NOCTURNE. I. The radio spectrum of narrow-line Seyfert 1 galaxies

TL;DR

This study investigates the radio spectrum of narrow-line Seyfert 1 galaxies using JVLA observations, revealing that their radio emission is mainly from outflows or star formation, with limited jet contribution.

Contribution

First systematic radio survey of NLS1 galaxies at frequencies above 10 GHz, identifying new jetted candidates and characterizing their spectral properties.

Findings

Most NLS1s show steep, optically thin spectra.

Two new candidate jetted NLS1s identified, including a high-frequency peaker.

No significant variability observed in repeated measurements.

Abstract

The origin of the radio emission in active galactic nuclei (AGN) is still debated. Multiple physical mechanisms can contribute to the spectrum at these frequencies, including relativistic jets, the jet base, outflows, star formation, and synchrotron emission from the hot corona. Recently, new extreme radio variability has been observed in the class of low-mass/high-Eddington AGN known as narrow-line Seyfert 1 (NLS1) galaxies, suggesting that another, more exotic mechanism may also play a role, especially at frequencies above 10 GHz. To investigate this relatively unexplored area of the radio spectrum, we observed a sample of 50 NLS1s with the Karl G. Jansky Very Large Array (JVLA), and 20 of them were observed twice. In this sample, 24 sources were not detected, while the others are typically characterized by a steep spectrum that can be modeled with a power law. We also identified two new candidate jetted NLS1s, including a high-frequency peaker, which is an extremely young relativistic jet. We found no significant variability in the sources observed twice. We conclude that the radio spectrum of NLS1s is typically dominated by optically thin emission, likely from low-power outflows, or by circumnuclear star formation, with a limited contribution from relativistic jets. Further studies at different spatial scales and at other wavelengths are necessary to fully constrain the origin of the radio emission in this class of active galaxies.