Close, bright, and boxy: the superluminous SN 2018hti

SN 2018hti was a very nearby (z = 0.0614) superluminous supernova with an exceedingly bright absolute magnitude of -21.7 mag in r band at maximum. The densely sampled pre-maximum light curves of SN 2018hti show a slow luminosity evolution and constrain the rise time to ~50 rest-frame d. We fitted synthetic light curves to the photometry to infer the physical parameters of the explosion of SN 2018hti for both the magnetar and the CSM-interaction scenarios. We conclude that one of two mechanisms could be powering the luminosity of SN 2018hti; interaction with ~10 M_⊙ of circumstellar material or a magnetar with a magnetic field of B_p~ 1.3 × 10¹³ G, and initial period of P_spin~ 1.8 ms. From the nebular spectrum modelling we infer that SN 2018hti likely results from the explosion of a ${\sim}40\, \mathrm{M}_\odot$ progenitor star.