VLBI detection of the HST-1 feature in the M 87 jet at 2 cm

Context. A bright feature 80 pc away from the core in the powerful jet of M 87 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. Aims. To examine whether HST-1 has a blazar-like nature, we analyzed λ2 cm VLBA archival data from dedicated full-track obser- vations and the 2 cm survey/MOJAVE VLBI monitoring programs performed between 2000 and 2009. Methods. We studied the subparsec scale structure of M 87 jet by using wide-field imaging techniques, after checking different weight- ing of the interferometric visibilities as a function of distance. The HST-1 region was imaged at milliarcsecond resolution. Results. We present the first 2 cm VLBI detection of HST-1 in observations performed between early 2003 and early 2007, and ana- lyze 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 λ2 cm and λ20 cm. Conclusions. We do not find evidence that HST-1 has a blazar nature.eros@uv.e