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

TL;DR

SDSS J110546.07+145202.4, a narrow-line Seyfert 1 galaxy, hosts an exceptional giant radio outburst with an amplitude of $>20$ at centimetre wavelengths over a span of about $18$ years and a sustained high-state for at least $7.6$ years, transitioning to a radio-loud state with a radio-loudness parameter exceeding $150$. The study combines historical radio data (FIRST, NVSS, GB6, VLASS, RACS) with new Effelsberg observations to map the light curve and determine a flat spectrum in the GHz regime, with a spectral index α ≈ $0.3$. Multi-wavelength analysis using WISE/NEOWISE infrared data and optical surveys finds no dramatic contemporaneous variability, arguing against TDEs and straightforward blazar-type swings. The authors discuss a plausible scenario in which a change in the accretion mode drives long-lived jet activity, and highlight the need for high-resolution radio follow-up and time-domain optical surveys to capture future similar events and constrain jet-disc coupling in NLS1s.

Abstract

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

Figures (2)

• Figure 1: Radio images of SDSS J1105$+$1452. The colour scale shows the $1.4$-GHz FIRST image first_white. The grey circle shows the restoring beam of the FIRST image in the lower left corner. The white contours represent the $3$-GHz VLASS SE image taken on August 18 2020. The peak intensity is $38.7$ mJy beam$^{-1}$. The lowest contours are drawn at $\pm 0.4$ mJy beam$^{-1}$ (corresponding to a $3 \sigma$ image noise level); further contours increase by a factor of two. The restoring beam of the VLASS SE image is shown as a white ellipse in the lower right corner. Its FWHM size is $2\overset{\prime\prime}{.}7 \times 2\overset{\prime\prime}{.}2$. The major axis of the position angle is $29^\circ$. The VLASS and FIRST radio emission is consistent with a point source and no extended jet structure is detected. Further, no other bright radio sources that could have affected the radio measurements are in the surrounding field. The black cross marks the Gaia DR3 optical position Gaia_DR3.
• Figure 2: Radio spectrum of SDSS J1105$+$1452. The black dots are from the RACS and the VLASS. The solid line is a power law fit to these points. The magenta diamond is from the FIRST. Green triangles show our Effelsberg measurements. The upper limits from NVSS and GB6 are displayed as black downward arrows with observing epochs indicated (for details see, Table \ref{['table:radioflux']}). The dashed magenta line shows the dramatic brightening at $1.4$ GHz.