Search

III Zw 2 shows dramatic radio outbursts roughly every five years. Here we present the light curves and VLBA observations of the latest flare with an excellent time sampling. We have detected superluminal motion with a lower limit for the apparent expansion speed of 1.25±0.09 c at 43 GHz. Spectral and spatial evolution are closely linked and the evolution of III Zw 2 in a turnover frequency vs. linear size diagram is similar to the correlation for GPS and CSS sources. Before and after this rapid expansion we observe a period of virtually no expansion. However, at 15 GHz III Zw 2 shows a constant slow expansion (∼0.6 c). The difference is qualitatively explained by optical depth effects in an ‘inflating-balloon model’, describing the evolution of radio lobes on an ultra-compact scale. The stop-and-go behaviour could be explained by a jet interacting with a molecular cloud or the molecular torus.

The cm-λ properties of BL Lac objects are discussed.

Forty AGN have been detected at high significance level by EGRET in the γ-ray band. Previous studies based on radio observations near or at the times of the EGRET detections suggest that there is a causal connection between individual events in these two wavebands. Here we examine the question of whether the cm-λ and the γ-ray activity are related.

We present ground-based data of the BL Lac object PKS 2155-305 obtained during a large international campaign spanning the electro–magnetic spectrum from the radio waves to X-rays in November 1991. For the complete description of the observations and data analysis we refer to the paper by Courvoisier et al. 1993, and references therein. The ground-based data include radio, infrared JHKL and UBVRI fluxes as well as optical and near IR polarimetry.The broad-band optical and near IR data from U to I exhibit the same behaviour in all bands: the flux nearly doubled over the well-covered period of 23 days. The cross-correlation function does not reveal any significant changes in the light-curves. Though significant variations in 24 hours have been recorded, the cumulated Fourier power spectrum drops to the noise level for periods shorter than 2.5 days. The spectral index remained constant.The polarised flux varied by a larger factor than the total flux and did not follow the same pattern. The degree of polarisation and polarisation angle are nearly independent of the wavelength and are strongly correlated in all filters.In the radio domain the spectral index increased from −0.1 on November 5 to +0.02 on 25-th.The absence of the lag between the optical and infrared bands and the polarisation variations are consistent with a model in which the variability is caused by micro-lensing of the source (Stickel, Fried and Kühr 1988). One would, however, expect in this model that the variation in the polarisation and the total flux are tightly correlated contrary to what is observed.The constant shape of the continuum spectral energy suggests that only the number of electrons whose emission is beamed towards the observer changes, rather than the arrival of fresh electrons that are being accelerated.The variability of the polarisation may be explained by changes in the geometry of the magnetic field (dominant direction). This is consistent with the observed variations of the polarisation angle.

To study whether the radio properties of BL Lacertae type objects and QSOs differ, we initiated a program in 1979 to monitor the total flux density and linear polarization at 14.5, 8.0 and 4.8 GHz of the strongest then known BL Lac objects which met the Hewitt-Burbidge criteria (Ledden, private communications) plus 3 subsequently identified high declination BL Lacs (Biermann et al. 1981). Results based on the behavior of the 45 brightest sample members are compared here with the properties of the QSOs in the flux-limited Pearson-Readhead sample (Pearson and Readhead 1988).

We have analyzed the cm-wavelength flux and polarization observations of four active objects (3C 279, OT 081, BL Lacertae, and 3C 446) for which the data exhibit signatures of shocks. Common characteristics of these sources are: high degrees of polarization, large changes in the polarized flux during outbursts; and stable position angles of the electric vector over long periods.

We report Very Large Array1 observations of 3C 58, the Crab Nebula's closest relative at radio wavelengths. We combined A, B, C, and D configurations (baselines ranging from 0.08 to 35 km) at frequencies of 1446 MHz and 4886 MHz, achieving resolutions of 1″.9 and 2″.5, respectively, and a sensitivity at both frequencies of about 70 μJy per beam. We deconvolved the point-source response from the images using a maximum-entropy technique. The 1446 MHz total-intensity image shows that filamentation not only dominates the spatial distribution of flux in the bright inner regions, but appears to continue into the faint outer envelope as well. This envelope fades smoothly into the noise over about half the circumference of the remnant; the lowest contours of our map show maximum extents in RA and δ of 10′.3 and 6′.3 respectively. However, in some places the edge is relatively well-defined, suggesting confinement of some kind. A remarkably jet-like extension can be seen protruding from the south central part of the remnant, with a position angle of about 150° with respect to the radio peak. Unlike the Crab's jet, this is wedge-shaped and somewhat resembles the brighter extension to the north. The X-ray point source (Becker, Helfand, and Szymkowiak 1982, Ap. J. 255, 557) lies about 30″ east of the peak in the radio map. No obvious radio feature coincides with it. We have made spectral-index maps of the remnant, and find only small variations, with little apparent correlation of spectral index with structure. The rms deviation from the remnant mean spectral index is 0.18, but profiles of filaments show less than 0.1 difference in spectral index between peaks and adjacent valleys. Thus the bright filaments are neither flatter nor steeper in spectrum than the remnant mean, ruling out a picture in which bright filaments are locations where shock acceleration produces a steeper particle energy spectrum. Filaments have brightness contrasts of factors of 1.5 to 2, steep edges (