Intra-night optical variability and radio characteristics of extremely radio-loud narrow-line Seyfert 1 galaxies

TL;DR

This study investigates the intra-night optical variability and radio properties of extremely radio-loud narrow-line Seyfert 1 galaxies, revealing blazar-like behavior and characteristics akin to flat spectrum radio quasars.

Contribution

It provides the first intra-night optical monitoring for most of these rare galaxies and links their optical and radio features to blazar-like activity.

Findings

High duty cycle of INOV up to 25% with large amplitudes.

RL-NLS1s are luminous, compact, flat spectrum radio sources.

They are low-redshift, low-luminosity analogs of flat spectrum radio quasars.

Abstract

Narrow-line Seyfert 1 galaxies (NLS1s) are generally known to be radio-quiet Active Galactic Nuclei (AGN), but a tiny subset of them are found to be extremely radio-loud with radio loudness parameter ($R_{\rm 1.4~GHz}$) $>$ 100. Given their rarity we investigated intra-night optical variability (INOV) and radio characteristics of a sample of 16 extremely radio-loud NLS1s. For all but four sample sources we report intra-night photometric monitoring for the first time with at least one monitoring session per source lasting for a minimum of 3.0 hours duration. In our sample, we detect INOV with a high duty cycle (up to 25 per cent) and large average amplitude ($\overline{\psi}$ $\sim$ 0.16) similar to that found in blazars. Using 3.0 GHz Very Large Array Sky Survey (VLASS) and auxiliary multi-frequency radio data we find that our RL-NLS1s are luminous ($L_{\rm 3.0~GHz}$ $\geq$ 10$^{24}$ W~Hz$^{-1}$), compact (less than a few kpc), variable, flat spectrum (${\alpha}_{\rm radio}$ $>$ -0.5) radio sources. The INOV, radio characteristics, and radio luminosity ($L_{\rm 1.4~GHz}$) versus super-massive black hole mass ($M_{\rm SMBH}$) plot infer that extremely radio-loud NLS1s are low-$z$ and low-luminosity analogs of flat spectrum radio quasars wherein the former are powered by, on average, one order-of-magnitude less massive SMBHs.