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

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.