The radio structure of the $\gamma$-ray narrow-line Seyfert 1 galaxy SDSS J211852.96$-$073227.5

TL;DR

This study presents high-resolution VLBA images of the gamma-ray NLS1 galaxy SDSS J211852.96-073227.5, revealing a core-jet structure, relativistic beaming, and spectral properties suggesting it is a low-mass, low-power blazar candidate.

Contribution

First high-resolution VLBA imaging of this gamma-ray NLS1, demonstrating its blazar-like properties and potential evolutionary link to blazars.

Findings

Core-jet morphology observed in VLBA images.

Significant flux density variability indicating relativistic beaming.

Spectral analysis shows inverted spectrum above 1 GHz and steep spectrum at lower frequencies.

Abstract

The $\gamma$-ray narrow-line Seyfert 1 (NLS1) galaxies can be considered to be the third class of $\gamma$-ray active galactic nuclei possessing relativistic jets. In this paper, we present multi-band high resolution Very Long Baseline Array (VLBA) images of the $\gamma$-ray NLS1, SDSS J211852.96$-$073227.5 (J2118$-$0732, $z=0.26$). We find a core-jet radio morphology and significant flux density variations in the radio core. The high brightness temperature estimated from VLBA images and core variability demonstrate that it exhibits substantial relativistic beaming effects. From considering radio emission in several bands, we find that the source has an inverted spectrum above 1 GHz but a steep spectrum at low frequencies from 74 MHz to 1 GHz; these may arise from the present activity and the old diffuse/extended emission, respectively. The core-jet morphology, significant flux density variations, and beaming effect make J2118$-$0732 resemble a blazar. Considering the low mass of its central black hole and ongoing merger environment, J2118$-$0732 may represent a low-mass, low-power counterpart of blazars, and may finally evolve to a blazar.