Parsec-scale jet direction evolution in AGNs

TL;DR

This study investigates the long-term variability of parsec-scale jet directions in AGNs across multiple frequencies, revealing significant jet rotation and constraining potential periodic mechanisms behind these variations.

Contribution

Developed an automatic method for measuring jet directions and analyzed a large AGN sample to characterize jet variability and its frequency dependence.

Findings

Strong jet direction variations in 25% of AGNs

Average apparent jet rotation speeds increase with frequency

Jet variability periods are constrained to less than 1000 years

Abstract

We analyze the variability of the parsec-scale jet directions in active galactic nuclei (AGNs). Our analysis involves 317 AGNs at frequencies ranging from 2 to 43 GHz, and is made possible by developing an automatic inner jet direction measurement procedure. We find strong significant variations in a one quarter of these AGNs; the effect is likely ubiquitous, and not detected in the rest due to a limited sensitivity and observations epoch coverage. Average apparent jet rotation speeds range from 0.21 deg/yr at 2 GHz to 1.04 deg/yr at 43 GHz. This strong frequency dependence indicates that the variability cannot be explained by jet components propagating ballistically without acceleration: more complex jet shapes or patterns are required. Still, we demonstrate that the apparent direction changes are predominantly caused by the jet nozzle rotations, and not by individual components propagating transversely to the jet. In this work, we focus on variability scales much longer than the times of observations, that is > 50 years. Using our measurements, we bound potential periods to less than 1000 years in the source rest frame for 90% AGNs in the sample. This allows us to constrain mechanisms causing these variations if they are periodic, such as instabilities, disk-driven precession, or binary black hole effects.