Jet Collimation and Acceleration in the Flat Spectrum Radio Quasar 1928+738

TL;DR

This study presents the first direct observation of an acceleration and collimation zone in an FSRQ jet, revealing how the jet's shape and speed evolve near the black hole, driven by magnetic energy conversion and external confinement.

Contribution

It provides detailed measurements of jet geometry and kinematics, identifying the acceleration and collimation zone in an FSRQ and linking it to black hole influence and magnetic energy conversion.

Findings

Inner jet is parabolic, outer jet is conical.

Jet accelerates up to Lorentz factor ~10.

Acceleration zone aligns with collimation region.

Abstract

Using time-resolved multifrequency Very Long Baseline Array data and new KaVA (KVN and VERA Array) observations, we study the structure and kinematics of the jet of the flat spectrum radio quasar (FSRQ) 1928+738. We find two distinct jet geometries as function of distance from the central black hole, with the inner jet having a parabolic shape, indicating collimation, and the outer jet having a conical shape, indicating free expansion of the jet plasma. Jet component speeds display a gradual outward acceleration up to a bulk Lorentz factor $\Gamma_{\rm max} \approx10$, followed by a deceleration further downstream. The location of the acceleration zone matches the region where the jet collimation occurs; this is the first direct observation of an acceleration and collimation zone (ACZ) in an FSRQ. The ACZ terminates approximately at a distance of 5.6$\times 10^6$ gravitational radii, which is in good agreement with the sphere of gravitational influence of the supermassive black hole, implying that the physical extent of the ACZ is controlled by the black hole gravity. Our results suggest that confinement by an external medium is responsible for the jet collimation and that the jet is accelerated by converting Poynting flux energy to kinetic energy.