2010A&A...515A..38C

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 because of these properties. To examine whether HST-1 has a blazar-like nature, we analyzed λ2cm VLBA archival data from dedicated full-track observations and the 2cm survey/MOJAVE VLBI monitoring programs performed between 2000 and 2009. We studied the subparsec scale structure of M87 jet by using wide-field imaging techniques, after checking different weighting of the interferometric visibilities as a function of distance. The HST-1 region was imaged at milliarcsecond resolution. We present the first 2cm VLBI detection of HST-1 in observations performed between early 2003 and early 2007, and analyze its evolution with time. Using the detections of HST-1, we find that the projected apparent speed is 0.61±0.31c. A comparison of the VLA and VLBA flux densities of this feature indicates that it is mostly resolved on milliarcsecond scales. This feature is optically thin (α~-0.8 for S∝ν^+α) between λ2cm and λ20cm. We do not find evidence that HST-1 has a blazar nature.