The subluminal parsec-scale jet of Markarian 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 Markarian 501. Data from the Very Long Baseline Array (VLBA) 2 cm survey, and 8 and 15 GHz data from the Radio Reference Frame Image Database, have been combined with data from a 5 GHz VLBI Space Observatory Programme observation to determine the apparent motions of jet components in this source. The combined data set consists of 12 observations between 1995 April and 1999 July. Four jet components are detected at most epochs, all of which are clearly subluminal (i.e., with apparent speeds less than the speed of light) and two of which appear stationary. The established TeV gamma-ray sources Mrk 501 and Mrk 421 thus both have subluminal parsec-scale jets, in contrast to the apparently superluminal jets of the majority of GeV 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 GeV gamma-ray sources.