Program: GN-2023B-FT-106

AT 2021lwx (aka “Scary Barbie”) is one of the most energetic and luminous transient events ever observed. From a spectroscopically measured redshift of 0.9945, and slowly fading g and r light curves spanning over 1000 observer-frame days that peak with an absolute magnitude of M_r = -25.7 mag, AT 2021lwx has an extraordinary peak pseudo-bolometric luminosity of log(Lmax/[erg/s]) = 45.7. As of June 2023, the transient has already surpassed a total radiative energy of 10^53 erg, and it continues decline slowly in a manner that will make it possible to monitor for years to come. Despite not having a detected host galaxy, multiple lines of evidence suggest that AT 2021lwx is most likely an extreme supermassive black hole (SMBH) accretion event. Favored interpretations include the tidal disruption event (TDE) involving a 15 Msun star and a SMBH with mass 10^8 Msun, or accretion of a giant molecular cloud on to a dormant 10^8 to 10^9 Msun SMBH. These scenarios are at the outer limits of other SMBH transients and make AT 2021lwx a unique opportunity to investigate an extreme extension of the known scenarios of black hole accretion. Here we request Gemini-N + GNIRS observations that can provide NIR spectroscopy needed to complement a multiwavelength data set that can be used to test proposed SMBH progenitor systems and accretion physics using radiative transfer and radiation hydrodynamics simulations.