2025ApJ...990...79S

Narrow-line Seyfert 1 galaxies (NLS1s) are generally known to be radio-quiet active galactic nuclei, but a tiny subset of them are found to be extremely radio-loud with radio loudness parameter (R_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 hr duration. In our sample, we detect INOV with a high duty cycle (up to 25%) and large average amplitude ($\overline{\psi }$∼ 0.16) similar to that found in blazars. Using 3.0 GHz Very Large Array Sky Survey and auxiliary multifrequency radio data we find that our RL-NLS1s are luminous (L_3.0 GHz_ ≥ 10²⁴ W Hz^–1), compact (less than a few kpc), variable, flat spectrum (α_radio> -0.5) radio sources. The INOV, radio characteristics, and radio luminosity (L_1.4 GHz_) versus supermassive black hole mass (M_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.