An optical perspective on early-stage active galactic nuclei with extreme radio flares

¹ Dipartimento di Fisica e Astronomia “G. Galilei”, University of Padova, Vicolo dell’osservatorio 3, 35122 Padova, Italy
² European Southern Observatory, Alonso de Córdova 3107, Casilla 19, 19001 Santiago, Chile
³ INAF – Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius, Italy
⁴ Departamento de Física y Astronomía, Universidad de La Serena, Av. Cisternas 1200 N, La Serena, Chile
⁵ Università degli studi dell’Insubria, Via Valleggio 11, 22100 Como, Italy
⁶ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, Italy
⁷ Laboratory of Instrumentation and Experimental Particle Physics, Av. Prof. Gama Pinto, 2 – 1649-003 Lisboa, Portugal
⁸ Department of Physics and Astronomy, Texas Tech University, Box 41051 Lubbock, 79409-1051 TX, USA
⁹ Homer L. Dodge Department of Physics and Astronomy, The University of Oklahoma, 440 West Brooks Street, Norman, 73019 OK, USA

^⋆ Corresponding author; luca.crepaldi.1@phd.unipd.it

Received: 15 July 2024
Accepted: 17 February 2025

Abstract

Over the last decade of active galactic nucleus (AGN) monitoring programs, the Metsähovi Radio Observatory has made multiple detections of seven powerful flaring narrow-line Seyfert 1 (NLS1) galaxies at 37 GHz. Several hypotheses have been proposed, but understanding this unique phenomenon is still far away. To look at the case from a different point of view, we performed an emission line analysis of the optical spectra, with the aim of identifying similarities among the sources, which in turn can possibly be tied with radio behavior. Our data were obtained with the Gran Telescopio Canarias. The results we obtained show that six out of the seven sources have typical properties for the NLS1 class, and one of them is an intermediate Seyfert galaxy. We found on average black hole masses above the median value for the class (> 10⁷ M_⊙), and a strong Fe II emission, which could be a proxy for an intense ongoing accretion activity. Although interesting, the characteristics we found are not unusual for this kind of AGN: the optical spectra of our sources do not relate with their unique radio properties. Therefore, further multi-wavelength studies will be necessary to narrow the field of hypotheses for this peculiar phenomenon.