Collimation, Acceleration and Recollimation Shock in the Jet of Gamma-Ray-emitting Radio-Loud Narrow-Line Seyfert 1 Galaxy 1H 0323+342

TL;DR

This study reveals the detailed jet structure of the gamma-ray emitting NLS1 galaxy 1H 0323+342, showing collimation, acceleration, and recollimation shocks similar to those in other active galactic nuclei, suggesting a common jet formation mechanism.

Contribution

It provides high-resolution multi-frequency VLBA observations of 1H 0323+342, identifying the jet's collimation break, recollimation shock, and acceleration zone, and compares these features to other AGN.

Findings

Jet has a parabolic shape near the core indicating collimation.

Recollimation shock is located at a bright stationary feature.

Jet structure is similar to that of M87 and blazars.

Abstract

We investigated the detailed radio structure of the jet of 1H 0323+342 using high-resolution multi-frequency Very Long Baseline Array observations. This source is known as the nearest $\gamma$-ray emitting radio-loud narrow-line Seyfert 1 (NLS1) galaxy. We discovered that the morphology of the inner jet is well characterized by a parabolic shape, indicating the jet being continuously collimated near the jet base. On the other hand, we found that the jet expands more rapidly at larger scales, resulting in a conical-like shape. The location of the "collimation break" is coincident with a bright quasi-stationary feature at 7 mas from core (corresponding to a deprojected distance of the order of $\sim$100pc), where the jet width locally contracts together with highly polarized signals, suggesting a recollimation shock. We found that the collimation region is coincident with the region where the jet speed gradually accelerates, suggesting the coexistence of the jet acceleration and collimation zone, ending up with the recollimation shock, which could be a potential site of high-energy $\gamma$-ray flares detected by the Fermi-LAT. Remarkably, these observational features of the 1H 0323+342 jet are overall very similar to those of the nearby radio galaxy M87 and HST-1 as well as some blazars, suggesting that a common jet formation mechanism might be at work. Based on the similarity of the jet profile between the two sources, we also briefly discuss the mass of the central black hole of 1H 0323+342, which is also still highly controversial on this source and NLS1s in general.