Supermassive Black Hole Transients: From Single Brush Strokes to the Entire Painting

In the past decade, a plethora of transients in galaxy centers has been discovered and their study has become a flourishing field of research. Many of these so-called “nuclear transients” (given their position in the nuclei of galaxies) are interpreted as tidal disruption events (TDEs) around supermassive black holes (SMBHs) and are used to study accretion physics, galaxy dynamics, SMBH growth and environments, stellar populations in galactic nuclei and more. However, important puzzles surrounding these transients – their nature, emission mechanisms and host-galaxy preference – remain unsolved largely due to their rarity (<100 events are known). We need larger samples to measure their rates, map their classes and outliers, test connections found with small-number statistics, and crystallize their host-galaxy preference, before using them to draw far-reaching conclusions across fields. The Rubin Observatory will detect thousands of nuclear transients, but they will be nearly indistinguishable in optical photometry from supernovae (SNe) during their first 30 days. With SNe outnumbering nuclear transients 40:1 in galaxy centers, spectral classification is unfeasible. In the UV, however, SNe and nuclear transients differ from day 0, making the UV key to discovering them at an early (and hence unbiased) phase by the thousands. Existing UV facilities can vet only a small fraction of candidates, but ULTRASAT, launching in 2027 with a 200-square-degree UV imager, will detect thousands of nuclear transients, changing the field in a revolutionary way. As chair of the TDE working group in ULTRASAT, I seek ERC funding to leverage this transformative opportunity. I will increase the sample of nuclear transients at all phases by an order of magnitude, constraining their nature, emission mechanisms, host-galaxy preference and environments. This will effectuate nuclear transients as probes of accretion theory, SMBH growth, and stellar dynamics and populations in galaxy centers.