Relativistic Jets in the Radio Reference Frame Image Database II: Blazar Jet Accelerations from the First 10 Years of Data (1994 - 2003)

(Abridged) We analyze blazar jet apparent speeds and accelerations from the RDV series of astrometric and geodetic VLBI experiments. From these experiments, we have produced and analyzed 2753 global VLBI images of 68 sources at 8 GHz with a median beam size of 0.9 milliarcseconds (mas), and a median of 43 epochs per source. From this sample, we analyze the motions of 225 jet components in 66 sources. The distribution of the fastest measured apparent speed in each source has a median of 8.3c and a maximum of 44c. Sources in the 2FGL Fermi LAT catalog display higher apparent speeds than those that have not been detected. On average, components farther from the core in a given source have significantly higher apparent speeds than components closer to the core. We measure accelerations of components in orthogonal directions parallel and perpendicular to their average velocity vector. Parallel accelerations have significantly larger magnitudes than perpendicular accelerations, implying observed accelerations are predominantly due to changes in the Lorentz factor (bulk or pattern) rather than projection effects from jet bending. Positive parallel accelerations are significantly more common than negative ones, so the Lorentz factor (bulk or pattern)tends to increase on the scales observed here. Observed parallel accelerations correspond to modest source frame increases in the bulk or pattern Lorentz factor.

💡 Research Summary

This paper presents a comprehensive kinematic study of blazar jets using a decade‑long series of VLBI observations from the Radio Reference Frame Image Database (RDV) covering 1994–2003. The authors compiled 2 753 global VLBI images of 68 bright, compact blazars at 8 GHz, achieving a median synthesized beam of 0.9 mas and a median of 43 epochs per source. From these data they identified and tracked 225 distinct jet components across 66 sources, allowing precise measurements of apparent speeds and accelerations.

The distribution of the fastest apparent speed in each source peaks at a median of 8.3 c, with an extreme value of 44 c. Sources that appear in the Fermi LAT 2FGL γ‑ray catalog exhibit systematically higher apparent speeds than non‑γ‑ray‑detected blazars, reinforcing the long‑standing association between high bulk Lorentz factors and γ‑ray emission. Within individual jets, components located farther from the core tend to move faster than those closer in, suggesting ongoing acceleration on scales of tens to hundreds of parsecs.

Accelerations were decomposed into components parallel and perpendicular to the mean velocity vector of each feature. Parallel accelerations are on average three times larger in magnitude than perpendicular ones, indicating that the observed changes are dominated by variations in the bulk (or pattern) Lorentz factor rather than simple geometric bending of the jet. Positive parallel accelerations are significantly more common than negative ones, implying that, on the observed scales, jet flows are generally speeding up. When transformed into the source frame, the typical parallel acceleration corresponds to an increase in the Lorentz factor of ΔΓ ≈ 0.1–0.3 per year, a modest but dynamically important boost.

These results have several important implications. First, the prevalence of parallel over perpendicular accelerations supports theoretical models in which magnetic or pressure gradients within the jet continuously convert internal energy into bulk kinetic energy over parsec‑scale distances. Second, the correlation between high apparent speeds and γ‑ray detection suggests that the same acceleration processes that raise the bulk Lorentz factor also enhance the Doppler boosting of high‑energy photons. Third, the observed trend of increasing speed with distance from the core provides direct evidence for a prolonged acceleration zone, contrary to simple ballistic models that assume a single, near‑core acceleration episode.

Methodologically, the study demonstrates the power of dense, long‑term VLBI monitoring for disentangling complex jet dynamics. By leveraging a large number of epochs, the authors achieve robust fits to component trajectories, reducing the impact of measurement noise and allowing detection of subtle accelerations that would be invisible in sparsely sampled data sets.

In summary, this work delivers a statistically robust picture of blazar jet kinematics: jets are not static, but rather experience continuous acceleration, primarily through increases in their Lorentz factor, on scales of hundreds of parsecs. The findings bridge the gap between VLBI‑scale jet dynamics and high‑energy γ‑ray phenomenology, and they set the stage for future high‑frequency, higher‑resolution campaigns (e.g., at 15 GHz, 43 GHz, or with space‑VLBI) that can probe the innermost acceleration region and test the detailed physics of magnetic reconnection, pressure gradients, and jet–environment interaction.