2018A&A...620A..67A

Context. Super-luminous supernovae (SLSNe) are rare events defined as being significantly more luminous than normal terminal stellar explosions. The source of the additional power needed to achieve such luminosities is still unclear. Discoveries in the local Universe (i.e. z<0.1) are scarce, but afford dense multi-wavelength observations. Additional low-redshift objects are therefore extremely valuable.
Aims. We present early-time observations of the type I SLSN ASASSN-18km/SN 2018bsz. These data are used to characterise the event and compare to literature SLSNe and spectral models. Host galaxy properties are also analysed.
Methods. Optical and near-IR photometry and spectroscopy were analysed. Early-time ATLAS photometry was used to constrain the rising light curve. We identified a number of spectral features in optical-wavelength spectra and track their time evolution. Finally, we used archival host galaxy photometry together with HII region spectra to constrain the host environment.
Results. ASASSN-18km/SN 2018bsz is found to be a type I SLSN in a galaxy at a redshift of 0.0267 (111Mpc), making it the lowest-redshift event discovered to date. Strong CII lines are identified in the spectra. Spectral models produced by exploding a Wolf-Rayet progenitor and injecting a magnetar power source are shown to be qualitatively similar to ASASSN-18km/SN 2018bsz, contrary to most SLSNe-I that display weak or non-existent CII lines. ASASSN-18km/SN 2018bsz displays a long, slowly rising, red "plateau" of >26 days, before a steeper, faster rise to maximum. The host has an absolute magnitude of -19.8 mag (r), a mass of M*=1.5_–0.33^+0.08x10⁹M_☉, and a star formation rate of =0.50_–0.19^+2.22M_☉/yr. A nearby HII region has an oxygen abundance (O3N2) of 8.31±0.01dex.