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 H II 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 (111 Mpc), making it the lowest-redshift event discovered to date. Strong C II 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 C II 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+0.08 × 109 M⊙, and a star formation rate of = 0.50−0.19+2.22 M⊙ yr −1. A nearby H II region has an oxygen abundance (O3N2) of 8.31 ± 0.01 dex.
- Astrophysics - High Energy Astrophysical Phenomena