Speaker
Description
Recent time-domain surveys are revealing a growing population of nuclear transients that trace rapid changes in the accretion state of supermassive black holes. These events offer a unique opportunity to study how previously quiescent or weakly active nuclei transition into phases of enhanced accretion and variability. In this contribution I will present recent multiwavelength results on a small sample of nearby galaxies that display different manifestations of such nuclear activity.
In the nearby LINER galaxy NGC 7213, new radio observations reveal evidence for a weak jet and compact hotspots associated with a previous episode of nuclear activity, suggesting recurrent jet launching on galactic scales. In the nuclear transient AT2021hdr, optical monitoring uncovered oscillations in the light curve with characteristic timescales of ∼60–90 days, accompanied by correlated variability in the UV and X-ray bands. This unusual behaviour challenges standard interpretations in terms of tidal disruption events or classical AGN variability and may instead point to the tidal disruption of a gas cloud in a binary supermassive black hole system.
Another remarkable case is the galaxy SDSS J1335+0728, which showed no detectable nuclear variability for decades before entering a new active phase in 2019. Multiwavelength observations reveal a rapidly evolving UV/optical and mid-infrared emission together with the recent appearance of soft X-ray radiation, suggesting that the central black hole may be in the process of turning on as an AGN. Follow-up observations have also revealed the presence of X-ray quasi-periodic eruptions (QPEs) in this source, placing it among the rare systems where extreme nuclear variability can be studied in real time.
Together, these objects illustrate the rich phenomenology of nuclear transients and highlight how time-domain surveys are uncovering a dynamic population of galaxies in which supermassive black holes intermittently switch between quiescent and active states. Studying these systems provides new insight into the physical mechanisms governing accretion instabilities, episodic jet activity, and the onset of AGN phases. In the coming years, facilities such as the Vera C. Rubin Observatory (LSST) and the Square Kilometre Array (SKA) will dramatically expand the discovery space of nuclear transients, enabling systematic studies of the onset and evolution of accretion and jet activity in galactic nuclei.