The kinematic of HST-1 in the jet of M87

TL;DR

This study investigates the structural dynamics of HST-1 in the M87 jet using VLBI observations, revealing superluminal motions and potential links to high-energy activity episodes.

Contribution

It provides detailed measurements of HST-1's component motions and structural changes, connecting these to VHE activity for the first time.

Findings

HST-1 detected at all epochs with complex multi-component structure.

Outer components exhibit superluminal motion around 4c.

Structural changes correlate with VHE activity episodes.

Abstract

Aims: We aim to constrain the structural variations within the HST-1 region downstream of the radio jet of M87, 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 M87 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 ~4c. 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 -65deg to -90deg, 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.