A two-component photoionization model where the high and the low ionization lines are allowed to arise from different kinds of clouds is explored. A spectral distribution of the incident radiation which tends to overestimate the X-ray flux around 3 keV is adopted in order to enhance the extent and heating of the excited HI region. It is concluded that the model is still unable to explain the observed ratio FeII/Hß.

While it is widely held that the densities of QSO and Seyfert broad-line regions lie below 1011 cm−3, there is a growing body of evidence, indicating that at least some of the broad-line gas has densities equal to or exceeding 1011 cm−3. Such evidence includes (1) the presence of a 3μm “bump” in roughly half of all QSOs, (2) the depression of Brγ in several Seyfert galaxies, (3) the broad-line variablity in some objects, and (4) the ability to produce the hydrogen spectrum theoretically.

We find indication of variability of the ratio Mg II 2934/2798 for 3C 232 and discuss the excitation mechanism of Mg II 2934. For 3C 249.1 we identified the following new lines: [Ne V] 3346, [Ne V) 3426 and He I 3884. Comments are made on continuum shapes; in both cases optical continuum variability is confirmed.

Narrow-band imaging of PG 1501+106 revealed no extra-nuclear [OIII] emission above a level of ∼ 20 μJy. The disagreement with the result of Boroson et. al., 1982 (who detected 40 μJy) can be explained by assuming a slight difference in angular size between the broad and narrow line emitting regions.

Nearly simultaneous FUV and optical spectrophotometry of X-ray selected Seyfert galaxies has revealed an average Ly α/H β ratio of 22, a positive correlation between the ratio Ly α/H β and the width of the lines, and additional Ly α emission in the wings of one source which is not matched by emission in the Balmer line wings. However, we find no distinguishing features in the continuum emission from these X-ray selected objects compared with other samples. If the correlation between Ly α/H β and the width of the lines is found to apply to larger samples of Seyferts, it may be that our objects appear Ly α bright because they are also broad-lined compared with other samples.

Analysis of ultraviolet spectra of quasars obtained with IUE indicate that the permitted emission lines in quasar spectra are variable. The upper limit for the delay between a change in the continuum and the consequential change in the emission lines is about 9 months. This rapid response of the permitted emission lines to changes in the continuum leads to a number of problems in understanding the nature of quasars.

Fitting technics are applied to study the variations of the 1550 line profile and adjacent features in the IUE spectrum of the Seyfert galaxy NGC 4151, from 1978 up to 1983. New important results are found concerning the kinematics and physics of the emission line gas at a few light-days from the continuum source.

From a set of spectrophotometric data covering 15 years, we observe a recurrent activity in the BLR of NGC 1566. Each period of activity lasts for 1300 days. It consists of successive bursts characterized by a rise-time less than 20 days and an exponential decay on a time scale of around 400 days. The total energy involved in one 1300 day period of activity is 1051 ergs. These variations can be understood in the framework of accretion disc models for AGN, as resulting from a limit cycle which operates in the inner regions of the disc.

Observations of active galactic nuclei are interpreted in terms of a theoretical model involving accretion onto a massive black hole. Optical quasars and Seyfert galaxies are associated with holes accreting near the Eddington rate and radio galaxies with sub-critical accretion. It is argued that magnetic fields are largely responsible for extracting energy and angular momentum from black holes and disks. Recent studies of electron-positron pair plasmas and their possible role in establishing the emergent X-ray spectrum are reviewed. The main evolutionary properties of active galactic nuclei can be interpreted in terms of a simple model in which black holes accrete gas at a rate dictated by the rate of gas supply which decreases with cosmic time. It may be worth searching for eclipsing binary black holes in lower power Seyferts.

In the Seyfert galaxy NGC 1566 and quasars 3C 345, 3C 446 long term (∼ 103 days) variations have been observed. We propose to explain them as a limit cycle behaviour, similar to that well studied in the case of dwarf novae outbursts. The key element of our analysis is the relationship between the accretion rate · and the surface density Σ.

Active Galactic Nuclei (AGN) and quasars have unique physical parameters among all the objects in the Universe. Undoubtedly it is the uniqueness of the physical conditions in these systems that gives rise to the peculiar physical processes in them.

Using the fact that the efficiency of the revived (Wagh et al 1985) Penrose process of energy extraction from black holes immersed in electromagnetic fields can be very high (Parthasarathy et al, 1986) we show that this process can comfortably power the ‘central engine’ in Active Galactic Nuclei. The microphysical Penrose process energized particles will be ultrarelativistic in the asymptotic frame. Hence the kinematical analysis of escaping photons by Piran and Shaham (1977) will be a good approximation to the kinematics of these particles. From this analysis one expects the energized particles to emerge within an angle∼ 40° above and below the equatorial plane. These energetic particles, which are collimated in the funnel of an accretion disk and further on by the magnetic field, then, form supersonic, relativistic, bilateral jets. The relativistic Y factor for such jets can be expected to be ∼ 2 since these ultrarelativistic particles will effectively mimick radiation in ‘dragging’ the matter already injected inside the funnel. Various implications of high energy extraction efficiency are illustrated.

Jets are found in many astrophysical phenomena, from young stellar objects and collapsed stellar cores to quasar and radiogalaxies. Therefore they must represent a common dynamical phenomenon, while different morphological characteristics arise from the interaction with specific environments (Ferrari and Tsinganos 1985). In order to investigate the basic aspects of jet dynamics and morphologies, we have proposed a simple analytical treatment based on the fluid theory of stellar winds (Parker, 1063), which allows a direct test of different physical effects (Ferrari et al. 1985, 1986). We discuss here implications for the physical theories of quasars.

First order Fermi acceleration is an efficient mechanism to produce relativistic particles in strong astrophysical shocks. We have developed a model using this theory to explain the non-thermal emission from non-relativistic jets in Quasars and AGNs. Fluid dynamical simulations of cosmic jets predict shock formation in them at quasi-periodic intervals. This is also suggested by observations of jets. We identify the shocks as the sites of reacceleration of the energetic particles required to explain the non-thermal emission of the jets. In order to facilitate the comparison with observations, we have done numerical simulations of the expected observational properties at high and low spatial resolution.

The equations describing the transport of suprathermal charged particles and electromagnetic radiation across accretion flows onto compact objects are solved analytically, including the effect of shocks in the flows. These solutions indicate (a) accretion flows with shocks accelerate particles very efficiently upto ultra-relativistic energies. (b) the emergent spectra of electromagnetic radiation from such flows reproduce the observed spectra of quasars from the infrared to the hard X-ray region.

An outline is given of a project to model thick accretion discs around a black hole. A simple shear viscosity is assumed and the effects of radiation transport and thermal conduction neglected. Preliminary results are given for the equatorial structure of the model discs.

Flare activity on the sun may be attributed to the distortion of coronal magnetic field caused primarily by vortical (Shearing) photospheric motion. If an accretion disk is permeated by a magnetic field, the strong differential rotation of the disk will lead to similar stressing of the magnetosphere, and one may expect similar flare activity to occur. It is likely that the density of plasma in the maganetosphere is sufficiently large that the “frozen flux” condition is satisfied, and sufficiently small that the magnetic field will be substantially in the force-free state.

Central cores of compact radio sources are believed to contain supermassive black holes accreting material from their vicinity which produce fast moving plasma in the form of directed beams (jets), with apparent opening angles ≥5° (Rees et al. 1981). In case, the jets are produced and their collimation is established on scales few times the SchwarzschiId radius (2m; m=GM/c2) of the central engine, deflection in particle trajectories in the curved spacetime () would be large enough to widen the beam and thereby reduce the particle density and effective luminosity in the beam (Fig.1).

Doubt is cast on the reality of the following four assumptions commonly made in the treatment of extragalactic radio sources: (1) The central engine is a black hole; (2) Electrons can be accelerated in situ in the knots and heads of the jets, to large Lorentz factors γ ≥ 102, with an efficiency exceeding 30%; (3) The (non-thermal) radiation emitted by the beam fluid is isotropic in some (comoving) Lorentz frame; and (4) The flow velocity in the jets of SS 433 is v = 0.26 c.