The Most Energetic Transients: Tidal Disruptions of High-Mass Stars

We present the class of extreme nuclear transients (ENTs), including the most energetic single transient yet discovered, Gaia18cdj. Each ENT is coincident with its host-galaxy nucleus and exhibits a smooth ($<$$10$% excess variability), luminous ($2\times$${10}^{45}$ to $7\times$${10}^{45}$ erg s${}^{-1}$), and long-lived ($>$$150$ days) flare. ENTs are extremely rare ($\ge$$1$$\times$${10}^{-3}$ Gpc${}^{-3}$ yr${}^{-1}$) compared to any other known class of transients. They are at least twice as energetic ($0.5\times {10}^{53}$ to $2.5\times {10}^{53}$ erg) as any other known transient, ruling out supernova origins. Instead, the high peak luminosities, long flare timescales, and immense radiated energies of the ENTs are most consistent with the tidal disruption of high-mass ($\gtrsim$$3$ M${}_{\odot }$) stars by massive ($\gtrsim$${10}^8$ M${}_{\odot }$) supermassive black holes (SMBHs). ENTs will be visible to high redshifts ($z\sim 4$ to $6$) in upcoming surveys, providing an avenue to study the high-mass end of the SMBH mass distribution, complementing recent studies of actively accreting SMBHs at high redshifts with the James Webb Space Telescope.