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B.G. Piner, S.C. Unwin (JPL), A.E. Wehrle (IPAC), P.G. Edwards (ISAS), A.L. Fey, K.A. Kingham (USNO)
We present thirty VLBI images of the TeV blazar Markarian 421 (1101+384) at fifteen epochs spanning the time range from 1994 to 1997, and at six different frequencies from 2.3 to 43 GHz. The imaged observations include a high-resolution 5 GHz VLBI Space Observatory Program (VSOP) observation with the HALCA satellite from 1997 November 14; full-track VLBA observations at 5, 15, 22, and 43 GHz from 1994 April, 1996 November, and 1997 May; six epochs of VLBA snapshot observations at 2, 8, and 15 GHz done as part of a study of the Radio Reference Frame; and five geodetic VLBI observations at 2 and 8 GHz from the archive of the Washington VLBI Correlator Facility located at the U.S. Naval Observatory. The dense time coverage of the images allows us to unambiguously track components 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 to 3 pc projected linear distance) to be 0.19±0.27, 0.30±0.07, and -0.07±0.07~c (H0=65 km s-1 Mpc-1, q0=0.1). 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 & Bå\aa th from three epochs of data from the early 1980's. Possible interpretations of these subluminal speeds in terms of the high Doppler factor demanded by the TeV variability of this source are discussed. Part of the work described in this paper has been carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.
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