VLBI detection of the HST-1 feature in the M87 jet at 2 cm
  A bright feature 80 pc away from the core in the powerful jet of M87 shows
highly unusual properties. Earlier radio, optical and X-ray observations have
shown that this feature, labeled HST-1, is superluminal, and is possibly
connected with the TeV flare detected by HESS in 2005. It has been claimed that
this feature might have a blazar nature, due to these properties. To examine
the possible blazar-like nature of HST-1, we analyzed lambda 2 cm VLBA archival
data from dedicated full-track observations and the 2 cm survey/MOJAVE VLBI
monitoring programs obtained between 2000 and 2009. Applying VLBI wide-field
imaging techniques, the HST-1 region was imaged at milliarcsecond resolution.
Here we present the first 2 cm VLBI detection of this feature in observations
from early 2003 to early 2007, and analyze its evolution over this time. Using
the detections of HST-1, we find that the projected apparent speed is 0.61 +/-
0.31 c. A comparison of the VLA and VLBA flux densities of this feature
indicate that is mostly resolved on molliarcsecond scales. This feature is
optically thin between lambda 2 cm and lambda 20 cm. We do not find evidence of
a blazar nature for HST-1.
