A multiwavelength perspective on extreme flares in narrow-line Seyfert 1 galaxies

A few decades have passed since the identification of narrow-line Seyfert 1 (NLS1) galaxies as a subclass of active galactic nuclei (AGN). NLS1s show a Seyfert 1-like spectrum, but with permitted emission line widths comparable to those of Seyfert 2 spectra. These peculiar traits suggest a combination of factors, high accretion rates near the Eddington limit and low-mass black holes, which indicate an early stage in the AGN life cycle. Although few, jetted NLS1s have been discovered. Recently, seven sources with an inverted radio spectrum and extreme radio variability were identified by the Metsähovi Radio Observatory, among a sample of radio-quiet and radio-silent NLS1s. They show rapid flares at 37 GHz that increase their flux density up to 9000-fold (Jy level) with an e-folding time-scale of a few hours. During quiescence and at lower frequencies, they achieve only mJy levels. As the main cause, relativistic jets would have been the most likely, but no jet emission traits were detected. Until now, their radio spectra are the only common feature. To explain the flares’ behavior, three main hypotheses have been suggested: jet-cloud/star interaction, relativistic jet and free-free absorption with moving clouds, and magnetic reconnection. In this thesis, I carried out three works to study the sample from different perspectives, with the goals of investigating the peculiarities of the sample and estimating the flares’ properties. In the first work, I performed a multi-epoch analysis for different optical bands, in particular through a variability and periodicity Fourier-based analysis. I used publicly available data retrieved from the catalogs of All-Sky Automated Survey for Supernovae and Zwicky Transient Facility surveys. For the second work, I derived the main physical properties of the sources in the sample by means of an optical spectra analysis. Here I used proprietary data acquired with the long-slit spectrometer Optical System for Imaging and low-Intermediate-Resolution Integrated Spectroscopy mounted on the Gran Telescopio Canarias. Finally, in the third work, I moved on to the radio regime to estimate the main characteristics of the flare episodes using both single-dish and interferometer data at multiple frequencies. The results revealed no apparent connection between the flare episodes detected in radio and the optical behavior. From the spectra, one of the sources turned out to be an intermediate Seyfert galaxy, while the remaining six showed typical values for the NLS1 class. On the other hand, the radio analysis results on the flares, with e-folding time-scales down to a few tens of minutes and variability brightness temperatures >10^15 K, were much more extreme and unexpected. Considering the multi-wavelength results, the magnetic reconnection in the black hole magnetosphere seems to be the most likely driver in the flare production. In particular, considering the radio behavior, the magnetic reconnection might be coupled with an emission mechanism as either an inverse Compton catastrophe or a coherent emission. To disentangle between these two main scenarios, and to further investigate also less viable hypotheses, follow-up radio observations very close in time will be fundamental.