The Radio Properties of Radio-Loud Narrow-Line Seyfert 1 Galaxies on Parsec Scales

TL;DR

This study uses VLBA observations at 5 GHz to analyze the parsec-scale radio structures of 14 radio-loud narrow-line Seyfert 1 galaxies, revealing diverse morphologies, non-thermal jets, and evidence for slow jet speeds, contributing to understanding jet physics in these active galaxies.

Contribution

First high-resolution VLBA imaging of 14 radio-loud NLS1 galaxies, revealing diverse jet morphologies and evidence for slow, mildly relativistic jets in these sources.

Findings

50% show core-only structures

50% exhibit core-jet structures

Brightness temperatures indicate non-thermal jets

Abstract

We present the detection of compact radio structures of fourteen radio-loud narrow line Seyfert 1 (NLS1) galaxies from Very Long Baseline Array observations at 5 GHz, which were performed in 2013. While 50\% of the sources of our sample show a compact core only, the remaining 50\% exhibit a core-jet structure. The measured brightness temperatures of the cores range from $10^{8.4}$ to $10^{11.4}$ K with a median value of $10^{10.1}$ K, indicating that the radio emission is from non-thermal jets, and that, likely, most sources are not strongly beamed, then implying a lower jet speed in these radio-loud NLS1 galaxies. In combination with archival data taken at multiple frequencies, we find that seven sources show flat or even inverted radio spectra, while steep spectra are revealed in the remaining seven objects. Although all these sources are very radio-loud with $R > 100$, their jet properties are diverse, in terms of their milli-arcsecond (mas) scale (pc scale) morphology and their overall radio spectral shape. The evidence for slow jet speeds (i.e., less relativistic jets), in combination with the low kinetic/radio power, may offer an explanation for the compact VLBA radio structure in most sources. The mildly relativistic jets in these high accretion rate systems are consistent with a scenario, where jets are accelerated from the hot corona above the disk by the magnetic field and the radiation force of the accretion disk. Alternatively, a low jet bulk velocity can be explained by low spin in the Blandford-Znajek mechanism.