We present optical photometry and spectroscopy of SN 2016iet ( =Gaia16bvd =PS17brq ) , an unprecedented Type I supernova ( SN ) at z = 0.0676 with no obvious analogue in the existing literature .
SN 2016iet exhibits a peculiar light curve , with two roughly equal brightness peaks ( \approx - 19 mag ) separated by about 100 days , and a subsequent slow decline by about 5 mag in 650 rest-frame days .
The spectra are dominated by strong emission lines of calcium and oxygen , with a width of only 3400 km s ^ { -1 } , superposed on a strong blue continuum in the first year .
There is no clear evidence for hydrogen or helium associated with the SN at any phase .
The nebular spectra exhibit a ratio of L _ { [ Ca II ] } / L _ { [ O I ] } \approx 4 , much larger than for core-collapse SNe and Type I SLSNe , but comparable to the so-called Ca-rich transients .
We model the light curves with several potential energy sources : radioactive decay , a central engine , and ejecta-circumstellar medium ( CSM ) interaction .
Regardless of the model , the inferred progenitor mass near the end of its life ( i.e. , the CO core mass ) is \gtrsim 55 M _ { \odot } and potentially up to 120 M _ { \odot } , 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 \approx 35 M _ { \odot } ejected in the final decade before explosion .
We further find that SN 2016iet is located at an unusually large projected offset ( 16.5 kpc , 4.3 effective radii ) from its low metallicity dwarf host galaxy ( Z \approx 0.1 Z _ { \odot } , L \approx 0.02 L _ { * } , M \approx 10 ^ { 8.5 } M _ { \odot } ) , 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 \approx 3 \times 10 ^ { -4 } M _ { \odot } yr ^ { -1 } , which is likely due to a dim underlying galaxy host or an \ion H2 region .
Despite the overall consistency of the SN and its unusual environment with PPISNe and PISNe , we find that the inferred properties of SN 2016iet challenge existing models of such events .