The kinematic of HST-1 in the jet of M87
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.
💡 Research Summary
The paper presents a comprehensive VLBI study of the HST‑1 region in the M87 jet, using 26 observations obtained between 2006 and 2011 with the VLBA at 1.7 GHz and the EVN at 5 GHz. By modelling the complex radio morphology of HST‑1 with multiple Gaussian components, the authors track the motion of individual features over a five‑year baseline. Two outermost components (designated A and B) exhibit a clear linear trajectory with an apparent proper motion of ~0.2 mas yr⁻¹, which translates to a super‑luminal speed of roughly 4 c when the distance to M87 (≈16.7 Mpc) is taken into account. This confirms earlier hints that HST‑1 hosts ultra‑relativistic plasma blobs capable of producing apparent faster‑than‑light motion due to projection effects.
A third, more upstream component (C) is detected intermittently. Its position changes are irregular and its signal‑to‑noise ratio is low, preventing a reliable speed estimate. The authors suggest that C may represent a nascent acceleration site or the remnants of a decaying blob, indicating that HST‑1 is not a static knot but a dynamically evolving region where new plasma condensations are continuously formed and older ones advect downstream.
Beyond individual component motions, the study reveals a systematic rotation of the overall position angle of the HST‑1 complex from –65° to –90° over the observing period, amounting to a ~25° swing. Simultaneously, the entire structure drifts more than 80 mas (≈6.5 pc) downstream. These trends point to a gradual bending of the jet flow and to pressure gradients or interactions with the external medium that steer the jet material.
Crucially, the authors compare the timing of the component evolution with two very‑high‑energy (VHE) gamma‑ray flares recorded in 2008 and 2010. In both cases, an increase in the brightness of the upstream component C precedes the VHE event, while the outer components A and B show a temporary acceleration. After the flares, the total radio flux of HST‑1 declines, suggesting rapid energy losses and possible re‑acceleration of particles. This temporal correlation supports a scenario in which particle acceleration initiates upstream, producing a compact high‑energy emitting zone, and the energized plasma then propagates downstream as the super‑luminal blobs observed in the radio band.
The authors conclude that HST‑1 should be regarded as a “multi‑blob accelerator” rather than a single stationary knot. Its internal dynamics—super‑luminal motions, position‑angle rotation, and episodic brightening—appear intimately linked to the production of VHE gamma‑rays. The paper emphasizes the need for higher‑cadence, multi‑frequency observations (including millimeter‑wave VLBI and X‑ray monitoring) to directly measure magnetic field strengths, particle energy distributions, and to refine models of shock‑driven acceleration in relativistic jets. In sum, the work provides robust observational evidence that the structural evolution of HST‑1 is a key driver of the extreme high‑energy phenomena observed in M87.