The microarcsecond structure of an AGN jet via interstellar scintillation

TL;DR

This paper introduces a novel method using interstellar scintillation to achieve microarcsecond resolution in studying AGN jets, revealing their structure and physical properties with unprecedented precision.

Contribution

The study presents a new technique for probing ultracompact AGN jets at microarcsecond scales through frequency-dependent scintillation analysis, enabling detailed core-shift measurements.

Findings

Resolved core-shift as a function of frequency below ~12 microarcseconds.

Found that jet size scales with frequency as d ∝ ν^{-0.64}.

Indicated the jet opening angle increases with distance, consistent with a hydrostatic confinement model.

Abstract

We describe a new tool for studying the structure and physical characteristics of ultracompact AGN jets and their surroundings with microarcsecond precision. This tool is based on the frequency dependence of the light curves observed for intra-day variable radio sources, where the variability is caused by interstellar scintillation. We apply this method to PKS1257-326 to resolve the core-shift as a function of frequency on scales well below ~12 microarcseconds. We find that the frequency dependence of the position of the scintillating component is r \propto \nu^{-0.1 \pm 0.24} (99% confidence interval) and the frequency dependence of the size of the scintillating component is d \propto \nu^{-0.64 \pm 0.006}. Together, these results imply that the jet opening angle increases with distance along the jet: d \propto r^{n_d}$ with n_d > 1.8. We show that the flaring of the jet, and flat frequency dependence of the core position is broadly consistent with a model in which the jet is hydrostatically confined and traversing a steep pressure gradient in the confining medium with p \propto r^{-n_p} and n_p > 7. Such steep pressure gradients have previously been suggested based on VLBI studies of the frequency dependent core shifts in AGN.