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

C. S. Chang¹, E. Ros^1,2, Y. Y. Kovalev^3,1 and M. L. Lister⁴

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: [cschang;ros]@mpifr.de
² Departament d'Astronomia i Astrofísica, Universitat de València, 46100 Burjassot, Spain
³ Astro Space Centre of Lebedev Physical Institute, Profsoyuznaya 84/32, 117997 Moscow, Russia e-mail: yyk@asc.rssi.ru
⁴ Department of Physics, Purdue University, 525 Northwestern Avenue, West Lafayette, IN 47907, USA e-mail: mlister@physics.purdue.edu

Received: 18 December 2009
Accepted: 12 February 2010

Abstract

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 observations 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 weighting 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 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 ) between λ2 cm and λ20 cm.

Conclusions. We do not find evidence that HST-1 has a blazar nature.