2023ApJ...948L..19S

We report multiwavelength observations and characterization of the ultraluminous transient AT 2021lwx (ZTF20abrbeie; aka “Barbie”) identified in the alert stream of the Zwicky Transient Facility (ZTF) using a Recommender Engine For Intelligent Transient Tracking filter on the ANTARES alert broker. From a spectroscopically measured redshift of 0.995, we estimate a peak-observed pseudo-bolometric luminosity of log(${L}_{\max }/[\mathrm{erg}\,{{\rm{s}}}^{-1}]$) = 45.7 from slowly fading ztf-g and ztf-r light curves spanning over 1000 observer-frame days. The host galaxy is not detected in archival Pan-STARRS observations (g > 23.3 mag), implying a lower limit to the outburst amplitude of more than 5 mag relative to the quiescent host galaxy. Optical spectra exhibit strong emission lines with narrow cores from the H Balmer series and ultraviolet semi-forbidden lines of Si III] λ1892, C III] λ1909, and C II] λ2325. Typical nebular lines in Active Galactic Nucleus (AGN) spectra from ions such as [O II] and [O III] are not detected. These spectral features, along with the smooth light curve that is unlike most AGN flaring activity and the luminosity that exceeds any observed or theorized supernova, lead us to conclude that AT 2021lwx is most likely an extreme tidal disruption event (TDE). Modeling of ZTF photometry with MOSFiT suggests that the TDE was between a ≈14M_⊙ star and a supermassive black hole of mass M_BH ∼ 10⁸M_⊙. Continued monitoring of the still-evolving light curve along with deep imaging of the field once AT 2021lwx has faded can test this hypothesis and potentially detect the host galaxy.