[Paper Review] Continuing EVN monitoring of HST-1 in the jet of M87

Kazuhiro Hada, M. Giroletti+10 more|arXiv (Cornell University)|Apr 8, 2015

Astrophysics and Cosmic Phenomena3 citations

TL;DR

This study presents ongoing EVN monitoring of HST-1 in M87's jet at 1.7 GHz, shifting from 5 GHz due to declining brightness. It reveals a new superluminal component (comp4) at the upstream edge and confirms complex, non-ballistic trajectories with apparent speeds up to ~10c, suggesting helical motion and possible magnetic field influences.

ABSTRACT

The relativistic jet in M87 offers a unique opportunity for understanding the detailed jet structure and emission processes due to its proximity. In particular, the peculiar jet region HST-1 at ~1 arcsecond (or 80 pc, projected) from the nucleus has attracted a great deal of interest in the last decade because of its superluminal motion and broadband radio-to-X-ray outbursts, which may be further connected to the gamma-ray productions up to TeV energies. Over the last five years, we have been doing an intensive monitoring of HST-1 with EVN at 5GHz in order to examine the detailed structural evolution and its possible connection to high-energy activities. While this program already yielded interesting results in terms of the detailed mas-scale structure, proper motion measurements and structural variations, the recent HST-1 brightness is continuously decreasing at this frequency. To counter this, we have shifted our monitoring frequency to 1.7GHz from October 2013. This strategy successfully recovered the fainter emission that was missed in the last 5GHz session. Moreover, we again discovered the sudden emergence of a new component at the upstream edge of HST-1, demonstrating that the use of EVN 1.7GHz is indeed powerful to probe the current weak nature of HST-1. Here we report early results from the 1.7GHz monitoring as well as further progress on the long-term kinematic study.

Motivation & Objective

• Investigate the kinematic evolution of HST-1 subcomponents in M87’s relativistic jet to understand their connection to high-energy γ-ray flares.
• Overcome declining brightness at 5 GHz by switching to 1.7 GHz to maintain sensitivity to weak, extended emission.
• Characterize the three-dimensional structure and motion of compact components in HST-1 using long-term VLBI monitoring.
• Examine whether curved trajectories and oscillating speeds indicate helical jet paths or magnetic field influences.
• Assess the extended jet structure around HST-1 to determine if bright components occupy only a fraction of the jet cross-section.

Proposed method

• Conduct long-term Very Long Baseline Interferometry (VLBI) monitoring of HST-1 using the European VLBI Network (EVN) at 1.7 GHz, following a shift from 5 GHz due to spectral steepening.
• Combine 1.7 GHz EVN data with archival 1.7 GHz VLBA data to achieve high angular resolution and sensitivity for structural and kinematic analysis.
• Perform least-squares linear fitting to distance-versus-time plots of subcomponents to measure apparent proper motions and speeds.
• Analyze two-dimensional sky-plane trajectories of components to detect deviations from straight-line motion and infer three-dimensional path geometry.
• Use deep EVN 1.7 GHz observations with eMERLIN to enhance dynamic range and detect fainter, extended emission around HST-1.
• Correlate kinematic features (e.g., direction changes, speed oscillations) with historical VHE γ-ray flares (2008, 2010) to test for causal links.

Experimental results

Research questions

• RQ1 Do the observed superluminal motions and curved trajectories of HST-1 components indicate three-dimensional helical paths in the jet?
• RQ2 Is there a correlation between the kinematic oscillations (in speed and direction) of HST-1 components and the timing of TeV γ-ray flares?
• RQ3 How does the extended jet structure around HST-1 compare to the parabolic collimation profile of the inner jet, and what does this imply about the jet’s cross-sectional size?
• RQ4 Can 1.7 GHz EVN observations resolve fainter, previously missed emission features that are critical for understanding HST-1’s overall structure?
• RQ5 Do the observed changes in component motion and trajectory support a recollimation shock model at HST-1?

Key findings

• The 1.7 GHz EVN monitoring successfully recovered fainter emission missed at 5 GHz, confirming the utility of lower-frequency observations for weak, steep-spectrum components.
• A new compact component (comp4) emerged at the upstream edge of HST-1, with a tentative apparent speed of ~10c (±5c), indicating rapid ejection.
• Long-lived components comp1, comp2, and comp3 exhibit superluminal motion with apparent speeds of 4.5c, 4.5c, and 5.1c, respectively, over the monitoring period.
• Trajectories of comp1, comp2, and comp3 are not straight lines but smoothly curved, with significant transverse motions (up to ~1c) perpendicular to the jet axis.
• Component comp3 showed a distinct turnaround, initially ejected at PA ~310°, then bending toward the jet axis, suggesting complex 3D dynamics.
• Oscillations in apparent speed and direction are observed on multi-year timescales, potentially indicating helical motion or magnetic field modulation.