9 epochs spectroscopy of type I supernova SN 201iet

Regardless of the model, the inferred progenitor mass near the end of its life (i.e., the CO core mass) is >~55M{sun} and potentially up to 120M{sun}, clearly placing the event in the regime of pulsational pair instability supernovae (PPISNe) or pair instability supernovae (PISNe). The models of CSM interaction provide the most consistent explanation for the light curves and spectra, and require a CSM mass of ~35M{sun} ejected in the final decade before explosion.

We further find that SN2016iet is located at an unusually large projected offset (16.5kpc, 4.3 effective radii) from its low-metallicity dwarf host galaxy (Z~0.1Z{sun}, L~0.02L*, M~108.5M{sun}), supporting the interpretation of a PPISN/PISN explosion. In our final spectrum at a phase of about 770 rest-frame days we detect weak and narrow H{alpha} emission at the location of the SN, corresponding to a star formation rate of ~3x10^-4^M{sun}/yr, which is likely due to a dim underlying galaxy host or an HII region.

Despite the overall consistency of the SN and its unusual environment with PPISNe and PISNe, we find that the inferred properties of SN2016iet challenge existing models of such events.