The Sub-Luminal Parsec-scale Jet of Mkn 501
  We have combined Very Long Baseline Interferometry (VLBI) data from several programs in order to resolve differences in reported parsec-scale jet speeds for the TeV gamma-ray source Mkn501. Data from the VLBA 2cm survey, and 8 and 15 GHz data from the Radio Reference Frame Image Database, have been combined with data from a 5 GHz VSOP Space VLBI observation to determine the apparent motions of jet components in this source. The combined data set consists of 12 observations between April 1995 and July 1999. Four jet components are detected at most epochs, all of which are clearly sub-luminal (i.e. with apparent speeds < c), and two of which appear stationary. The established TeV gamma-ray sources Mkn 501 and Mkn 421 thus both have sub-luminal parsec-scale jets, in contrast to the apparently super-luminal jets of the majority of >100 MeV sources detected by EGRET. No new VLBI component has emerged from the core following the extended TeV high-state in 1997, again in contrast to the general behavior of >100 MeV gamma-ray sources. 
P. G. Edwards and B. G. Piner