What powers the most energetic transients in the Universe?

Phil Wiseman (Southampton)

Ambiguous nuclear transients (ANTs) are extremely luminous events occurring in the centres of galaxies. The most energetic of all, AT2021lwx, at redshift 1, shows a single optical brightening to a luminosity close to 10^46 erg/s, followed by a smooth decline over more than a year in the rest frame. X-ray and mid-infrared flares accompany the optical transient. The luminosity and timescale indicate that a large supermassive black hole is likely the central engine, but how the energy is released, and from what, remains a mystery.   Despite their duration and luminosity, luminous ANTs are frequently missed by follow-up and classification efforts. AT2021lwx, and a handful of less-luminous analogues (Frederick et al. 2021), were discovered serendipitously, and post-peak. In this talk I present a systematic search for luminous, long-duration nuclear transients. I reveal a number of exciting candidates, some in known active galactic nuclei, and others in unclassified point-sources (likely quasars) and some with no previous signatures of accretion at all. All show delayed mid-infrared flares, clear signatures of dust reprocessing. Neither regular gas accretion nor tidal disruptions of stars explain the data well. I then discuss the exciting but complex challenges that searching for ANTs presents as we enter the era of the Vera Rubin Observatory, distinguishing ANTs from regular active galactic variability, and strategies for spectroscopic follow up with facilities such as 4MOST.