The kinematic of HST-1 in the jet of M 87

M. Giroletti¹, K. Hada^2,3, G. Giovannini^1,4, C. Casadio⁵, M. Beilicke⁶, A. Cesarini⁷, C. C. Cheung⁸, A. Doi⁹, H. Krawczynski⁶, M. Kino³, N. P. Lee¹⁰ and H. Nagai³

¹ INAF Istituto di Radioastronomia, via Gobetti 101, 40129 Bologna, Italy
e-mail: giroletti@ira.inaf.it
² The Graduate University for Advanced Studies (SOKENDAI), 2-21-1 Osawa, Mitaka, 181-8588 Tokyo, Japan
³ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, 181-8588 Tokyo, Japan
⁴ Dipartimento di Astronomia, via Ranzani 1, 40127 Bologna, Italy
⁵ Instituto de Astrofisica de Andalucia, CSIC, Apartado 3004, 18080 Granada, Spain
⁶ Department of Physics, Washington University, St. Louis, MO 63130, USA
⁷ School of Physics, National University of Ireland, University Road, Galway, Republic of Ireland
⁸ National Research Council Research Associate, National Academy of Sciences, Washington, DC 20001, resident at Naval Research Laboratory, Washington DC 20375, USA
⁹ Institute of Space and Astronautical Science, JAXA, 3-1-1 Yoshinodai, Sagamihara, 229-8510 Kanagawa, Japan
¹⁰ Smithsonian Astrophysical Observatory, 60 Garden St., Cambridge MA 02138, USA

Received: 9 January 2012
Accepted: 26 January 2012

Abstract

Aims. We aim to constrain the structural variations within the HST-1 region downstream of the radio jet of M 87, in general as well as in connection to the episodes of activity at very high energy (VHE).

Methods. We analyzed and compared 26 VLBI observations of the M 87 jet, obtained between 2006 and 2011 with the Very Long Baseline Array (VLBA) at 1.7 GHz and the European VLBI Network (EVN) at 5 GHz.

Results. HST-1 is detected at all epochs; we model-fitted its complex structure with two or more components, the two outermost of which display a significant proper motion with a superluminal velocity around ~4 c. The motion of a third feature that is detected upstream is more difficult to characterize. The overall position angle of HST-1 has changed during the time of our observations from −65° to −90°, while the structure has moved by over 80 mas downstream. Our results on the component evolution suggest that structural changes at the upstream edge of HST-1 can be related to the VHE events.