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 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 days to more than +340 days from the r-band maximum. Combining our BVgri-band photometry with Swift UVOT optical/ultraviolet photometry, we calculated the peak luminosity as ∼ 3.5×10^44 erg s^-1. Modeling 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=1.8×10^13 G having an initial spin period of P_0=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_ZAMS≳ 25 M_⊙.