The compact jets of TeV blazars

I present multi-epoch 15, 22, and 43 GHz VLBA images of Mkn 421 and Mkn 501 at several epochs each, following outbursts of TeV radiation. The resoluti on ranges from 0.2 to 0.6 mas in the direction of the most compact structur e, corresponding to a linear resolution of 0.15 to 0.4 pc (projected onto t he plane of the sky) for a Hubble constant of 65 km s(-1) Mpc(-1). In Mkn 4 21, the jet is very weak relative to the core, and components are difficult to identify across epochs. Nevertheless, the data indicate outward motion at about 0.9-1.35 mas yr(-1), or 2-3c. In Mkn 501, the jet is more prominen t, with a bend by about 90 degrees about 3 mas from the core. A resolved co mponent between 0.5 and 1 mas from the core has a proper motion of 0.96 +/- 0.1 mas yr(-1), or 2.5 +/- 0.3c. In neither object is there any indication of bright components moving down the jet that would have been ejected by t he TeV events. The magnetic field of the jet in Mkn 501 is perpendicular to the jet near the axis and parallel near the boundary, while that of Mkn 42 1 is parallel. I interpret the rather low superluminal apparent speeds and lackluster vari ability properties of the radio jets as evidence that the bulk flow of the jets decelerates from the TeV emitting section to the radio emitting region . In Mkn 501, other authors have found that the apparent speed is even lowe r a few mas from the core, beyond the bend. This weakening of the jet with distance from the central engine is readily explained by energy and forward -momentum loss of the relativistic electrons and positrons (if the latter a re much more numerous than protons). This would occur in cases such as TeV blazars in which the slope of the energy distribution of the electrons is f latter than -2. The parallel magnetic field at the boundary is then explain ed by shearing caused by interaction with the external medium. (C) 1999 Els evier Science B.V. All rights reserved.