VSOP and Ground-based VLBI Imaging of the TeV Blazar Markarian 421 at Multiple Epochs

We present 30 VLBI images of the TeV blazar Markarian 421 (1101+384) at 15 epochs spanning the time range 1994-1997 and at six different frequencies from 2.3 to 43 GHz. The imaged observations include a high-resolution 5 GHz VLBI Space Observatory Programme (VSOP) observation with the HALCA satellite on 1997 November 14; full-track Very Long Base Array (VLBA) observations from 1994 April, 1996 November, and 1997 May at frequencies between 5 and 43 GHz; six epochs of VLBA snapshot observations at frequencies between 2 and 15 GHz from Radio Reference Frame studies; and five geodetic VLBI observations at 2 and 8 GHz from the archive of the Washington VLBI Correlator Facility located at the US Naval Observatory. The dense time coverage of the images allows us to track components unambiguously in the parsec-scale jet over the observed time range. We measure the speeds of three inner jet components located between 0.5 and 5 mas from the core (0.3-3 pc projected linear distance) to be 0.19 ± 0.27c, 0.30 ± 0.07c, and -0.07 ± 0.07c (H0 = 65 km s-1 Mpc-1). If the sole 43 GHz image is excluded, all measured speeds are consistent with no motion. These speeds differ from tentative superluminal speeds measured by Zhang & Bååth from three epochs of data from the early 1980s. Possible interpretations of these subluminal speeds in terms of the high Doppler factor demanded by the TeV variability of this source are discussed.