Letter to the Editor

Discovery of a giant radio outburst of the narrow-line Seyfert 1 galaxy SDSS J110546.07+145202.4

¹ Department of Astronomy, Institute of Physics and Astronomy, ELTE Eötvös Loránd University, Pázmány Péter sétány 1/A, H-1117 Budapest, Hungary
² HUN-REN–ELTE Extragalactic Astrophysics Research Group, ELTE Eötvös Loránd University, Pázmány Péter sétány 1/A, 1117 Budapest, Hungary
³ Konkoly Observatory, HUN-REN Research Centre for Astronomy and Earth Sciences, Konkoly-Thege Miklós út 15-17, 1121 Budapest, Hungary
⁴ CSFK, MTA Centre of Excellence, Konkoly-Thege Miklós út 15-17, 1121 Budapest, Hungary
⁵ Institute of Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudzidzka 5, 87-100 Toruń, Poland
⁶ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
⁷ Institute of Physics and Astronomy, ELTE Eötvös Loránd University, Pázmány Péter sétány 1/A, H-1117 Budapest, Hungary

^⋆ Corresponding author: k.gabanyi@astro.elte.hu

Received: 7 August 2025
Accepted: 25 September 2025

Abstract

Aims. We have identified a high-amplitude radio outburst in the course of a large-sample study of the radio properties of narrow-line Seyfert 1 (NLS1) galaxies.

Methods. We have analysed previous radio data and obtained new radio observations with the Effelsberg 100 m telescope, in order to measure the properties and understand the nature of the high-amplitude radio variability. We have also searched for signs of variability in the infrared and optical bands using archival data.

Results. We report the discovery of a rare high-amplitude radio outburst of a NLS1 galaxy, SDSS J110546.07+145202.4, with an amplitude of a factor of > 20 at centimetre wavelengths within 18 yr, and remaining at high-state for at least 7.6 yr. Thus, the object transitioned to a radio-loud state with a radio-loudness parameter exceeding 150. The radio spectrum measured at gigahertz frequencies during the 2020s is flat. We did not find indications of a similar increase in brightness in optical surveys or in the infrared measurements of the Wide-field Infrared Survey Explorer.

Conclusions. The variability characteristics are inconsistent with tidal disruption events, and hard to reconcile with blazar variability.