that the bulge stellar populations of IRAS Seyfert galaxies are similar to those of normal spirals, except that they are more reddened by E(B-V)i ∼ 0.20. Population-subtracted emission line ratios indicate on average stronger reddening for the narrow-line region (E(B-V)l ∼ 0.8. From photoionization models a power-law index for the ionizing continuum α=1.5, and a metallicity larger than solar are obtained. The most luminous IRAS galaxy of the sample (IRAS555) is discuss in detail: in order to be compatible with the observed IRAS fluxes and the optical stellar continuum, the ionizing continuum must be reddened by AV > 10 magnitudes. Consequently a dust structure in this galaxy appears to be increasingly affecting stars and gas towards the galaxy center.