SN 2018hti: a nearby superluminous supernova discovered in a metal-poor galaxy

SN 2018hti is a Type I superluminous supernova (SLSN I) with an absolute g-band magnitude of -22.2 at maximum brightness, discovered by the Asteroid Terrestrial-impact Last Alert System in a metal-poor galaxy at a redshift of 0.0612. We present extensive photometric and spectroscopic observations of this supernova, covering the phases from ∼-35 d to more than +340 d from the r-band maximum. Combining our BVgri-band photometry with Swift UVOT optical/ultraviolet photometry, we calculated the peak luminosity as ∼3.5 × 10⁴⁴ erg s^-1. Modelling the observed light curve reveals that the luminosity evolution of SN 2018hti can be produced by an ejecta mass of 5.8 M_⊙ and a magnetar with a magnetic field of B = 1.8 × 10¹³ G having an initial spin period of P₀ = 1.8 ms. Based on such a magnetar-powered scenario and a larger sample, a correlation between the spin of the magnetar and the kinetic energy of the ejecta can be inferred for most SLSNe I, suggesting a self-consistent scenario. Like for other SLSNe I, the host galaxy of SN 2018hti is found to be relatively faint (M_g = -17.75 mag) and of low metallicity (Z = 0.3 Z_⊙), with a star formation rate of 0.3 M_⊙ yr^-1. According to simulation results of single-star evolution, SN 2018hti could originate from a massive, metal-poor star with a zero-age main sequence (ZAMS) mass of 25-40 M_⊙, or from a less massive rotating star with M_ZAMS ≈ 16-25 M_⊙. For the case of a binary system, its progenitor could also be a star with $M_\mathrm{ZAMS} \gtrsim 25\, \mathrm{ M}_\odot$ .