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 \(\sim -35\) days to more than +340 days from the \(r\)-band maximum. Combining our \(BVgri\)-band photometry with {\it Swift} UVOT optical/ultraviolet photometry, we calculated the peak luminosity as \(\sim 3.5\times10^{44}\) 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 \(M_\odot\) and a magnetar with a magnetic field of \(B=1.8\times10^{13}\)~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 (\(Z=0.3~Z_\odot\)), with a star-formation rate of 0.3 \(M_\odot\) 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 \(M_\odot\), or from a less massive rotating star with \(M_\mathrm{ZAMS} \approx 16\)--25 \(M_\odot\). For the case of a binary system, its progenitor could also be a star with \(M_\mathrm{ZAMS} \gtrsim 25\) \(M_\odot\).