We present results of observations of two BL Lac objects, 0735+178 and 0723–008, with the European VLBI Network (EVN) at 1.7 GHz. These objects have a distorted morphology, characterized by a large misalignment between their inner and outer radio structures. Our EVN observations at intermediate resolution reveal the presence of a highly curved jet in 0735+178, providing evidence for continuity of the radio emission from milliarcsecond to arcsecond scales. No indication of connecting radio structure is found in the case of 0723–008.

The structural evolution of the BL Lac object OJ287 has been studied with milliarcsecond resolution by using 8.4 GHz geodetic VLBI data from the Crustal Dynamics Program. Such data provide valuable maps which are useful to track superluminal components on short time scales (Charlot 1992). We have mapped and model-fitted OJ287 at ten epochs separated by intervals of a few months between 1985.4 and 1988.1 (Fig. 1a). The structure of OJ287 over this time span consists of two components, one of which is the source core, while the other one is identified with the knot K3, ejected during the 1983–1984 optical outburst. Our models indicate that the knot K3 moves along a non-radial path, with evidence for an apparent deceleration by mid-1986 and a reacceleration afterwards (Fig. 1b). We explain this peculiar motion by a projection effect due to a helical morphology of the jet. Our proposed helical model, estimated by considering the sky positions of K3 together with those of the earlier knots K1 and K2 (Roberts et al. 1987, Gabuzda et al. 1989), corresponds to a helix with a pitch angle of 20° on a narrow cone of half opening angle 3.4°, and central axis inclined at 17° relative to the line of sight. Ejection of VLBI components appears to occur simultaneously with optical outbursts and at mid-time between them. This finding is consistent with the supermassive binary black hole model previously proposed to explain those periodical outbursts (Sillanpää et al. 1988). Comparison of our helical fit with hydrodynamical models (Hardee et al. 1994) provides estimates of the radius (R ≃ 0.015 pc at a distance of 5 pc from the core), opening angle (ψ ≃ 0.1°), and Mach number (M ≃ 60) of the jet, and shows that the orbital 9.0 yr period of the binary black hole system is adequate to drive the helical perturbation if the jet propagates in a hot ambient medium with an external sound speed of 5000 km/s.

It seems now important to consider bulk velocity gradients inside jets (Blandford, 1993; Biretta et al, 1995) and recent results from tomography technique suggest the presence of different components in radio sources (Rudnick, this symposium). A few jet models take explicitly into account two components with different bulk velocities (Smith, Raine, 1985; Baker et al, 1988; Sol et al, 1989; Achatz et al, 1990). A fast beam comes from the vicinity of the black hole while a slower collimated wind is emitted by the accretion disk. When stable, the two components can survive along the jet. If a fast instability develops at a distance Dc from core, the beam is destroyed over some relaxation length. Its energy and momentum are transmitted to the wind. Apparent change in the flow regime such as slowing down, decollimation, local bending or discontinuity in surface brightness is the expected observational signature of such critical zones on radio maps. The exact appearance of the zone depends on the parameters of the two flows and on their interaction regime.

BL Lac objects often show a quite distorted radio morphology. Almost 75% of the BL Lacs for which the information is available show an apparent misalignment angle ΔPA between the VLBI jet and the large scale radio structure larger than 45 degrees. This can be explained by strong enhancement of slight bending due to projection effects, especially if BL Lacs are the most highly beamed sources. However we recently performed a statistical analysis of misalignment angle histograms for 155 extragalactic radio sources of different types and found that the intrinsic distortion is significantly more important in BL Lacs than in quasars and even CSS sources. Indeed the best fits of the δPA histograms by a simple bend model correspond to γɸ = 123° for BL Lacs, 37° for quasars and 36° for CSS sources, where ɸ and γ are the jet typical intrinsic bend and Lorentz factor within a given class of sources (Appl et al, 1995). If, as currently thought, jets in BL Lacs have smaller Lorentz factors than in quasars, high intrinsic bending and misalignment appear to be the rule in BL Lac sources.

VLBI polarization data show magnetic field configuration parallel to the nuclear jet in quasars and perpendicular to it in BL Lac objects. It appears difficult to account for this contrast within the unified scheme for AGN. To investigate a direct explanation of this peculiarity of BL Lac objects, we study the possibilities of propagation and radiation of beams of particles in transverse ambient magnetic field. High energy streams with kinetic energy density larger than the ambient magnetic one, Ekin > Emag, can easily propagate with enhancement of the transverse magnetic field at the leading edge of the stream and reconnection of magnetic lines in its wake. Synchrotron radiation in front shocks naturally leads to the observed polarization. Moreover self-polarization, with formation of charge layers and E × B drift velocity, allows substantial propagation for even lower energy streams with Ekin < Emag, as long as their density no is large enough, typically κ = 4πnomic2/B2 > 1. Such low energy streams are non diamagnetic and do not modify the ambient field. Any high energy tail of the total particle distribution in the jet therefore radiates in the transverse field pattern. This concerns for instance the BL Lac object W Comae if we assume a proton-electron jet with bulk velocity vo = 0.1c (as the source does not require relativistic beaming so far), an equipartition magnetic field B = 0.02 G and a density of radiating particles nr = 0.05cm−3 at about 7 pc from the nucleus (knot K3). For nr/no = 10−3, one gets the stream density no = 50cm−3 which allows good propagation as κ reaches 2 × 103, and still corresponds to a moderate mass outflow of 0.06 M⊙ /year for a VLBI jet cross-sectional area of 2pc2.