The analysis of the line ratios along the profiles show that CIV/Lα line ratio is low in the center of lines but high in the wings although the Lα/Hβ line ratio has changed the opposite way. The line ratios have been modeled using the photoionization code CLOUDY. On the bases of the photoionization modeling along the line profiles we discuss the possible geometry of the emitted gas.

The possibility of an attenuated continuum for the gas ionization in the BLR is discussed. It is shown that “line ratio” problem for the low ionization lines in NGC 5548 could be successfully solved when the attenuation by the column density Nh = 1022 cm−2 are taken into consideration.

We present a brief summary of the ongoing Abastumani Blazar Monitoring Program started in the May of 1997. More than 50 000 frames have been collected during 498 observing nights for about 50 blazars.

We present CCD opbservations of BL Lacertae obtained from August 1997 to August 1998. The maximum amplitudes in BVRI bands during observing period included most part of great ouburst and second one are equal to 3.00 (rms=0.03), 2.71 (0.02), 2.53 (0.01) and to 1.25 (0.040), 1.11 (0.03), 1.02 (0.01) 0.91 (0.03), respectively. The amplitude for variability is larger toward the blue end of the spectrum.

The accumulation of evidence now strongly favours interstellar scintillation (ISS) as the principal mechanism causing intra-day variability (IDV) at cm wavelengths. While ISS reduces the implied brightness temperatures, they remain uncomfortably high. The distance to the scattering screen is an important parameter in determining the actual brightness temperature encountered. The high brightness temperatures, the presence of strong and variable circular polarization and the observed lifetimes of a decade or more for several IDV sources, pose significant problems for synchrotron theory.

We present new WSRT observations of the micro-arcsecond quasar J1819+3845. All short term variations are attributed to interstellar scintillation of a source which is at most 30 micro-arcseconds in diameter. The timescale of the modulations changes over the year, which we interpret as due to a peculiar velocity of the scattering medium. The scintillation behaviour can be used to determine sub-structure in the source.

We discuss conditions which have to be satisfied for observable signatures of diffractive scintillation in intraday variable (IDV) sources. We apply this discussion to the quasar PKS 0405-385 which had a scintillating component with angular size of 5 μas (Kedziora-Chudczer et al. 1997).

We discuss the detection of variable circular polarization (CP) in several scintillating radio sources: PKS 1519-273, PKS 0405-385 and Sgr A*. In all cases the CP is variable, with the fractional variability in the CP greatly exceeding that in total intensity.

Radio monitoring of the gravitational lens system B0218+357 reveals it to be a highly variable source with variations on timescales of a few days correlated in both images. This shows that the variability is intrinsic to the background lensed source and suggests that similar variations in other intraday variable sources can also be intrinsic in origin.

We investigate the structural evolution of the IDV source BL Lac 0716+714 from 10 epochs of global VLBI and VLBA observations. Our study reveals two possible component identification schemes, in which the jet components move either slow or fast. Although the fast motion would fit better to the observed IDV, the quality of our data does not allow a final decision between the slow and the fast picture.

The NRAO seven-beam receiver was used on the former Green Bank 91 m telescope, during 1986 November and 1987 October, to repeatedly survey the declination band (0° < δ < +75°) at 4.85 GHz. This data base has been used to extract variability information for sources in the GB6 catalog, a large unbiased sample of sources derived from the combined survey images. Long term variability information (one year time base) is available for 97% of the 75,162 GB6 sources and short term variability (few day) is available for 20,963 sources. This large variability data base is available to the astronomical community through http://pulsar.physics.ubc.ca/gregory/index.html.

Results of the first measurements of a VLBI structure of 0524+034 at 6 cm at the EVN+HartRAO in 1999 and recent broad-band spectra monitoring data are reported. We suppose that precession of the relativistic jet can be one of the possible mechanisms to account for the observed milliarcsecond structure and the strong long-term variations of the spectrum. The detected rapid changes may be interpreted as a propagation of a shock in the inhomogeneous jet plasma, but interstellar scintillations can not be ruled out on the basis of available data.

VLBI observations can now be used to examine the milliarcsecond properties of mJy and sub-mJy radio sources. Here we describe a snapshot survey of compact mJy sources selected from the FIRST survey and the first EVN observations of a 6 arcmin square field centred on the Hubble Deep Field. At the few mJy level, a significant number of steep-spectrum sources appear to be very young low-luminosity radio galaxies. Older and fainter examples of this type of source are seen in the HDF, and are detected by the EVN. The VLBI observations of the HDF support the view that at least one of the putative distant, dusty starbursts harbours an AGN.

We present first results from the analysis of multi-epoch VLBI observations of 241 sources in the CJ-F sample (Caltech Jodrell-Flat-spectrum, Taylor et al. 1996). We have been obtaining 5 GHz VLBI (global and VLBA) snapshot observations of the CJ-F sources (complete with 293 sources) since 1990 in order to create a valid database for thorough statistical tests of pc-scale jet motion in AGN. All 241 CJ-F AGN have been observed at least three times with the VLBA, which enables us to investigate jet component motions and paths. In particular, we concentrate on the analysis of those properties supposed to be essential for gamma-ray production, i.e., superluminal motion and bending.

So far all relativistically boosted jets with superluminal motion have only been detected in normal radio galaxies that have early-type host galaxies. We have now discovered superluminal motion in the Seyfert 1 galaxy III Zw 2, classified as a spiral. Spectral and spatial evolution are closely linked. Since III Zw 2 is part of a sample of so-called radio-intermediate quasars (RIQ), it confirms earlier predictions of superluminal motion for this source, based on the argument that RIQs could be relativistically boosted jets in radio-weak quasars and Seyfert galaxies.

Compact Symmetric Objects (CSOs) represent an early stage in the life of radio galaxies. Our kinematic studies over a 20 year timebase have unambiguously demonstrated that they expand fast and that the radio activity is recently born. The CSO evolution with time is however still a matter of investigation. If they are progenitors of the “classical double” radio sources, then in order to fit the the observed statistics of radio sources, CSOs must have decrease their luminosity and expansion speed with time. It is also possible that CSOs represent a phase of periodic activity of the central engine; in this case the presence of the old activity in the form of extended weak emission, should be observed.

VLBI observations of reported TeV gamma-ray sources are described and the inferences that can be drawn on the jet behaviour between the TeV variability-scale and the VLBI parsec-scale discussed.

We report VLBI observations of 15 EGRET-detected AGNs with European VLBI Network (EVN) at 5 GHz. All sources in the sample display core-jet structures.

The VLBI Space Observatory Programme (VSOP) combines an orbiting radiotelescope with arrays of ground radio telescopes to extend the Very Long Baseline Interferometry (VLBI) technique to baselines up to almost three Earth diameters. In this paper, we present results from VSOP observations of active galactic nuclei (AGN) at 1.6 and 5 GHz from the first 3.5 years of the mission.

The VSOP Survey Program contains 402 bright, small diameter extragalactic radio sources. Pre-launch observations indicated that 113 sources were probably undetectable on space-ground baselines, and so VSOP observations are being undertaken at 5 GHz of the remaining 289 sources. Progress to date is described.